Development of Ferromagnetic/Insulator/Ferromagnetic ...

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Development of Ferromagnetic/Insulator/Ferromagnetic Devices For Digital Magnetic Data Storage and Magnetic Field Sensors Thesis Submitted to the Faculty of Physics and Astronomy Ruhr-Universität Bochum For the degree of Doktor der Naturwissenschaften (Dr. rer.nat.) By Emad Azmy Sultan Girgis Institute For Thin Film and Ion Technology Forschungszentrum Juelich Germany 2000

Transcript of Development of Ferromagnetic/Insulator/Ferromagnetic ...

Page 1: Development of Ferromagnetic/Insulator/Ferromagnetic ...

Developmentof

Ferromagnetic/Insulator/Ferromagnetic DevicesFor

Digital Magnetic Data Storage and Magnetic Field Sensors

ThesisSubmitted to the Faculty of Physics and Astronomy

Ruhr-Universität Bochum

For the degree ofDoktor der Naturwissenschaften (Dr. rer.nat.)

By

Emad Azmy Sultan Girgis

Institute For Thin Film and Ion TechnologyForschungszentrum Juelich

Germany

2000

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I. Introduction ....................................................................................................................................................4

II. Literature ........................................................................................................................................................6

II.1. Tunnel magnetoresistance (TMR) ....................................................................................................6

II.1.1. Electron tunneling ....................................................................................................................6

II.1.2. Spin dependent tunneling .......................................................................................................8

II.1.3. Preparation of tunnel magnetoresistance devices (TMR) .............................................10

II.1.3.1. Preparation of the electrodes ..................................................................................10

II.1.3.2. Preparation of the barrier .........................................................................................11

II.1.4. Applications of tunnel magetoresistance devices ............................................................13

II.1.4.1. Magnetic data storage ...............................................................................................13

II.1.4.2. Magnetic field sensors ..............................................................................................16

III. Experimental ................................................................................................................................................18

III. 1. Preparation of the samples ........................................................................................................18

III.1.1. Natural oxidation method ................................................................................................18

III.1.2. Thermal oxidation method ..............................................................................................19

III.1.3. Ultraviolet oxidation assisted in oxygen method .....................................................19

III.2. Processing the samples .................................................................................................................21

III.2.1. Optical lithography ............................................................................................................21

III.2.2. Electron beam lithography ..............................................................................................22

III.3. Materials characterization methods .......................................................................................23

III.3.1. Alternative gradient magnetometer (AGM) ...............................................................23

III.3.2. SQUID magnetometer ......................................................................................................25

III.3.3. Atomic force microscopy (AFM) ..................................................................................26

III.3.4. Magnetic force microscopy (MFM) .............................................................................26

III.3.5. Magnetic Optic Kerr Effect (MOKE) ..........................................................................27

III.3.6. X-ray Photoemission Spectrum (XPS) .......................................................................28

III.4. TMR devices characterization methods ................................................................................28

III.4.1. Characterization of the electrical properties of TMR devices .............................28

III.4.2. Characterization of the magnetic properties of TMR devices...............................29

III.4.3. Realibility and breakdown of TMR devices ..............................................................30

III.4.4. Micromagnetic simulation of TMR devices based

on solution of Landau-Lifshitz-Gilbert equation .........................................................30

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III.4.5. Characterization of the TMR elements using MRAM architecture ....................32

IV. Results and Discussions .............................................................................................................................34

IV.1. Characterization of the substrate ......................................................................................................34

IV.1.1. Surface roughness for Si/SiO2 wafer using AFM .........................................................34

IV.2. Characterization of the magnetic materials ....................................................................................35

IV.2.1 Characterization of the Cobalt thin film ..............................................................................36

IV.2.1.1. Magnetic switching characteristics of thin film Cobalt

in nanometer-scale patterned arrays of elements...........................................36

IV.2.1.2. Magnetic switching characteristics of thin film Cobalt

at different temperature .........................................................................................39

IV.2.1.3. Images of Cobalt thin film patterned element arrays

by AFM and MFM microscopes.........................................................................42

IV.2.1.4. Magnetic switching simulation of Cobalt thin film

at different aspect ratios ......................................................................................45

IV.2.2. Characterization of Nickel-Iron thin film .........................................................................47

IV.2.2.1. Magnetic switching characteristics of Nickel-Iron thin

film in nanometer-scale patterned arrays of elements..................................47

IV.2.2.2. Images of Nickel-Iron thin film patterned element arrays

by AFM and MFM microscopes .......................................................................50

IV.2.2.3. Magnetic switching simulation of Nickel-Iron thin film

at different geometry .............................................................................................51

IV.3. Characterization of the barrier .........................................................................................52

IV. 3.1. Barrier thickness variations......................................................................................52

IV.3.2. Structural characterization of the barrier using (XPS) .....................................54

IV.4. Characterization of TMR devices .....................................................................................55

IV.4.1. Magnetic switching characteristics of NiFe/Al2O3/Co trilayers

in nanometer-scale patterned arrays of elements................................................55

IV.4.2. Images of NiFe/Al2O3/Co trilayers patterned element

arrays by AFM and MFM microscopes ................................................................59

IV.4.3. Magnetic Optic Kerr Effect (MOKE) for NiFe/Al2O3/Co trilayers ..............60

IV.4.4. Magnetic switching simulation of NiFe/Al2O3/Co trilayers

at different aspect ratios .............................................................................................61

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IV.4.5. Electrical and magnetic properties of the TMR devices

at different oxidation methods ..................................................................................63

IV.4.6. Electrical and magnetic properties of the TMR devices

at different temperatures ............................................................................................65

IV.4.7. Further Parameters which effect on the tunnel magnetoresistance

devices .............................................................................................................................68

IV.4.8. Realibility and breakdown of TMR devices ........................................................73

IV.4.9. MRAM array of tunnel magnetoresistance elements characterization.........75

Summary..................................................................................................................................................................82

References.............................................................................................................................................................. .85

Zusammenfassung ...............................................................................................................................................90

Acknowledgment...................................................................................................................................................98

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I. Introduction

In science and technology, as well as in daily life, magnetism plays an important role. One of the

applications of magnetism in the present technology can be found in the ability of magnetic

materials to store information. This includes hard disks, floppy disks, optical storage devices and

magnetic strips on credit cards. Recent advances in this area have been driven in many respects

by the improvements of the magnetic materials.

A special class of the magnetic materials are the so-called magnetic multilayers or

superlattices, consisting of alternate layers of magnetic and non-magnetic metallic layers with

typical thickness of the order of nm (10-9m). In 1988 it has been discovered that a multilayer

consisting of alternating magnetic and non-magnetic layers shows a large magnetoresistance

effect when a magnetic field was applied upon the system, denoted as “giant magnetoresistance

or GMR “ [1]. These effect results from spin dependent scattering of electrons within the

magnetic layers or at their interfaces. This effect arises as the result of the mutual alignment of

the magnetization directions of the magnetic layers which varies as a function of applied

magnetic field: antiparallel in the absence of a field but parallel in a sufficiently large external

field. Since the scattering of the electrons is spin dependent, the two spin currents lead to

different resistances for both situations resulting in the observed magnetoresistance. The

discovery of this effect led to an enormous increase of effort in scientific research on layered

magnetic films. The GMR effect was found in many other combinations of

magnetic/nonmagnetic (e.g. Co/Cu/Nife) layers. The effect exists irrespective of the direction of

the current with respect to the layers, for which two configurations are identified: current in the

layer plane (CIP) and current perpendicular to the layer plane (CPP). The magnetoresistance

effect is larger in the CPP than in the CIP while the resistance is extremely small.

More recently, the development of magnetoresistive devices is focused on tunnel devices,

which consist of two ferromagnetic metal layers, separated by a very thin insulation layer (type

CPP). The electron transport is perpendicular to the plane of layers and is determined by

tunneling of electrons through the insulator barrier [2]. The magnetoresistivity is based on a spin-

dependent tunneling probability caused by an energetic splitting of the energy bands with spin-up

and spin-down electrons. The magnetoresistivity is directly proportional to the polarization

values of the electrons at the two-insulator/metal interfaces, which might be different from the

bulk polarization of the two magnetic films. Nowadays, the tunnel magnetoresistance is

becoming more important when patterned into micron or submicron structure which might serve

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the high technology industry in two major ways, as a means of storing information and as sensors

to read out this stored information [3]. In the present time different types of storage are used. The

difference between the types of the storage based on the relation between the capacity vs. average

access time. One of the different types of memories which is widely used is dynamic random

access memory (DRAM), which is based on Si technology. One of disadvantage of this memory

is the backup battery, which is important to refresh the memory. It means that when users turn on

their computers, they must wait for transfer the information form the hard disc to the memory

during the “boot up “ process (and vice versa during shutdown). Nowadays there is great interest

in the possibility of fabricating DRAM, which retains its memory even after removing power

from the device. Non-volatile memories do not have this problem; therefore these memories have

important applications, e.g. for missiles as satellites. Magnetoresistive Random Access Memory

(MRAM) is one of the non-volatile memories, which is the newest technique based on the

integration of Si CMOS with magnetic memory element. One of the most important

characteristics of MRAM is the fact that it uses the spin of an electron, rather than the charge.

MRAM has unlimited read and write endurance, and could enable truly non-volatile RAM with

both the high speed of today’ s Static RAM and the high density of DRAM. Recent advances in

giant magnetoresistance (GMR) give MRAM the potential for high speed, low operating

voltages, and high density. Nowadays there is a great interest to fabricate the MRAM based on

magnetic tunnel junctions

In this thesis, the results of experiments on the spin dependent transport in structured

multilayered systems are presented. The first part deals with the characterization of the magnetic

materials using different techniques. In the second part results on the fabrication of tunnel

magnetoresistance devices with different oxidation methods are presented. These different

oxidation methods are natural oxidation and ultraviolet radiation assisted oxidation in oxygen,

which is used in the preparation of tunnel magnetoresistance devices for the first time worldwide.

Also the electrical and magnetic properties of the magnetic tunnel junctions compared with

micromagnetic modeling are presented.

The third part is the focused on the applicability of magnetoresistive devices as

magnetoresistive random access memory (MRAM) and magnetic field sensors. However, before

these structures will be actually applied in commercial devices, a good physical understanding of

their properties and their limitations is needed, which is the aim of the research described in this

thesis.

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II. Literature

II.1. Tunnel magnetoresistance (TMR)

The Magnetoresistance effect was discovered in 1857, which is defined as the changes in the

electrical resistivity of a conductor carrying a current as a function of external magnetic field.

The magnetoresistance is expressed as ∆ρ/ρo, where ∆ρ=ρ (B) - ρo is the change in resistivity

when an external magnetic field is applied, where ρo is the resistivtity at zero external magnetic

field [4]. While the research work was carried out to understand the physical importance of this

effect, different types of the magnetoresistance effect were discovered like anisotropy

magnetoresistance (AMR) [5], Giant magnetoresistance (GMR) [6] and tunnel magnetoresistance

(TMR) [7].

The tunnel magnetoresistance, which is the newest type of the magnetoresistance effect, has

attracted more interest than AMR and GMR because of its high magnetoresistance ratio at room

temperature. The multilayer device of the tunnel magnetoresistanec structure consists of two

ferromagnetic electrodes separated by a very thin nonmagnetic insulator layer. The tunnel current

through the insulator layer depends on the magnetization direction of the two ferromagnetic

electrodes relative to each other in the presence of an external magnetic field. The tunnel current

for a given voltage is higher if the magnetization directions of both electrodes are aligned in

parallel while the tunnel current for the same given voltage is lower if the magnetization of both

electrodes are aligned in antiparallel. The change of the tunnel current when the magnetization

directions of the two electrodes are parallel and antiparallel as a function of external magnetic

field is called tunnel magnetoresistance [8]. The essential effect of the TMR is based on the spin

state of the electron, therefore the tunnel magnetoresistance effect is called spin dependent

tunneling. More details about the tunneling process of the electron in metal/insulator/metal and in

ferromagnetic/insulator/ ferromagnetic film structures will be described in the next part.

II.1.1. Electron tunneling

The phenomenon of electron tunneling in metal/insulator/metal (MIM) structures describe the

electron transport from one metal to the other through the barrier. This phenomenon was studied

long time ago [9], in which the insulator (barrier) of the MIM junction was fabricated with a

thermally grown oxide on the bottom electrode metal. In the literature different models describe

the electron tunneling phenomenon; the simplest model for the electronic potential across a

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tunnel junction can be expressed as the electronic potential of the two regions of the electrode,

which is equal to the bulk materials potential. The two electrodes are separated by a barrier

region in which the potential is flat and higher than the Fermi levels in the electrodes. The barrier

height Φ is defined as the height of the barrier above the Fermi level, and d is the barrier

thickness. Different pairs metal/oxide have different Φ, it is approximately linear with the

metal/vacuum work function for a given oxide [10]. It is noted that Φ is not equal to the work

function of the material in vacuum, it depends on the electronic structure of the insulator (e.g.

size of the band gap) and the detailed electronic structure at the interface. Fig. 1-a shows the

energy diagram for such a metallic junction when no external voltage is applied. In this case the

Fermi level of both electrodes is at the same energy level, so no tunneling occurs.

Figure 1: Schematic representation of the energy diagrams of a metallic tunnel junction. (a) A square barrier withzero applied voltage, (b) the same barrier as in (a) with an applied voltage V. (c) An asymmetric barrier, with twodifferent barrier heights Φ1 and Φ2 (at zero V). The energy range within the electrodes for which electron states areoccupied (E<EF) is shaded.

The sign and the direction of the applied voltage on the MIM play an important role in the

tunneling process. If the right electrode is positively biased with voltage V, an energy difference

of eV appears across the barrier. The probability of the electrons to tunnel from the top of the

conduction band of the grounded metal to the empty levels of the positively biased metal

increased. This situation is depicted in Fig. 1-b. In this case a net electron current will flow from

the left electrode to the right electrode. The number of empty levels that can receive electrons is

proportional to the bias, so the tunnel current flow is also proportional to the bias. When two

different metals, which give rise to different barrier potentials, are used as electrodes, the barrier

will be asymmetric. For asymmetric barriers, the I-V characteristic is different to (a and b). The

minimum of the differential conductance is shifted from zero to a finite voltage, depending on the

side at which the barrier height is the lowest. Fig. 1-c depicts the energy diagram of an

EF,1EF,2

d

electrode1 electrode2

EF,1

EF,2

EF,1 EF,2

Φ0 Φ0Φ1

Φ2

eV

(a) (b) (c)

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asymmetric barrier with two different barrier heights Φ1, Φ2 for the left and right electrodes,

respectively, at zero applied voltage. The voltage dependent tunnel current can be evaluated by

Simmons’s formula, where the tunnel barrier is treated as a square barrier [11]. The occupation

probability of electron states is defined by the Fermi-Dirac distribution, which is taken into

account for calculating the rate of tunnel electrons at a certain applied voltage and temperature.

II.1.2. Spin dependent tunneling

As the electron has charge, it has spin also. The spin of the electron has two states, which are

called spin up and spin down. The nonmagnetic metal such as copper has equal numbers of

electrons with spin up and down. Therefore it has no net moment and the current carrying

electrons at the top of the Fermi level, is unpolarised [12]. The ferromagnetic metals such as

cobalt has unequal numbers of electrons with spin up and spin down see Fig. 2. The magnetic

moment of cobalt is simply proportional to the difference between the occupations of the two

spin bands. In the spin dependent tunneling of the two ferromagnetic films separated by a very

thin insulator, the electron of one spin state at the Fermi level of the first film can tunnel through

the barrier to the unoccupied states of the same spin state at the Fermi level of the second film.

Figure 2: Scheme showing the density of spin up and spin down states for nonmagnetic and magnetic metal.

If the two ferromagnetic films were magnetized parallel to each other, then the minority (spin

state up or down) of the electrons would go into minority states, and majority electrons would

pass into the majority states which leads to high tunnel current. If the two films were magnetized

in opposite directions, the identity of majority and minority would be reversed, minority electrons

Energy Energy Fermi level EF

Density of States N (E) Density of States N(E)

Copper Cobalt

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from the first film would seek empty majority states in the second, and the majority electrons

from the first film would seek minority empty states in the second which leads to low tunnel

current, see Fig. 3.

The effective density of states for the tunneling electrons with spin up and down for both

electrodes are (D1 up & D1 down) and (D2 up & D2 down) respectively. The tunnel current for

the parallel (Ip) and antiparallel (Ia) case can be expressed as:

Ip α D1↑ D2 ↑ +D1 ↓ D2 ↓ (1.a)

Ia α D1↑ D2 ↓ + D1 ↓ D2 ↑ (1.b)

The effective D↑ and D↓ are not the real densities of states of the ferromagnetic, because the

tunneling electrons are influenced by the interface between electrode and insulator [7]. The

asymmetry in the effective density of states of the up and down electrons is described by the

polarization P of the ferromagnetic:

P= (D↑ – D↓)/ (D↑ + D↓) (2)

The magnetoresistance ratio, can be written as

∆R/R= 2P1P2/ (1-P1P2) (3)

where P1 and P2 represent the polarization of the tunneling electrons at the first and the second

electrode. The TMR ratio becomes infinite if a material has a polarization of 100 % [13,14].

Figure 3: Scheme of spin-dependent tunneling: the similar density of states for a parallel alignment of themagnetization direction of the ferromagnetic layers (top) leads to a greater conductance as compared to theantiparallel alignment (bottom).

e-EF

M1 M2

EF

M1 M2

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II.1.3. Preparation of tunnel magnetoresistance devices (TMR)

Evaporation deposition and sputter depositions are two methods, which were used to prepare the

electrodes and the barrier of the TMR devices. In the case of evaporation, a crucible with source

material is heated, leading to evaporation of the source atoms. Due to the low vacuum pressure,

the evaporated atoms move collision-free through the vacuum chamber and condense on the

substrate. In the case of sputter deposition, the source atoms are sputtered from a target due to the

impact of highly energetic Ar ions. The chamber is filled with Ar gas at a low pressure, and

because the substrate is close enough to the source (i.e. 5cm), the atoms will be deposited on the

substrate with few collisions in the sputter gas. In this case the atoms have a much larger kinetic

energy when deposited compared to evaporation. This results in a high mobility of the incoming

atoms on the surface and a chance of the impinging atoms being back scattered or displacing

substrate atoms. The higher mobility results in fewer nucleation sites as compared with

evaporation since the atoms are able to move around to find an island, instead of nucleating

almost at the site of incidence. Generally, sputtered layers are polycrystalline, have a low surface

roughness, and relatively large grains. The preparation process for TMR devices with the

electrodes and tunnel barrier are deposited as follows; the first electrode is sputter deposited on

the top of Si/SiO2 followed by a very thin aluminum film, which is oxidized using different

oxidation methods [15,16]. Finally the top electrode is sputter deposited on the top of the

aluminum oxide. The junction is defined using optical and electron beam lithography with

different size areas. The junction is surrounded by SiOx in order to insulate the top and the

bottom electrodes, followed by depositing a gold film on the top of the junction as contact pads to

measure the tunnel current.

II.1.3.1. Preparation of the electrodes

A flat and smooth bottom electrode will improve growth of a flat and pinhole free barrier,

therefore a special care is taken when growing the bottom electrode. The magnetron sputtering

method is used to deposit the two electrodes in which the effect of the magnetron can be

described as a closed drift path of crossed electric and magnetic fields for electrons in a plasma

discharge. For a simple planar magnetron cathode, the arrangement consists of the planar cathode

(target) backed by permanent magnets that provide a torodial field, with field lines forming a

closed path on the cathode surface as shown in Fig. 4. The difference in the mobilities of the ions

and the electrons causes a positive ion sheath to be developed close to the target cathode, floating

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at a negative potential relative to the plasma. Because of the field due to the ion sheath at the

cathode, ions are extracted from the plasma and accelerated to strike the target, resulting in the

sputtering of the target material. The produced secondary electrons, upon entering the region of

crossed electric (E) and magnetic (B) fields, are trapped in orbits that permit long travel distances

close to the cathode. In the zones of the efficient electron trapping, the electron density reaches a

critical value, at which the ionization probability due to the trapped electrons is at its maximum.

This means that a higher rate of secondary electron production by high-energy positive ions is not

necessary for effective sputtering. Most magnetron sources operate in the pressure range from 1-

20 mbar and a cathode potential of 300-700V. The sputtering rates are primarily determined by

the ion current density at the target, and the deposition rates are affected by factors such as

applied power, source-substrate distance, target material, pressure, and sputtering gas

Composition [17, 18].

Figure 4: Scheme of a magnetron sputtering apparatus.

II.1.3.2. Preparation of the barrier

After depositing the bottom electrode the aluminum target rotate to the central position above the

sample and a very thin aluminum film is deposited on top of the bottom electrode using the radio

frequency (RF) sputtering method. The usefulness of RF methods for sputtering nonconducting

materials is based upon the fact that a self-bias voltage, negative with respect to the plasma

floating potential, develops on any surface that is capacitively coupled to a glow discharge. When

an alternating voltage is applied to such an electrode, more electron current flows when the

+

-

+

-

V c

V c

V acuumPu m ps

U nifo rmM agn eticF ie ld

Ca rbo nBo ttom Plate

M a gnetic F ie ldL ines

Carbon SteelO uter C y lind er

C y linde rical M a gn etro nSp u tte r ing So u rce

ST op P la teM ag netic

F ield C oils

Carbon teel

Stanless SteelVacuum ChamberWall

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electrode is positive relative to the floating potential than ion current flows when the electrode is

negative relative to the floating potential. Fig. 5 shows a schematic drawing of a typical RF

planar-diode-sputtering configuration in which a nonconducting target is placed over one

electrode and substrates are placed on the other one [19].

Figure 5: Schematic drawing of a RF sputter apparatus.

In recent experimental research the oxidation of the metals was discussed; as many metals or

perhaps all of them show very similar behavior when exposed to oxygen at a sufficiently low

temperature. Oxidation is initially extremely rapid, but after a few minutes or hours drops to very

low or negligible rates and a stable film is being formed with a thickness of 2-10 nm. This is the

behavior of the aluminum at room temperature while Copper, Iron, Barium and a few other

metals have the same behavior at the temperature of liquid air. The first explanation of this

behavior was given by Mott [20], who explains the formation of the oxide film is due to a contact

potential difference between the metal and the adsorbed oxygen, which enables the metal ions to

move through it.

In this work the aluminum oxide was used as a barrier, which is made from aluminum thin layer

oxidized by different oxidation methods (Natural oxidation, Thermal oxidation, Ultraviolet

radiation assisted in oxygen). In the literature different oxidation methods were used to prepare

the aluminum oxide, which are discussed in Table 1.

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Barrier First author, year Preparation method RA

(Ωµm2)

MR

(%)

T

(K)

Al2O3

Al2O3

Al2O3

Al2O3

Al2O3

Al2O3

Miyazaki, 95 [21]

Moodera, 95 [22]

Lu, 98 [23]

Sousa, 98 [24]

W.Opets, 99[25]

This work

Oxidation of Al in air for 24 h

Plasma oxidation of 1.5 nm Al

Plasma oxidation of Al

Plasma oxidation of Al, anneal at 500K

Plasma oxidation of 1.5 nm Al

UV oxidation of 1-2 nm Al

6.4x103

6x107

5x105

3x104

6x106

200-10k

18

13

28

36

23

20

300

300

300

300

300

300

Table 1: Different oxidation methods of the aluminum thin film.

In 1995 Miyazaki et al presented results with Al2O3, which is formed by subjecting a thin Al

layer for a certain time (of hours) in air, Al oxidized slowly by natural oxidation. The oxidation

rate decreases logarithmically with time and the process stops when the oxidation rate becomes

low. This method shows MR ratio of 18% at room temperature.

Then Moodera presented an in-situ fabrication with Al2O3 as a barrier in which the Al is oxidized

by an oxygen glow discharge (referred to as plasma oxidation). This method shows MR ratio of

13% at room temperature.

Sousa observed that magnetoresistance values increased by annealing of the complete junction

for 30min. up to 500 K and the obtained MR ratio was 36%.

In 1999 W.Oepts et al presented results with Al2O3 is formed by plasma oxidation for 40 sec. and

the structure was 3.5nm Ta/3.0nm NiFe/20nm FeMn/2.5nm NiFe/1.5nm Co/10nm NiFe/3.5nm

Ta, the obtained MR was 23% at room temperature.

II.1.4. Applications of tunnel magnetoresistance devices

II.1.4.1. Magnetic data storage

Storing the information using Magnetic Random Access Memories (MRAM) based on the TMR

becoming more attractive in the last two years [26,27,28]. The binary information “1” or “0” is

stored in the magnetic tunnel junction by aligning the magnetization of the two magnetic

electrodes either parallel or antiparallel.

The structure of the tunnel magnetic junction consists of two magnetic electrodes with different

coercive magnetic fields, which are separated by a thin insulating barrier (e.g. A12O3) as shown

in Fig. 6. By applying magnetic field on the TMR element, the magnetic layer which has low

coercivity will rotate and the magnetization in the two electrode are antiparallel which leads to

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large resistance and then the binary information “0” can be stored, after that the magnetization of

the second electrode will rotate and the magnetization of the two electrodes will be parallel,

which leads to small resistance and in this case the binary information “1” can be stored.

Figure 6: Two tunnel magnetic junctions have two states of the magnetization either parallel or antiparallel.

Figure 7: Shows the MRAM arrays based on TMR junctions.

The MRAM array based on tunnel magnetoresistance elements consists of an array of tunnel

magnetic junctions, which are connected, in series with silicon diodes, in this array each junction

is connected with one diode. In this case, since current only passes through a single magnetic

tunnel junction. All junctions are connected at the top with a conductor and a write line and at the

Bit line

Word line

TMR element

M1

M2

M1

M2

“1”“0”

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bottom with a bit line as shown in Fig. 7. In the writing operation a high current is applied

through the write line and the top conductor which generates a magnetic field high enough to

switch the soft layer (first electrode) of the magnetic tunnel junction in two states either parallel

to the hard layer (second electrode), then the binary information “1” can be stored or antiparallel,

then the binary information “0” can be stored. By turning the diode on or off, the single tunnel

magnetic junction is selected for reading. In the reading operation it needs a small current than

writing, which is applied only at the top conductor, which generates a small magnetic filed

sufficient to rotate the soft layer and the resistance of the element is measured. If the element

resistance is large then the stored binary information will be “1” while if the resistance is small

then the binary stored information will be “0”. With this simple method the stored information

can be read and write [29,30,31].

Properties SRAM DRAM FLASH MRAMRead Time fast Mod. Mod. Mod-fast

Write Time fast Mod. slow Mod.-fast

Non-Volatile no no yes yes

Minimum Cell Size large small small small

Low Voltage yes limited No yes

Table 2: Shows the major parameters for the different types of RAMs.

In Table 2 the differences between nonvolatile memories and other semiconductor memories are

described. Each memory in this table has important advantages. For example, SRAM has very

fast read and write speeds, but it is volatile and requires a minimum of four transistors per cell.

Therefore, the current state-of-the-art SRAM requires a relatively large cell area of approximately

40F2, where F is minimum feature geometry. DRAM cell architecture is simpler and denser than

SRAM, requiring one pass transistor and a storage capacitor per cell. DRAM has moderate

read/write speeds and the cell size is currently about 10F2. The charge in the capacitor leaks

through the pass transistor and needs to be refreshed in millisecond time intervals. Flash is a

high-density (8F2) nonvolatile memory technology in which the charge is stored in a dummy

gate. In read mode, Flash has unlimited endurance, operates at low voltages, and has moderate

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access times. However, in write mode, Flash has limited endurance of 105-106 cycles, requires

high voltage (5-12 V), and has slow program (ms) and erase (sec) times. Disadvantage of Flash;

the ratio of memory core to peripheral support circuitry, does not compete with DRAM because

of the high voltage circuit requirements.

II.1.4.2. Magnetic field sensors

Sensors are smart electronic devices which can “see”, ”hear”, ”smell”, ”taste”, and “touch”, by

converting non-electrical physical or chemical quantities into electrical signals. Microsensors

have a wide market in telecommunication, computer technology, environmental monitoring,

health care, and agriculture. A magnetic field sensor is a device, which generates electronic

signals due to the magnetic field, whereby the generated electronic signal is correlated to the

magnitude of the magnetic field and its direction [32,33].

The tunnel magnetoresistance field sensors are not available until now. Philips developed the first

magnetic read head based on TMR [34]. Actually magnetic read heads (which are used to read

the stored information from the magnetic recording medium) which based on GMR (yoke-type)

is used since a few years. For typical head design parameters, TMR- based heads are 3-5 times

more flux-efficient than GMR-based heads. In yoke-type read heads, the MR element is not

indirect contact with the medium [35]. For GMR- based yoke-type heads the efficiency of flux

transport through the yoke to the MR element is limited by the requirement of electrical

insulation of the element. However as described in the literature by Philips, the TMR-based yoke-

type heads can be very efficient, because the yoke can be at an extreme small distance from the

free magnetic layer in the TMR element. Fig. 8 shows a yoke-type read head based on a TMR

element, in which the upper and lower flux guides are soft magnetic thin films. The free magnetic

layer of the junction (F1) bridges the gap in the yoke and its easy axis is oriented perpendicular to

the gap. The magnetization of the second magnetic electrode (F2) which is pinned by the

exchange interaction with a metallic antiferromagnet (AF) in the direction perpendicular to the

medium.

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17

Figure 8: Yoke-type read head based on a magnetic tunnel junction.

Back upperflux guide(at V=0)

Test WindingBottom flux guide

Contact lead (at V≠0)

AF

F2Insulator

F1

Front upper flux guide

Read gap

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18

III. Experimental

III.1. Preparation of the samples

The samples were prepared by different oxidation methods and characterized by electrical and

magnetic measurements. Silicon wafers are chosen as substrate because of its smooth surface,

which is thermally oxidized. Typical oxide thickness varies between 50-150 nm. This oxide layer

is needed for electrical insulation between the junctions on the same wafer. For these oxide layers

the surface roughness increases slightly proportional to the oxide thickness. The bottom magnetic

layer is the most critical one for different reasons:

(a) The surface of this layer needs to be smooth.

(b) For patterning reasons the thickness of this layer needs to be fairly large, at least 10 nm.

(c) The deposited aluminum should exhibit a wetting on this surface. For the deposition of the

three layers Co/Al2O3/NiFe a magnetron sputter deposition system was used for the processing

with three targets of cobalt, aluminum and permalloy the layers were sputter deposited

consecutively without breaking the vacuum, at base pressure of 2 x 10-6 mbar. During the short

sputter time the substrate remains at the temperature of the water-cooled substrate holder. After

the sputter deposition of the first magnetic film a thin Al film is sputtered. The aluminum layer is

oxidized either by an ex-situ natural oxidation method (i.e. outside the chamber) in air at room

temperature or by in-situ oxidation at pure oxygen (i.e. inside the chamber) at room temperature

or by thermal oxidation or by ultraviolet radiation assisted in an oxygen atmosphere. Finally the

second magnetic film is deposited on the top of Al2O3. With optical and electron beam

lithography the cross-section of the junctions are defined and then etched by ion beam etching. A

low Ar ion energy of 250 eV was used in order to keep the sidewall damage as low as possible,

the etching process is stopped when the bottom electrode appears. The laterally structured layer

stack is then covered with a SiOx layer, which insulates the junction, followed by gold on top of

the junction. The electrical measurements were done using the four points method between the

gold contact pads and the bottom electrode. Systematically, the junction areas were varied within

the limits of 1-600 µm.

III.1.1. Natural oxidation method

The bottom electrode (NiFe or Co) was sputter deposited on top of Si/SiO2 wafer followed by

very thin aluminum film which is oxidized in air (ex-situ oxidation method i.e. out of the

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19

chamber) or in pure oxygen (in-situ oxidation method) see Fig. 9. The oxidation time varied

between 24 hours to 2 weeks for natural oxidation ex-situ [36,37,38]. The oxidation time for in-

situ varied between 12 to 48 hours in pure oxygen under pressure 130mbar.

Figure 9: Schematic representation of the natural oxidation method.

III.1.2. Thermal oxidation method

After deposition of the bottom electrode the aluminum layer was sputter deposited and oxidized

in the chamber (in situ oxidation method) in the presence of pure oxygen under pressure of 100

mbar. The oxidation temperature varied between 60 C° and 90 C° see Fig. 10. The thermal

oxidation time varied between 6 and 18 hours [39,40].

Figure 10: Schematic representation of the thermal oxidation method.

III.1.3. Ultraviolet oxidation assisted in oxygen method

After deposition of the bottom electrode the aluminum layer was sputter deposited. Followed by

an in-situ oxidation method using an ultraviolet lamp in the chamber with 100 mbar of high

purity oxygen [41,42]. By mounting a light bulb beside one of the targets, the sputter deposition

facility was modified. The low-pressure mercury bulb emits visible and UV light from a distance

of 5 cm to the substrate and the 4 W light bulb is switched on for 4-60min. The illumination is

fairly homogeneous if the substrate is positioned properly with respect to the bulb. The light of an

AlCo

SiO2

O2 O2 O2O2

O2

Hot plate 60 - 90 C°

Si

AlCo

SiO2

O2 O2 O2O2

O2

Si

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20

UV lamp can act via two possible mechanisms. The first proposed by Cabrera [43], which is due

to excitation of metal electrons, so they can pass through the oxide by photoemission or an

increased tunnel current. Thus the electric field inside the oxide is enhanced which leads to a

stronger oxidation compared to in-situ oxidation. The second effect is the generation of ozone:

light dissociates the O2 bond and liberates atomic species and leads to ozone (O3) formation,

which is a more reactive environment.

O2 + hv 2O

O 2 + O O3

where hv >5.1 eV, corresponding to a wavelength shorter than 242-nm. Both mechanisms play

the major role in the oxidation method, which is determined by the absorption of UV light by the

oxygen atmosphere. Fig. 11 shows the UV oxidation method, where the UV lamp is surrounded

by polished Al surface which act as a mirror to reflect the UV radiation and concentrate the UV

radiation on the sample surface. After oxidation of the aluminum film the top electrode was

deposited using dc magnetron sputtering.

Figure 11: Schematic representation of the ultraviolet oxidation method.

The observed oxidation enhancement indicates that the rate of oxidation of Al in oxygen at room

temperature is not limited by diffusion of Al atoms through the already formed Al2O3 but by the

kinetics of the solid /gas reaction.

AlCo

SiO2

O2O2 O3 OO

O

UV lamp

O3

(+)(-)Al Cover

Si

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21

III.2. Processing the samples

The fabrication of an integrated circuit requires a technique that enables the various thin-film

materials used to build up the device on a semiconductor substrate to be patterned. This technique

is the lithographic process, which developed for semiconductor devices. The lithographic process

involves transferring the circuit pattern-as might, for example, be contained in a photomask-into a

polymer film (termed a resist) and subsequently replicating that pattern in an underlying thin

conductor or dielectric film. Photolithography, which uses ultraviolet radiation 360-410 nm to

transfer the pattern from the mask to a photosensitive resist, is the dominant technology today for

integrated circuit fabrication. Nano-lithography becomes very important technique for different

applications like magnetic read heads and magnetic data storage, because the increase in density

is achieved partly by reducing the grain size in the current granular magnetic media. Thus, the

grain size must be reduced in order to increase the density of the magnetic data storage. From

different types of lithography (Photolithography, X-ray lithography, Electron-Beam lithography,

Ion-Beam lithography, stamping), only two methods are used in this work which are

photolithography and electron-beam lithography.

III.2.1. Optical Lithography

The sample has a dimension of 10x10mm, which is annealed before the lithography process, in

order to remove the water from the surface. The used photoresist AZ5214 by Hoechst can be

processed in two different ways: a positive or negative process. In the positive process the parts

exposed to light are soluble in the developer. After an additional heat treatment and overall

exposure, the parts, which were unexposed during the first exposure, are soluble. The chrome

mask was used to define the junctions, which varied between 1–600 µm. The structures were

etched with an argon ion beam (IBE). The progress of the etching was controlled by a secondary

ion mass spectrometer (SIMS), which has sensitivity for elements with high ionization

probability. Fig. 12 shows the steps necessary for preparation the trilayers and patterning the

junction. The bottom electrode is defined by etching through the complete trilayer. The second

etching process through the top electrode and the barrier is used to define the junctions. Before

removing the photoresist after the second etching, insulating of 100-nm SiOx layer is evaporated

onto the sample. The junction and the contact pads are exposed by lift-off and finally 300nm of

gold are sputter-deposited as a contact pads.

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22

Figure 12: Preparation process and patterning of the junction.

III.2.2. Electron beam lithography

The electron beam can be used to write patterns of very high resolution surface which is coated

with a radiation sensitive resist. This high-resolution capability can originally led to the

development of electron beam lithography as a tool for fabricating integrated circuits. Electron

irradiation of the polymeric resist film produces microstructure changes, such as cross-linking,

that enable a pattern corresponding to the original electron-beam exposure pattern to be

developed [44]. As the resist is frequently electron beam-sensitive organic polymers, usually

Co = 15 nmAl = 1.3 nm

SiSiO2

Co = 15 nmAl = 1.3 nm + Oxidation

SiSiO2

Co = 15 nmAl2O3

SiSiO2

NiFe = 20 nm

Co = 15 nmAl2O3

SiSiO2

NiFe = 20 nm

Resist + Etcing

Co = 15 nmAl2O3

SiSiO2

NiFe = 20 nmJunction + Contact pads

Co = 15 nmAl2O3

SiSiO2

NiFe = 20 nmAu 300 nm

SiOx

Current direction

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23

PMMA (polymethyal metacrylate) is used. During the positive exposure the electron beam splits

the carbon-carbon link at one or more places along the chain molecule and thus shortens the

molecule chains of the polymer. The developer dissolves the areas with low degree of

polymerization and does not dissolve the unexposure areas. Before using electron beam the

photoresist was hardened at 120C° for two min. The structure was defined by electron beam

exposure followed by a developing process. The dimensions of the sample were 3.5x3.5mm,

which were patterned into array of elements. Each sample has 108 elements and the element

dimension varies between 100 x 150 nm – 600 x 1800 nm and the distance between the elements

varies between 500 nm – 5µ depending on the structure.

III.3. Materials characterization methods

Before preparing the tunnel magnetoresistance devices, the materials used in the two electrodes

and the barrier were investigated using different techniques. The switching characteristic of the

used materials for the two electrodes is one of the important parameter in designing the array of

MRAM and magnetic field sensors. This parameter was investigated using different techniques

like Alternative Gradient Magnetometer (AGM) at room temperature and Superconducting

Quantum Interference Devices (SQUID) at different temperatures. The samples for AGM and

SQUID were prepared by electron beam lithography. The structures were investigated by Atomic

Force Microscopy (AFM) and the magnetization behavior by Magnetic Force Microscopy

(MFM). One of the problems in the switching characteristic is the magnetic coupling between the

electrodes. Magnetic Optic Kerr Effect (MOKE) investigated this coupling after sputtering and

before patterning, the dimensions of the samples were 10x10 mm. One of the parameter, which

plays an important role in the quality of the barrier, is pinhole, which can be detected by X-ray

Photoemission Spectroscopy (XPS). Detail information about the different techniques (AGM,

SQUID, AFM, MFM, MOKE and XPS) which were used for characterization of the used

materials will be described in the next paragraphs.

III.3.1. Alternative gradient magnetometer (AGM)

Different types of magnetometers are used to measure the magnetic moment. One of these

methods is the alternating-gradient magnetometer (AGM), which has a sensitivity exceeding (10-8

emu). The magnetic sample is mounted on the end of a cantilevered rod that incorporates a

piezoelectric element. The sample is magnetized by a DC field and simultaneously subjected to a

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24

small alternating field gradient. The alternating field gradient exerts an alternating force on the

sample, proportional to the magnitude of the field gradient and the magnetic moment of the

sample. The sample is mounted on the tip of a vertical extension rod, which is oriented along the

z-axis, and the gradient field is along the x or the y-axis see Fig. 13-a, b. The top end of the

sample rod is attached to the bottom end of the piezoelectric element as shown in Fig. 13-c. The

force of the field gradient on the magnetized sample produces a bending moment on the

piezoelectric element, which generates a voltage proportional to the force on the sample. The

output from the piezoelectric element is synchronously detected at the frequency of the gradient

field. The amplitude of this voltage is proportional to the magnetic moment of the sample, which

can be varied by changing the applied DC field Hx. The sensing element is composed of two

polarized sheets of a metallized piezoelectric, which are cemented back to back to both sides of a

thin brass vane [45,46]. The preamplifier is the front ends of a lock-in the amplifier, which

automatically tracks the frequency of the AC current in the gradient coils as shown in Fig. 13-d.

Figure 13: Configuration of the magnetization and gradient fields (a) and (b); the bimorph, extension, and sample(c); and the overall system (d).

A , B

Lock-IN

INREF

A

BSIG

IN

OSC

GradientCoils

OUT

(d)

Z

X

Y

-hxhx

Fx = mx dhx / dy

(a)

Clamp

Bimorph

Extension

SampleAc Field Gradient

Electrodes

Z(c)

-hx

hx

Fx = mx dhx / dy

(b)

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25

III.3.2. SQUID magnetometer

Superconducting magnetometers are based on superconducting quantum interference devices

(SQUIDs). There are two types of SQUID, the DC-SQUID and the RF-SQUID. These are the

most sensitive instruments available for the measurement of magnetic fields. The DC SQUID

consists of two Josephson junctions mounted in parallel on a superconducting loop, see Fig. 14.

The device is shown in Fig. 15, which is constructed from films of Nb, with Al2O3 barriers for the

junctions. A constant current I0 larger than the maximum zero-voltage current of the two

junctions biases the junctions in the Voltage State. This voltage is periodic in the magnetic flux φ,

applied to the loop with a period of one flux quantum φ0. However, one can detect the change δØ,

which is very small compared with φ0 using the flux-locked loop. The feedback circuit generates

a current in a coil coupled to the SQUID so as to generate a flux, δφ, thereby maintaining the

total flux in the SQUID at a constant value. The output voltage V0 is proportional to δφ typically,

the dynamic range is as high 107 Hz1/2 in a 1 Hz bandwidth, and the flux resolution approaches

10-6 φ0 Hz-1/2 at frequencies above 1Hz [47,48].

Figure 14: Simplified schematic of flux-locked dc SQUID.

Figure 15: Direct-current SQUID with enclosed magnetic flux φ.

Oscillator

Lock-in detector and integratoramplifier

I0

XY

Z

V0

I

I

Superconductor

Tunnel Barrier

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26

III.3.3. Atomic force microscopy (AFM)

Atomic force microscopy is used to study the morphology and the microstructure of surfaces at

the atomic level. The atomic force microscopy scanned the surface of the sample using a

sharpened tip, which is attached to a cantilever see Fig. 16. The atomic force microscopy tip

usually consisted of a micron-sized silicon nitride pyramid on the end of a silicon nitride

microcantilever has a few hundred microns long and fabricated by semiconductor lithography

and etching techniques. The electronic force between the tip and the surface atoms cause a

deflection of the cantilever. These deflections of the cantilever can be monitored using laser

beam, which gives the image of the surface topography or morphology at the atomic level

[49,50]. The atomic force microscopy is used to study the atomic structure on crystal surfaces,

thin films of semiconductor, metals, etc…

Figure 16: Shows a schematic diagram for Atomic Force Microscopy.

III.3.4. Magnetic force microscopy (MFM)

The variation of the magnetic force interaction between a magnetic tip and the sample, which is

measured by mounting a magnetic tip at the end of an AFM-style cantilever, yields the magnetic

force image of the magnetic material. The tip for MFM is different from the AFM, which has a

ferromagnetic material at the top of the tip [51]. The tip is scanned several tens or hundreds of

nanometers above the sample, avoiding contact. Magnetic field gradients exert a force on the

magnetic moment at the tip, and monitoring the tip/cantilever response gives the magnetic force

image. To enhance the sensitivity, most MFM instrument oscillates the cantilever near its

PiezoScanner

Phase, Amplitude Adjustor

OpticalDetector

O ptica lSource

C an tileverSam ple

Piezo

Tip

Display

Computer and Feedback

Controller

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27

resonant frequency (around 100 kHz) with a piezoelectric element. Both hard and soft magnetic

materials can be imaged by the magnetic force microscopy.

III.3.5. Magnetic Optic Kerr Effect (MOKE)

All magneto-optical properties of magnetic material can be described basically in terms of the

anisotropy induced in the optical parameters of the material by its magnetization.

For convenience of description, it is usual to classify the magneto-optic effects according to

whether the effect is observed in radiation transmitted through the material or in radiation

reflected from it. The former is termed the "Faraday Effect" and the latter the "Kerr Magneto-

Optic Effect". Both of these effects are usually subdivided into three categories as seen in Fig. 17,

according to the orientation of the magnetization relative to the plane of incidence of the radiation

and to the plane of the boundary surface of the sample [52]. For the longitudinal and polar

orientation, if the incident radiation is plane polarized in or perpendicular to the plane of

incidence, the reflected and transmitted radiation is generally polarized elliptically.

Figure 17: Shows the orientation of the magnetization.

Moreover, the rotation of the major axis of the ellipse and its ellipse are both proportional to the

magnetization of the material. In this case of the transverse orientation, the intensities of the

reflected and transmitted radiation are simply modulated by amounts proportional to the

magnetization. In the elementary treatment of the effect, the angel of rotation X of the major axis

of the ellipse is given as (X = K M d) and (X = KR M) for the Faraday and Kerr effects

respectively; M is the magnetization, d the thickness of the film and K & KR are constants. The

Kerr and Faraday rotation and ellipticities for a particular material depend on:

1- Orientation of the magnetization.

2- Thickness of the film.

3- State of polarization of incident light.

4- Angle of incidence.

MMM

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28

III.3.6. X-ray Photoemission Spectrum (XPS)

X-ray photoelectron spectroscopy (XPS) technique is used to study the surface of the materials.

The excitation source in XPS is X-ray photons, in which the photons impinge on the surface of a

material and the photoelectrons are emitted from the surface atoms of the material, see Fig. 18.

These photoelectrons originate from discrete electronic energy levels associated with these atoms

that are unique and characteristic of the atoms. The emitted photoelectrons can in turn give

information about the local electronic environment. Since these energy levels can be dependent

on the oxidation state or the electronic state of the atoms. The advantage of the XPS technique is

the determination of the oxidation or valance states of atoms as well as the detection of their

presence. In this work the XPS is used to detect whether the aluminum layer is thick enough to

cover the bottom electrode without pinholes [53,54].

Figure 18: Schematic diagram for XPS radiation.

III.4. TMR device characterization methods

III.4.1. Characterization of the electrical properties of TMR devices

The electrical measurements carried out comprise I–V of the tunnel magnetoresistance junctions.

In both cases the junction is measured using the four points probe geometry as shown in Fig. 19.

With this method, measurement errors due to possible contact resistances are eliminated. All

measurements on tunnel junctions presented in this thesis were carried out at different

temperatures. For I–V measurements of the junctions, a computer controlled current is applied

and the voltage across the barrier is measured. The current apparatus used was a Keithley 224

programmable current source. The voltage is measured using a Keithley 2000 multimeter. The I–

V curve is normally taken by sweeping the current from a negative value to a positive value.

Typically I–V diagram are taken in the voltage range of (-500 to 500 mV). The I-V curves were

measured at different temperatures varied between 4-300K.

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29

Figure 19: Shows the TMR devices. Top view shows the four points measurements and the cross section shows thestructure of the TMR devices.

III.4.2. Characterization of the magnetic properties of TMR devices

Magnetoresistance measurements were carried out using I–V characteristic apparatus described

above where the sample was surrounded by magnetic coils. The magnetic field sweep is

computer controlled by a calibration source model DIGISTANT type 6705. A field of up to

240kA/m can be applied. The field is measured with a digital teslameter model DTM 132, which

is recorded by the computer. During the magnetoresistance measurement, the junction is biased

with a constant current and the voltage is measured. The bias dependence of the

magnetoresistance is obtained by either measuring a magnetoresistance curve at various voltages,

or by measuring the resistance (V/I) as a function of the swept magnetic field (from positive to

negative and vice versa) at the situation of parallel and anti-parallel alignment of the

magnetization direction of the two ferromagnetic layers. The latter is only possible if the junction

has two well-defined states. The magnetoresistance effect is depending on the change of the

coercivity of the two layers. If the top electrode has a lower coercivety than the bottom electrode,

then its magnetization will start to rotate before the bottom electrode. So the resistance will

change as a function of the magnetization direction in the two electrodes. The magnetoresistance

ratio was measured at different temperatures varied between 4-300K.

Au

Al-OxidNiFe

Co

JunctionContact Pad Contact Pad

Cross Section AA

I

V

Current Source Bottom Electrode

Junction

Voltage Source

SiOx

Top Electrode

Top View

A A

SiO2

Si

SiOxSiOx

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30

III.4.3. Realibility and breakdown of TMR devices

The low resistive tunnel magnetoresistance devices are interesting for many applications like read

heads and memory elements. A strongly reduced RC time constant will lead to a better dynamic

performance. Moreover, for an improved signal-to-noise ratio, a low resistance in combination

with higher working voltages (0.3 to 0.5 V) would be required. The physical limit will be the

dielectric breakdown voltage, which is typically 1 V for ferromagnetic tunnel-junctions. [55]. In

the breakdown measurements two methods were used. The first method, the device is stressed

using fixed voltage (0.3,0.4,0.5 volts, etc.…) and the breakdown time is recorded. Such

procedure require very long times, i.e. days or months, depending on the applied voltage and

junction area. In the second method a high voltage (> 1V) is generally applied. Therefore, the

breakdown is measured by applying different voltage ramp speed, in which the applied voltage

increases monotonically with time, and the breakdown voltage is recorded. The breakdown is

investigated with different ramp speed (dV/dt) as a function of the junction area.

III.4.4. Micromagnetic simulation of TMR devices based on solution of Landau-Lifshitz-Gilbert equation

Understanding how the domain walls move in a magnetic material while being driven by an

applied magnetic field is a problem common to many magnetic devices such as: transformers,

magnetic recording read/write heads and various other magnetic sensors. Micromagnetic

structure, such as that present in surface domain walls, can be extracted with standard methods

for the solution to the Landau-Lifshitz-Gilbert equation. Such methods have been given in the

literature by Brown [56], LaBonte [57], Aharoni [58], Hubert [59] and Schabes [60]. The

equilibrium magnetization configuration results from the minimization of the total system free

energy. The energy of a ferromagnetic system is composed of (a) the magnetocrystalline

anisotropy energy EK which describes the interaction of the magnetic moments with the crystal

field characterized by the constant Kv (erg/cm3); (b) the surface magnetocrystalline anisotropy

energy Eks which corrects the broken symmetry near surfaces in the interaction of the magnetic

moments with the crystal field which, characterized by the constant Ks erg/cm 2; (c) the

magnetostatic self-energy Es which arises from the interaction of the magnetic moments with the

magnetic fields created by discontinuous magnetization distributions both in the bulk and at the

surface; (d) the external magnetostatic field energy Eh which arises from the interaction of the

magnetic moments with any externally applied magnetic field; and (e) the magnetostrictive

Page 32: Development of Ferromagnetic/Insulator/Ferromagnetic ...

31

energy Er which arises when mechanical stress (strain) is applied to a ferromagnetic material

which introduces effective anisotropy into the system characterized by Km erg/cm3. The

ferromagnetic equations are based upon the assumption that the bulk saturation magnetization Ms

(emu/cm3) is constant microscopically throughout the ferromagnet. The parameter Ms represents

saturation magnetization at room temperature. For most practical systems being considered (Fe,

Co or Permalloy), there is little deviation in Ms at room temperature from K=0 value. The value

of the magnetization vector M(r) at each point within the ferromagnet is the saturation

magnetization multiplied by the direction cosines, that is

M(r) = (Mx (r), My (r), Mz (r) ) = Msα (r) (4)

= Ms (α (r), β (r),γ (r) ).

The constraint equation implied by the constant magnetization assumption is α(r) =1.

The individual contributions to the energies in this continuum model are calculated by integrating

the energy expressions over the structure in question. The energy integrals below are integrated

over the appropriate dimension, dV. The external field energy Eh for an applied field of H0 is

simply given by Eh = ∫dV H0 . α . MS (5)

In order to calculate the magnetic microstructure of ferromagnets, the time evolution of a

magnetization configuration inside a ferromagnet, which is described by the Landau Lifshitz

Gilbert equation, should be evaluated. The Landau-Lifshitz-Gilbert equation has been examined

experimentally and theoretically [61,62,63] and found to yield an accurate description of the time

evolution for a magnetic moment of fixed magnitude in a magnetic field. This equation has the

following form dM/dt = (ω/(1+λ2)) . M . Heff +( (ω λ)/(1+ λ2) . M . M . Heff) (6)

Here, the gyromagnetic ratio ω = g ωe/2 is determined from the free electron value of ωe

and the spectroscopic splitting factor g= 2. The gyromagnetic ratio ω, the damping parameter λ

and the magnitude of the effective fields determine the time scales of interest. For time domain

simulations, we use the free electron gyromagnetic value of ω = 1.78 x 107 Oe sec-1. The

damping parameter λ is not well known. We have used values between 0.005 and 2.0 for λ and

found that for the calculation of equilibrium magnetization configurations in domain walls and

uniform ferromagnetic systems it is not important. The effective magnetic field on each magnetic

moment is determined from the total system energy Etot as

Heff = -dEtot /(d(Ms α)) (7)

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32

III.4.5. Characterization of the TMR elements using MRAM architecture

The electrical and magnetic properties of MRAM arrays based on tunnel magnetic junctions were

investigated. The processing of the MRAM arrays is divided into three steps. The first step is the

preparation of the GaAs diode, which is important to select the TMR junction.

The second step is preparation of the tunnel magnetoresistance junctions, in which the structure

of the sample was NiFe 20nm/Al2O3 1.3nm/Co 15nm, the Al was oxidized by UV oxidation

method for one hour. The third step is pattering the MRAM arrays, in which the mask consists of

3x3 tunnel junction cells of different sizes (40-60µm2). During the patterning processes, the bit

lines and the word lines must be defined in which the bit lines were the bottom electrode 25-60

µm wide and 20 nm thick. The word line on the top of the arrays was separated from the junction

by a SiOx passivation layer (~ 200nm thick). The thickness of the Au word line is 150nm of

width 14-40 µm.

Figure 20: Array of 3x3 magnetic tunnel junctions.

Fig. 20 shows the MRAM array of 3x3 magnetic tunnel junctions. The arrows represent the

current directions through the storing information process (writing) at junction (a), while at (b)

the junction will receive only the field, which generated by the vertical current-line, but at (c) the

junction didn‘t receive any field at all. Fig. 21 shows a schematic cross section of full integration

an MRAM cell in which the diode and the tunnel junction of each cell are combined [64]. This is

a general configuration of a memory cell including a tunnel junction. The three terminal devices

on the bottom of the drawing represent the diodes, which are connected to the ground terminal

and the other contacts are connected to the bottom electrodes of the tunnel junctions. The

a

b c

Bottom Electrode

Top Electrode

Junction

Page 34: Development of Ferromagnetic/Insulator/Ferromagnetic ...

33

schematic is for a stacked cell concept, where the tunnel junction and the diode are vertically

integrated on the top of each other is presented in Fig. 22.

Figure 21: Schematic cross section of an MRAM cell that combines a diode and a tunnel junction in each cell.

To fabricate the MRAM arrays, which are based on tunnel magnetic junctions a magnetic tunnel

junction is sputtered deposition on the top of the diode, which was a GaAs diode. The GaAs p-n

diode have doping levels of 10-17 cm-3 for both p and n regions and the diode with dimension

320µm2 and the magnetic tunnel junctions dimension varies between 1µm x 3µm and 4-6 µm x

10µm [65].

Figure 22: MRAM cell contains diode and tunnel junction.

Page 35: Development of Ferromagnetic/Insulator/Ferromagnetic ...

34

IV. Results and Discussions

IV.1. Characterization of the substrate

IV.1.1. Surface roughness for Si/SiO2 wafer using AFM

The surface roughness is one of the critical parameters of the preparation the integrated magnetic

and semiconductor devices. In this work the used substrate was a Si wafer, which is thermally

oxidized to SiO2. This oxidized layer (SiO2) is important for the electrical insulation between the

devices on the same wafer. Before preparing the tunnel magnetoresistance devices the surface

roughness of the Si/SiO2 wafer (the substrate) was investigated by AFM. Fig. 23 shows two AFM

images with different thickness of a SiO2, the left image shows smooth surface with 50 nm layer

of SiO2, while the right image shows a rough surface and many small islands on the top of 1 µm

SiO2 layer.

Figure 23: Two AFM images: left SiO2 of 50nm, right SiO2 of 1µm.

Fig. 24 presents the relation between the thickness of the SiO2 and the surface roughness, which

shows that with increasing the thickness of the SiO2 the surface roughness is also increased. Fig.

25 depicts a system of Si/SiO2/Co/Al layers, which shows the effect of the surface roughness of

the substrate at preparation the tunnel magnetoresistance junctions. If the substrate is rough and

contains islands on the top, the Co/Al layers also have highly rough interfaces between the two

materials which generate a magnetic coupling (orange coupling). This leads to a reduction of the

magnetoresistance effect. In addition, these islands can create pinholes in the aluminum layer,

which is the main reason for frequently observed short circuits of the tunnel magnetoresistance

device [66]. According to this AFM investigation 50nm SiO2 layer was used in this work.

Page 36: Development of Ferromagnetic/Insulator/Ferromagnetic ...

35

Figure 24: The surface roughness as a function of SiO2 thickness.

Figure 25: Schematic diagram of aluminum and cobalt layers on the top of Si/SiO2 wafer.

IV.2. Characterization of the magnetic materials

Magnetic films patterned into nanometer-scale can serve as a means of storing information [67].

At present, magnetic random access memories (MRAM), such devices are under development,

which are based on tunnel magnetoresistance (MR) elements. To compete against existing

memories, a new memory technology with high-density cells is required. Performance of these

MR elements will depend critically on the switching characteristics from one state to other.

Understanding the switching behavior of these elements as a function of its size is essential to

push the density limits of the storing technology. As recording densities increase, element sizes

decrease and therefore, the individual grains (ferromagnetic particles) that form each element

have to be smaller. The switching characteristics depend sensitively on the geometry of the

element (length, width and film thickness) and the material properties (exchange constant,

crystalline anisotropy, etc.) [68,69]. In this part, a systematic study of magnetic switching

characteristics for different magnetic films is presented. The magnetic films are patterned into

0 200 400 600 800 1000

0.2

0.4

0.6

0.8

1.0

Sur

face

Rou

ghne

ss (

nm)

SiO2 Thickness (nm)

Al

SiO2Co

Si

Page 37: Development of Ferromagnetic/Insulator/Ferromagnetic ...

36

arrays in nanometer-scale with different lateral shapes; different widths of 100nm, 200nm,

600nm and aspect ratios (length/width) = 1.5, 2, 3, and 4.

IV.2.1. Characterization of the Cobalt thin film

IV.2.1.1. Magnetic switching characteristics of thin film Cobalt in nanometer-scalepatterned arrays of elements

Fig. 26 shows the switching behavior of cobalt film 15nm thick at aspect ratios of 1.5, 2, 3 and 4

of 100 nm width as a function of applied magnetic field. The elements with aspect ratio of 1.5

and 2 show low remanence and distorted hysteresis curves, while the elements with higher aspect

ratio of 3 and 4 do not display such behavior. This distortion in the hysteresis curve is due to the

magnetization behavior, which suffers some disturbance at small aspect ratio; this behavior will

be shown in the section summarizing the MFM results. The magnetization behavior at small

aspect ratio has no preferred orientation to rotate and the magnetic flux closes on itself leaving

low remanence (vortex state). While increasing the aspect ratio provides a well-defined

anisotropy axis, which hinders flux closure through vortex formation. At widths 100 nm and 200

nm, the magnetization vortices are observed at aspect ratios of 1.5 and 2, while at 3 and 4 the

magnetization vortices are not observed. However, elements of width 600nm display a weak

tendency for vortex formation at aspect ratio of 1.5, which completely disappear at higher aspect

ratios [70]. This means that there is a strong relation between the magnetization vortices, the

aspect ratio and the element width. Fig. 27 shows the micromagnetic simulation image of the

magnetization behavior at vortex state, in which the magnetization has no preferred direction to

rotate. Fig. 28 shows the magnetization behavior curves and the MFM images at different aspect

ratio of 1.5, 2 and 3 with width =200 nm. Fig. 28 shows at (a1 and b1) the magnetization behavior

at aspect ratio of 1.5, in which the hysteresis curve is distorted and the MFM image shows that,

most of the elements (the dark area) have no magnetic image (switched) i.e. most elements are

found in a vortex state. The early switching behavior of these low aspect ratio elements is due to

the majority of elements entering a vortex state. Fig. 28 shows at (a2 and b2) an aspect ratio of 2, a

few elements are found in a vortex state, this explains the higher remanence and sharper

hysteresis curve [71]. Fig. 28 shows at (a3 and b3) an aspect ratio of 3 in which no elements in a

vortex state at H applied = 0 Oe are presented. All elements of this size are aligned along their

easy axis. Fig. 29 displays the switching field as a function of the aspect ratio for different widths

of 100 nm, 200 nm, and 600 nm. The relatively flat behavior of the 600nm elements, with respect

Page 38: Development of Ferromagnetic/Insulator/Ferromagnetic ...

37

to aspect ratio, reflects the stability of these elements to vortex formation. This means that the

switching behavior of the elements is strongly dependent on the aspect ratio i.e. elements of

width 100nm up or less than 200nm, have a higher propensity for vortex formation. This explains

the curved behavior in the figure. In the Stoner-Wohlfarth single domain model [72], switching

characteristics of a single domain element are simply determined by their geometric factors and

magnetization (M). For rectangular thin film elements with the switching field in the easy axis

direction, Hc can be approximated to a flat ellipsoid and expressed as:

Hc = t/w f (e), and e = (1 – W2 / L2 )1/2 ... (8)

Where L, W and t are the element length, width and thickness respectively, and L ≥ W>> t; f (e)

contains complete elliptic integrals, which is function of (e) only. For a fixed aspect ratio L/W the

single domain Hc should be inversely proportional to the element width W. Compared with our

results, this single domain model is do not follow that for low aspect ratio elements. This again

suggests that these elements have a vortex state.

Figure 26: Magnetization as a function of the applied magnetic field at different aspect ratios of 100nm width.

-150 -100 -50 0 50 100 150

-1.0

-0.5

0.0

0.5

1.0

Hs

L/W = 3

M/M

s

Applied Magnetic Field µ0H (mT)

-150 -100 -50 0 50 100 150

-1.0

-0.5

0.0

0.5

1.0

Hs

L/W = 4

M/M

s

Applied Magnetic Field µ0H (mT)

-150 -100 -50 0 50 100 150

-1.0

-0.5

0.0

0.5

1.0

Hs

L/W = 1.5

M/M

s

Applied Magnetic Field µ0H (mT)

-150 -100 -50 0 50 100 150

-1.0

-0.5

0.0

0.5

1.0

Hs

L/W = 2

M/M

s

Applied Magnetic Field µ0H (mT)

Page 39: Development of Ferromagnetic/Insulator/Ferromagnetic ...

38

Figure 27: Magnetization behavior at vortex state.

(a1) (b1)

(a2) (b2)

-100 -50 0 50 100

-1.0

-0.5

0.0

0.5

1.0

Hs

L/W= 2

M/M

s

Magnetic Field µ0H (mT)

-100 -50 0 50 100

-1.0

-0.5

0.0

0.5

1.0 L/W= 1.5

Hs

M/M

s

Magnetic Field µ0H (mT)

Page 40: Development of Ferromagnetic/Insulator/Ferromagnetic ...

39

(a3) (b3)

Figure 28: Magnetization as a function of the applied magnetic field for 200 nm width (a1-3), the MFM images forarrays with different aspect ratio for 200 nm width (b1-3).

Figure 29: Switching field as a function of the aspect ratio (length/width) for different widths.

IV.2.1.2. Magnetic switching characteristics of thin film Cobalt at different temperatures

With the macroscopic method, SQUID magnetometer, the magnetization was measured as a

function of the applied magnetic field at various temperatures. The specimens are arrays of

elements of the cobalt film 15nm thick of dimensions 100nm x 150nm and 100nm x 400nm, and

each array contains 108 elements. Fig. 30 shows the magnetization curves, which obtained by

measuring the magnetic moment of the sample as a function of external magnetic field at

-100 -50 0 50 100

-1.0

-0.5

0.0

0.5

1.0

Hs

L/W= 3

M/M

s

Magnetic Field µ0H (mT)

1.5 2.0 2.5 3.0 3.5 4.0

-20

-10

0

10

20

30

40

100 nm 200 nm 600 nm

Sw

itchi

ng F

ield

µ0H

(mT

)

Aspect ratio (%)

Page 41: Development of Ferromagnetic/Insulator/Ferromagnetic ...

40

different temperatures 400K, 300K and 4K. Fig. 30 (left) shows the magnetization curves of the

elements have an aspect ratio of 1.5. Again the distorted magnetization curves are observed, in

which with decreasing the temperature the distortion in the magnetization curve is more less than

at higher temperature, while the switching field value does not change dramatically within this

range of the temperature. Fig. 30 (right) shows the magnetization curves of the elements have an

aspect ratio of 4, in which the switching field shows temperature dependence i.e. with increasing

the temperature the switching field is decreased and vice versa. Fig. 31 shows the switching field

as a function of aspect ratio (Length/Width = 1.5, 2, 3 and 4) for 100nm width with different

temperatures. It is shown that the highest aspect ratio has the highest switching field and with

decreasing the temperature, the switching field of the elements increases for larger aspect ratio

[73]. These results might be due to the magnetic anisotropy, which has temperature dependence

i.e. the magnetic anisotropy decreased with increasing the temperature and increase with

decreasing the temperature. Also the magnetic anisotropy depends on the aspect ratio of the

element i.e. with decreasing the aspect ratio, the magnetization has no preferred ordination to

rotate and the magnetic anisotropy decreased i.e. the magnetic anisotropy is small at aspect ratio

of 1.5 than 4 which leads to a small changes of the switching field at different temperature at

aspect ratio of 1.5 than 4. By contrast at small aspect ratio of 1.5 the magnetic anisotropy was

very small, which leads to low temperature dependence while with increasing the aspect ratio the

magnetic anisotropy increase and show temperature dependence. This explains at low aspect

ratios there are no changes at the switching field as a function of the temperature compared with

large aspect ratio.

Page 42: Development of Ferromagnetic/Insulator/Ferromagnetic ...

41

Figure 30: Shows the magnetization as a function of the applied magnetic field for two arrays with dimension of100nm x 150nm (left) and 100nm x 400nm at different temperatures.

-150 -100 -50 0 50 100 150-1.5

-1.0

-0.5

0.0

0.5

1.0

1.5

400 K

M/M

s

Applied Magnetic Field µ0H (mT)

-150 -100 -50 0 50 100 150-1.5

-1.0

-0.5

0.0

0.5

1.0

1.5

4 K

M/M

s

Applied Magnetic Field µ0H (mT)

-150 -100 -50 0 50 100 150-1.5

-1.0

-0.5

0.0

0.5

1.0

1.5

300 K

M/M

s

Applied Magnetic Field µ0H (mT)

-100 -50 0 50 100-1.5

-1.0

-0.5

0.0

0.5

1.0

1.5

300 K

M/M

s

Applied Magnetic Field µ0H (mT)

-100 -50 0 50 100-1.5

-1.0

-0.5

0.0

0.5

1.0

1.5

400 K

M/M

s

Applied Magnetic Field µ0H (mT)

-150 -100 -50 0 50 100 150-1.5

-1.0

-0.5

0.0

0.5

1.0

1.5

(c)

(b)

(a)

4 K

M/M

s

Applied Magnetic Field µ0H (mT)

Page 43: Development of Ferromagnetic/Insulator/Ferromagnetic ...

42

Figure 31: Shows the switching field as a function of the aspect ratio at different temperatures.

IV.2.1.3. Images for Cobalt thin film patterned element arrays by AFM and MFM microscopes

Fig. 32 shows the MFM (left) and AFM (right) images for cobalt 15nm thin film which were

patterned into element arrays of nanometer-scale. The images show the AFM and MFM for

elements with width = 200 nm at different aspect ratios of 1.5 at (a), of 2 at (b) and of 3 at (c).

Fig. 32 (right) depicts the AFM images, which shows sharp and well-defined shapes of the

elements, which are arranged in regular arrays. The spots on the top are residual resist from

processing during electron beam lithography. Due to the dimensions of the elements, the

magnetization lies predominantly on the film plane. Furthermore, because the elements are twice

as long as they are wide, the “easy” axis (or the preferred axis where the magnetization would

orient) is more or less parallel to the long dimension (horizontal). The MFM images (left) for

cobalt elements show a rich and complex magnetic structure behavior. The elements in Fig. 32a-c

show bright and dark areas which can be regarded to the north and south poles of these nano-

magnets. Elements with a single distinct bright/dark pair are known as “single domains”, while

those that have more complex structures are commonly referred to as “multidomains” [74]. At (a)

the MFM image shows an array of elements with dimension 200nm x 300nm (aspect ratio of 1.5)

in which a few elements have single domain magnetization and the rest of the elements haven’t

any magnetic image (this means the elements are switched) which indicate that most of the

elements are found in a low moment vortex state. At (b) the MFM image shows elements with

1.5 2.0 2.5 3.0 3.5 4.0

-20

0

20

40

60

c 4Kb 300 K

c

b

a

a 400K

Sw

itchi

ng F

ield

µ0H

(m

T)

Aspect ratio (L/W) for Width=100 nm

Page 44: Development of Ferromagnetic/Insulator/Ferromagnetic ...

43

dimension 200nm x 400nm (aspect ratio of 2), which have a few elements are found in a vortex

state, these elements are clearly switched. At (c) the MFM image shows elements with dimension

200nm x 600nm (aspect ratio of 3) which have no elements in a vortex state at H _applied = 0

(Oe). Also the south poles of all elements are on their one side while the north poles of all

elements in the other side, which means that the magnetic moment of all elements are in the same

direction. Fig. 33 shows the difference between the multidomain and single domain, the top Figs.

show elements at (a, b) have multidoamin, which contains two or three domains. Also due to the

nonuniformity of the magnetization vector elongated elements, the magnetization vector tends to

be tangential to the edges of the magnetic element to prevent the creation of free poles (and

increased demagnetization energy) [75]. At bottom Fig. (a) element shows the single domain

which has two poles south and north. While at (b) the image shows an element didn’t present any

magnetic image, which indicate that the element is switched.

(a)

(b)

Page 45: Development of Ferromagnetic/Insulator/Ferromagnetic ...

44

(c)

Figure 32: MFM (left) and AFM (right) images with width 200nm at aspect ratio of 1.5 at (a), 2of at (b) and 3 of at(c).

Figure 33: MFM images for single domain (bottom) and multidomain (top).

a

b

aba

a

b

a

b

Page 46: Development of Ferromagnetic/Insulator/Ferromagnetic ...

45

IV.2.1.4. Magnetic switching simulation of cobalt thin film at different aspect ratios

A micromagnetic model utilizing the Landau-Lifshiz-Gilbert equation has been used to

investigate the switching field variation in patterned film elements. A patterned film element is

modeled by discretizing it into a two dimensional arrays of tetragonal cells.

Each cell has a size of 10nmx10nmxfilm thickness. To study the magnetic switching properties,

an external field is always applied along the length direction of the film element in the film plane.

The amplitude of the field is varied step-wise with 2 Oe field steps. The switching field is

investigated for patterned Co thin film 15nm thickness as a function of aspect ratio of 1.5, 2, 3

and 4 for width 100nm, 200nm and 600nm. Considering at this thickness, the crystalline structure

of Co film usually is ffc. Cubic crystalline anisotropy is therefore assumed with anisotropy

constant of k1=-3.5x105 erg/cm3 (K2 is assumed to be zero). The crystalline orientation for each

discretization cell is assumed to be three-dimensional (3D) random [76]. Fig. 34-a, b and c shows

the switching field as a function of experimental and modeling aspect ratio using LLG formula

for 100nm, 200nm, 600nm width. The switching field of the experimental values is different from

the modeling values. The difference can be explained as, when one applies an external field to a

particulate medium each particle is sensitive not only to this field but also to the field created by

the nearest neighbor particles, in which this creation field is exist only at real particle.

Domain structures of real samples on the other hand, are not uniform, instead they display some

magnetization variation especially near sample edges. Fig. 35 shows the schematic diagram for

the real and ideal magnet [77], in which the arrows represent the direction rotation of the

magnetization which is different at the ideal and the real magnet due to the demagnetization

effect at the real magnet.

Page 47: Development of Ferromagnetic/Insulator/Ferromagnetic ...

46

Figure 34: Shows the switching field experimental and modeling values for Co 15nm thick as a function of the aspectratio of width 100nm at (a), 200nm at (b) and 600nm at (c).

1.5 2.0 2.5 3.0 3.5 4.0

-15

0

15

30

45

60

75(a)

i

j

j Modelingi Expermintal

Sw

icth

ing

Fie

ld µ

0H (

mT

)

Aspect ratio (length/width)

1.5 2.0 2.5 3.0 3.5 4.0

-5

0

5

10

15

20

25

30

35

(b)

i

j

j Modeling

i Expermintal

Sw

itch

ing

Fie

ld µ

0H (

mT

)

Aspect ratio (length/width)

1.5 2.0 2.5 3.0 3.5 4.0

9

12

15

18

21

(c)

i

jj Modelingi Expermintal

Sw

itch

ing

Fie

ld µ

0H (

mT

)

Aspect ratio (length/width)

Page 48: Development of Ferromagnetic/Insulator/Ferromagnetic ...

47

Figure 35: Schematic drawing for the real and the ideal magnet.

IV.2.2. Characterization of Nickel-Iron thin film

IV.2.2.1. Magnetic switching characteristics of Nickel-Iron thin film in nanometer-scalepatterned arrays of elements

The AGM results for NiFe layer thin film patterned into nanometer-scale arrays using electron

beam lithography are presented, the dimension of the sample was 3.5x3.5mm which have 108

elements. Fig. 36-top shows the hysteresis loop for NiFe thin film, the dimension of each element

was 100nm x 150nm of thickness 2nm, 3nm and 4nm. It is shown that with decreasing the

thickness of the film the magnetic moment decreases, which lead to decrease the switching field

of the sample. For a fixed aspect ratio (L/W) the switching field Hc should be inversely

proportional to the element width and directly proportional to the thickness according to the

Stoner-Wohlfarth eq. (8). Fig. 36 bottom shows the hysteresis loop for 600nm width with

different aspect ratio at fixed thickness 3nm, which shows that with decreasing the aspect ratio,

the magnetic moment decreased [78].

Fig. 37 shows the results of the magnetic switching field of NiFe thin film as a function of aspect

ratio of 1.5, 2, 3 and 4 at element widths 100nm, 200nm and 600nm of thickness 2nm, 3nm and

4nm. It is shown that the switching field of the element at 2 nm thickness shows the lowest

switching field value than thickness of 3nm and 4nm. The switching field value is decreased with

increasing the width from 100nm to 600nm [79]. So in contrast the switching field decreased

with increasing the width and decreased with decreasing the thickness of the film (become more

sensitive to the magnetic field). These results are very important in designing the MRAM matrix

and magnetic field sensors.

Real magnet Ideal magnet

Page 49: Development of Ferromagnetic/Insulator/Ferromagnetic ...

48

Figure 36: The magnetic moment as a function of the applied magnetic field at different thickness for dimension 100x 150nm (top), and at different aspect ratios of 600nm width (bottom).

-8 -6 -4 -2 0 2 4 6 8

-2.0x10-5

-1.0x10-5

0.0

1.0x10-5

2.0x10-5 100 nm x150 nm

2 nm 3 nm 4 nm

M (e

mu)

Applied Magnetic Field µ0H (mT)

-6 -4 -2 0 2 4 6

-3.0x10-5

-2.0x10-5

-1.0x10-5

0.0

1.0x10-5

2.0x10-5

3.0x10-5

NiFe 3nm

0.6x1.2 0.6x2.4 0.6x0.9

M (

emu

)

Applied Magnetic Field µ0H (mT)

Page 50: Development of Ferromagnetic/Insulator/Ferromagnetic ...

49

(a)

(b)

(c)

Figure 37: The switching field as a function of aspect ratio for NiFe thin film of widths 100nm, 200nm, and 600nm ofthickness 2nm, 3nm and 4nm.

1.5 2.0 2.5 3.0 3.5 4.00

1

2

3

4

5

6

k

j

ik 4nmj 3nmi 2nm

Sw

itch

ing

Fie

ld µ

0H (

mT

)

Length/Wdith (Width=200 nm)

1.5 2.0 2.5 3.0 3.5 4.0-10123456789

k 4nmj 3nm

k

j

i

i 2nm

Sw

itchi

ng

Fie

ld µ

0H (

mT

)

Length/Width (width = 100nm)

1.5 2.0 2.5 3.0 3.5 4.00

1

2

3

4

k

j

i

k 4nmj 3nmi 2nm

Sw

itchi

ng F

ield

µ0H

(m

T)

Length/Width (Width=600 nm)

Page 51: Development of Ferromagnetic/Insulator/Ferromagnetic ...

50

IV.2.2.2. Images of Nickel-Iron thin film patterned element arrays by AFM and MFMmicroscopes

Fig. 38-a, b shows the MFM image (left) and AFM image (right) for NiFe layer of dimensions at

(a) 600nm x 2400nm with 2nm and at (b) 600nm x 1800nm 4nm thickness. The MFM images

were taken in the absence of magnetic field H = 0, with decreasing the thickness (2nm, 3nm) the

magnetic moment is decreased, in which it is very low signal to be detected by MFM. The

600nmx2400nm image with 2nm thickness shows a very low contrast. This can be observed

within the interior regions is similar to the background areas outside of the patterns. For the

dimension of 600nmx1800nm and thickness of 4nm image shows a strong contrast which can be

observed within the interior regions to the background area outside the patterns compared with

dimension of 600nmx2400nm and thickness of 2nm image, which means that with increasing the

element thickness the magnetic moment increases, then the element has a contrast and a stable

magnetization direction along the length of the element [80]. The MFM results show the same

behavior as AGM results with decreasing the thickness the magnetic moment decrease.

(a)

(b)

Figure 38: Shows the MFM (left) and AFM (right) images for NiFe thin film at different geometry.

Page 52: Development of Ferromagnetic/Insulator/Ferromagnetic ...

51

IV.2.2.3.Magnetic switching simulation of Nickel-Iron thin film at different geometry

The micromagnetic program using LLG equation was used to study the magnetization behavior

and switching field for NiFe thin film at different widths 100nm, 200nm and 600nm for different

aspect ratio of 1.5, 2, 3 and 4 with thickness 2nm, 3nm, 4nm. By comparing the simulation value

of NiFe thin film at different aspect ratio with the experimental value it shows that at 2nm, 3nm

and 4nm thickness the switching experimental and modeling value of the small aspect ratio are

close compared with the large aspect ratio. The modeling switching value has the same behavior

like experimental switching value, which means with increasing the aspect ratio, the switching

field value increase. At 600nm width, the modeling and experimental values are close compared

with 200nm and 100nm widths, which means that with decreasing the dimension the structure is

complex and the difference between the modeling and the experimental is high while at large

dimension the difference is small [81]. This difference between the modeling and the measured

value is due to the demagnetization created by the nearest neighbor particle.

1.5 2.0 2.5 3.0 3.5 4.0

0

2

4

6

8

10

12

14 (a) 100nm Exp. 100nm Mod. 200nm Exp. 200nm Mod. 600nm Exp. 600nm Mod.

Sw

itchi

ng

Fie

ld µ

0H (

mT

)

Aspect ratio (length/width)

Page 53: Development of Ferromagnetic/Insulator/Ferromagnetic ...

52

Figure 39: The switching field as a function of the aspect ratio for thickness 2nm at (a), 3nm at (b) and 4nm at (c).

IV.3. Characterization of the barrier

IV.3.1. Barrier thickness variations

The barrier thickness determines the resistance of the magnetic tunnel junctions. For optimal

reproducibility on wafer level, the barrier thickness must be controlled almost on an atomic level.

Important barrier properties such as the thickness and the potential height can be obtained fairly

well from the results of electric transport measurements using the theoretical models for spin

1.5 2.0 2.5 3.0 3.5 4.002468

101214161820

(b) 100nm Mod. 100nm Exp. 200nm Exp. 200nm Mod. 600nm Exp. 600nm Mod.

Sw

itchi

ng

Fie

ld µ

0H (

mT

)

Aspect ratio (length/width)

1.5 2.0 2.5 3.0 3.5 4.0

02468

10121416182022

(c) 100nm Exp. 100nm Mod. 200nm Exp. 200nm Mod. 600nm Exp. 600nm Mod.

Sw

itchi

ng

Fie

ld µ

0H (

mT

)

Aspect ratio (length/width)

Page 54: Development of Ferromagnetic/Insulator/Ferromagnetic ...

53

dependent transport in magnetic tunnel junctions. Fig. 40 shows the difference between an ideal

and the more realistic tunnel barrier. The realistic barrier has thickness variations due to the

growth of the bottom electrode and the initial roughness of the substrate. Thickness variations

will strongly influence the tunnel current distribuation on the junction area, since the tunneling

probability is exponentially dependent on the barrier thickness. Thus it may be possible that

almost all the electron transport in a tunnel junction takes place in a small fraction of the total

junction area [82].

Figure 40: Schematic drawing of an ideal barrier at (a) and a realistic barrier at (b).

In literature several oxidation processes have been described. All processes can yield tunneling

barriers, but having different barrier properties. The density of the oxide in the barrier seems to

be the key issue and therefore, a thin Al layer is deposited first and then oxidized, either (in-situ)

by a glow discharge or in oxygen ambient, or (ex-situ) in air (natural oxidation). An optimum

oxidation state has to be achieved as too thin aluminum film can result in some oxidation of the

ferromagnetic underlayer, which has a strong implication on the spin-polarization of this layer. A

too thick aluminum film will weaken the spin-dependent tunneling due to scattering in the

remaining aluminum film. For practical purposes the most important parameter is the resistance

of the micro-scale tunnel junction itself. To understand the relation between the barrier height and

the barrier thickness, Simmon’s model was used for a junction of 1 µm2 area as shown in Fig. 41.

The resistance (Ω) was calculated as a function of two parameters, the barrier thickness (from 0.5

to 2 nm) and the barrier height 0.5 to 3 eV. This model is widely used to compare the different

barrier types. Another parameter which has an effect the barrier height is the oxidation method in

which the aluminum by one oxidation method is completely oxidized while by other oxidation

method is not complete [83].

(a) (b)

Page 55: Development of Ferromagnetic/Insulator/Ferromagnetic ...

54

Figure 41: Junction resistance (Ω) for 1 µm2 area as a function of the two parameters in the model: barrier heightand thickness.

IV.3.2. Structural characterization of the barrier using (XPS)

Investigating structure, composition and thickness of the barrier is a direct way, which lead to

more information and understanding of the relation between the MR effect and the barrier

properties. The main topic of the barrier structure is the composition of the A12O3 (i.e. if the

oxidation time is not sufficient, this leads to leftover Al and if the oxidation time is too high, this

leads to formation the bottom electrode oxide). One of the methods used, which characterize the

barrier, is X-ray Photoemission Spectrum (XPS). The samples have the same thickness of the

bottom electrode, which was 20 nm cobalt film while the aluminum thickness changed in each

sample, which were 0, 1, 2 and 3 nm thick. Fig. 42 shows four XP-spectra in the Co 2p core level

region of the Co film without aluminum on top at (A), and with an aluminum layer on top of

thickness 1 nm at (B), 2 nm at (C) and 3 nm at (D), respectively. All the samples were oxidized

by natural oxidation for 1 month. The spectra of the single Co film clearly exhibit the cobalt

oxide peak at a binding energy of 782 eV and the metallic Cobalt 2p3/2 peak, perceptible as a

shoulder at 779 eV. The cobalt oxide peak is already drastically reduced in the spectrum at (B).

This result is a strong evidence that only 1 nm aluminum layer is sufficient to suppress nearly or

completely the formation of Co oxide. The spectra with 2 nm aluminum at (C) and 3nm

aluminum at (D) exhibit no Co oxide peak anymore. The above results show that less than 2 nm

0.5

0.8

1.1 1.

4 1.7 2.

0

0.50.60.70.80.91.01.11.21.31.41.51.61.71.81.92.02.12.22.32.42.52.62.7

2.82.93.0

1 .00E-01

1.00E+00

1.00E+01

1.00E+02

1.00E+03

1.00E+04

1.00E+05

1.00E+06

1.00E+07

1.00E+08

1.00E+09

1.00E+10

1.00E+11

1.00E+12

1.00E+13

1.00E+14R

esis

tan

ce

Thickness (nm)

Barrier Height (eV)

Page 56: Development of Ferromagnetic/Insulator/Ferromagnetic ...

55

aluminum is sufficient to cover the Co surface without pinholes and to suppress the Co oxide

formation [84].

Figure 42: Co 2p spectra of an air-oxidized polycrystalline Co film without Al layer at (A), Air-oxidized Co/Aloverlayer structures with 1 nm Al at (B), 2 nm at (C) and 3 nm at (D) on the top of 20 nm Co.

IV.4.Charactarization of TMR devices

IV.4.1. Magnetic switching characteristics of NiFe/Al2O3/Co trilayers in nanometer-scalepatterned arrays of elements

In this part the results of magnetic trilayers patterned into arrays using electron beam lithography

(each array contains 108 elements) are presented. Fig. 43 shows the magnetic switching as a

function of external magnetic field measured by AGM method with different aspect ratio of 1.5,

2, 3 and 4 for trilayers of NiFe 20nm/Al2O3 0.8nm/Co 15nm of width of 200nm. The barrier

thickness of 0.8nm was prepared by ultraviolet radiation assisted in oxygen for 1 hour oxidation

time under oxygen pressure of 100 mbar. From Fig. 43 it is shown that with increasing the aspect

ratio the magnetization curve against the applied magnetic field is changed. Fig. 43 a-d shows the

switching field for both NiFe and Co layers at different aspect ratio of 1.5, 2, 3 and 4

respectively. It is shown clear that the switching field for both NiFe and Co layers is changed

with increasing the aspect ratio. The switching field of a free magnetic single domain stripe is

directly proportional to the magnetization and the thickness of the magnetic film and inversely

810 800 790 780 770

Co 2p 1/2

Cobalt Oxid

Co 2p3/2

Binding Energy [eV]

(A)

(C)

(D)

(B)

100

200

300

400

Page 57: Development of Ferromagnetic/Insulator/Ferromagnetic ...

56

proportional to the width of the stripe. The non distinguishable switching field for both soft layer

(I) and hard layer (II) as a function of the aspect ratio due to the magnetostatic coupling (dipole-

dipole interaction) between the two magnetic layers and the magnetization vortex for the Co

single layer which is strong at small aspect ratio see Fig. 43-a (inside curve). The inside curve

shows the magnetization curve of Co 15nm single layer in which the distortion in the curve (due

the magnetization vortex) leads to distortion in the trilayers curve for the same aspect ratio . The

experimental results on the switching fields of the TMR devices are summarized in Fig. 44 in

which the switching field of the soft layer and the hard layer increase with increasing the aspect

ratio. This behavior for soft and hard layers due to the dipole-dipole interaction between the

layers which provides a well pronounced change of magnetization of the two layers. The

resulting forces per volume are stronger for the smallest elements [85]. This finding can be

considered in designing MRAM matrix based on TMR elements.

(a)

-100 -50 0 50 100

-1.0

-0.5

0.0

0.5

1.0

200nmx300nm

(II)

(I)

M/M

s

Magnetic Field µ0H (mT)

-100 -50 0 50 100

-1.0

-0.5

0.0

0.5

1.0

Co single layer

L/W= 1.5

Hs

M/M

s

Magnetic Field µ0H (mT)

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57

(b)

(c)

-150 -100 -50 0 50 100 150

-1.0

-0.5

0.0

0.5

1.0

(I)

(II)

200nm x 400nm

M/M

s

Magnetic Field µ0H (mT)

-60 -40 -20 0 20 40 60

-1.0

-0.5

0.0

0.5

1.0 200nm x 600nm

(II)

(I)

M/M

s

Magnetic Field µ0H (mT)

Page 59: Development of Ferromagnetic/Insulator/Ferromagnetic ...

58

(d)

Figure 43: Shows the AGM results for NiFe/Al2O3/Co trilayers with different aspect ratios of 200 nm width.

Figure 44: Dependence the switching field on the aspect ratio for the soft layer (NiFe) and the hard layer (Co).

-60 -40 -20 0 20 40 60

-1.0

-0.5

0.0

0.5

1.0 200nm x 800nm

(II)

(I)

M/M

s

Magnetic Field µ0H (mT)

1.5 2.0 2.5 3.0 3.5 4.0

-60

-45

-30

-15

0

15

30j

ij Co layeri NiFe layer

Sw

itchi

ng

Fie

ld µ

0H (

mT

)

Length/Width ratio ( width= 200 nm)

Page 60: Development of Ferromagnetic/Insulator/Ferromagnetic ...

59

IV.4.2. Images of NiFe/Al2O3/Co trilayers patterned element arrays by AFM and MFM microscopes

Fig. 45 shows the AFM (right) and MFM (left) images for NiFe 20nm/Al2O3 0.8nm /Co 15nm,

elements of 200nm width with different aspect ratio. Ideally, the MFM measures the net

magnetization of the two magnetic layers. It is shown clearly that all elements are patterned along

the element length. Fig. 45-a shows the MFM image (left) for dimension of 200nmx300nm in

which some of the elements have the south pole on the left side while in other elements on the

right side, which means the magnetization of the elements have random distribution at aspect

ratio of 1.5. Fig. 45 shows at (b and c) the effect of increasing the aspect ratio of 2 and 3 on the

MFM images, in which the south poles start to have one direction in some elements at

200nmx400nm and increased at 200nmx600nm in which most of the elements have south poles

in one direction. This means at H= 0 Oe the magnetic moments of all elements start to be in one

direction [86], which means that the magnetization doesn’t suffer any distortion at large aspect

ratio. The MFM result is compatible with the AGM result, in which the magnetization curves is

dependence of the aspect ratio and in accordance with dipole-dipole interaction. The dipole-

dipole interaction is strong interaction at small aspect ratio and the magnetization vortex for Co

single layer which appear at small aspect ratio and disappear with increasing the aspect ratio.

(a)S N N S

Page 61: Development of Ferromagnetic/Insulator/Ferromagnetic ...

60

(b)

(c)

Figure 45: AFM and MFM images for NiFe 20nm/Al2O3 0.8nm/Co 15nm trilayers at 200nm width and at aspectratio of 1.5 at (a), of 2 at (b) and of 3 at (c).

IV.4.3. Magnetic Optic Kerr Effect (MOKE) for NiFe/A2O3/Co trilayers

The MOKE loops for a NiFe 15nm single film and a Co 10nm single film are shown in Fig. 46-a

and 46-b, respectively. They depict simple square loops, in which the switching fields of the

single films differ from the Co 10nm/A12O3 1.3nm/NiFe 15nm trilayers as shown in Fig. 46-c.

The MOKE loop for trilayers clearly shows two distinct switching fields, in which the small step

in the magnetization curve corresponds to the switching of the NiFe electrode (soft layer), the

large step corresponds to switch the Co electrode (hard layer), as can be readily deduced from

the ratio of the magnetic moments of the respective layers. In the trilayers curve, the switching

field of the NiFe electrode is larger and the switching field of the Co electrode is smaller than the

N SN S

S N N S

Page 62: Development of Ferromagnetic/Insulator/Ferromagnetic ...

61

switching field in case of single film [87]. This indicates that there is weak coupling between the

ferromagnetic electrodes in the trilayers. This weakness of the coupling suggests that the

insulating barrier is almost pinhole free which supports the XPS results on similar samples.

(a) (b)

(c)

Figure 46: MOKE measurement for NiFe single layer at (a), Co single layer at (b), and Co 10nm/A12O3 1.3nm/NiFe15nm trilayers at (c).

IV.4.4. Magnetic switching simulation for NiFe/Al2O3/Co trilayers at different aspect ratios

The magnetic switching of the trilayers (NiFe/Al2O3/Co) was determined by micromagnetic

modeling based on Landau-Lifshiz-Gilbert equation with different aspect ratios. Fig. 47 shows

the magnetic switching value (modeling and experimental) of the trilayers (NiFe 20nm/Al2O3

0.8nm/Co 15nm) as a function of the aspect ratio, in which the modeling value is not equal to the

measured value, this is due to the sensitivity of the element which is not only to the external

magnetic field but also to the field created by the nearest neighbor particles, in which this

-4 -2 0 2 4

Mag

netiz

atio

n (a

.u.)

Applied Magnetic Field µ0H (mT)

-4 -3 -2 -1 0 1 2 3 4

Mag

netiz

atio

n (a

.u.)

Applied Magnetic Field µ0H (mT)

-4 -3 -2 -1 0 1 2 3 4

Co

NiFe

Mag

net

izat

ion

(a.u

.)

Applied Magnetic Field µ0H (mT)

Page 63: Development of Ferromagnetic/Insulator/Ferromagnetic ...

62

creation field is exist only at real particle. From the previous results of NiFe and Co single layer it

is shown clearly that the switching field modeling and experimental values are different which

lead to the difference between the switching field modeling and experimental values for the

trilayers.

Fig. 48 shows the micromagnetic modeling images of the magnetization behavior for soft layer

(NiFe) and hard layer (Co). In each layer the magnetization at the opposite ends of the element

are antiparallel to each other [88]. When the two layers started to switch, the observed

configurations by micromagnetic simulation show that, when the hard layer (Co) start to switch a

deviation from homogeneous magnetization occurs only at the layer boundary while at switching

the soft layer (NiFe) a deviation from homogeneous magnetization occurs towards the center of

the layer.

Figure 47: Switching field value, experimental and modeling, as a function of aspect ratio for NiFe/Al2O3/Cotrilayers.

Figure 48: The magnetization configuration of soft and hard layers using micromagnetic modeling.

1.5 2.0 2.5 3.0 3.5 4.0

-60-40-20

020406080

100120140 Soft layer Exp.

Soft layer Mod. Hard layer Exp. Hard layer Mod.

Sw

itchi

ng F

ield

µ0H

(m

T)

Aspect ratio (length/width)

Soft Layer (NiFe) Hard Layer (Co)

Page 64: Development of Ferromagnetic/Insulator/Ferromagnetic ...

63

IV.4.5. Electrical and magnetic properties of the TMR devices at different oxidationmethods

The first measurement of the TMR devices is always the electrical measurements I-V that gives

information about the quality of the barrier. The device was measured using the four

measurements points. The nonlinearity of the I-V curves indicate that the barrier is pinhole-free

(pinholes would lead to a short circuit between the two electrodes). The left parts of Fig. 49-a, b

and c presents the typical I-V curves at room temperature for three junctions. At (a) the junction

area was 10x10 µm. It was oxidized by natural oxidation (ex-situ) at room temperature for

oxidation times of 96 hours. The junction structure was NiFe 20nm/Al2O3 1.3nm/Co 15nm. At

(b) the junction was oxidized by natural oxidation (in-situ) inside the chamber filled by pure

oxygen under pressure of 130 mbar for 12 hours. The junction structure was FeMn 12nm/CoFe

6nm/Al2O3 1.3/ CoFe 2nm/ NiFe 8nm and the junction area was 2x4 µm [64]. At (c) the junction

was oxidized by UV oxidation method for 1 hour under pressure of 100 mbar. The junction

structure was NiFe20nm/ Al2O3 1.3nm/Co15nm and the junction area was 10x4 µm. In all three

cases the characteristic shows a non-linear behavior, which is similar to the theoretical I-V curves

calculated by Simmon (1963) [11]. The non-linearity is caused by the decrease in the effective

barrier height with increasing applied voltage. By fitting such measured I-V curves with

Simmon’s expression the correlated values for the barrier height and thickness could be obtained.

The I-V curves have less pronounced non-linearity. This result is consistent with the calculation

of Simmons model. In the right part of Fig. 49-a, b and c the measurements of the resistance as

function of the external applied magnetic field are shown. Clearly the states with parallel and

anti-parallel magnetization vectors of the hard magnetic Co and the soft magnetic NiFe are

separated. The measured resistance values at parallel state (Rp) and antiparallel state (Ra) lead to

MR ratio of 6.4 % at natural oxidation (ex-situ) at room temperature at (a), an MR ratio of 18.7%

with natural oxidation (in-situ) at room temperature at (b) and an MR ratio of 20% at room

temperature with ultraviolet oxidation at (c). At Fig. 49-a and c the two ferromagnetic electrodes

with different coercivity of NiFe and Co which used in a tunnel junction as top and bottom

electrodes. At a high positive field, the magnetization of both electrodes NiFe and Co are parallel,

when the field is lowered and raised in the negative direction, the magnetization of the electrode

with the smallest coercivity of NiFe will rotate first. The net moment of both layers is lowered, or

become zero if the saturation magnetization of both layers is identical. The NiFe electrode will

rotate till the magnetization in the two electrodes are antiparallel, which leads to maximum MR

Page 65: Development of Ferromagnetic/Insulator/Ferromagnetic ...

64

ratio. Then, with increasing the negative field, the magnetization of the electrode with highest

corecivity Co will switch, and again a parallel alignment which leads to minimum MR ratio. In

the second case at Fig. 49-b the switching from parallel to antiparallel by exchange coupling of

one of the layers to an antiferromagnet was obtained, which is called exchange-biasing [90]. The

magnetization of the electrode will only rotate when the exchange-anisotropy field is reached

while the other free electrode has a low coercive field. At low positive field the free layer will

switch to antiparallel alignment. This big changes between the parallel and antiparallel of the two

electrodes leads to a large value change in the magnetoresistance. Further experimental findings

that the failure rate of devices on a substrate is fairly large, 30% of the samples showed tunnel

behavior of MR effect at natural oxidation ex-situ while at in-situ more junctions show MR

effect. In addition, the samples degraded with time where after weeks short circuits were

observed. While, no changes of the resistance or the MR ratio were observed even after two years

the junction prepared with UV oxidation method. With comparing the oxidation methods for the

same electrode thickness of NiFe and Co and for the same Al thickness 1.3nm the MR ratio was

6.5% with natural oxidation ex-situ, 8% with natural oxidation in-situ and 20% with UV

oxidation method at room temperature.

-10 -8 -6 -4 -2 0 2 4 6 8 10-1

0

1

2

3

4

5

6

7Ra

Rp

(a)MR= 6.4%T= 300KEx-situ oxid.

MR

rat

io (%

)

Applied Magnetic Field µ0H (mT)-0.3 -0.2 -0.1 0.0 0.1 0.2 0.3

-0.9

-0.6

-0.3

0.0

0.3

0.6

0.9(a)

dI/d

V (

mA

/V)

Tun

nel c

urr

ent (

mA

)

Applied Voltage (V)

0

1

2

3

4

5

6

7

-0.4 -0.2 0.0 0.2 0.4-0.3

-0.2

-0.1

0.0

0.1

0.2

0.3(b)

dI/d

V (

mA

/V)

Tun

nel C

urre

nt (

mA

)

Applied Voltage (V)

0.57

0.58

0.59

0.60

0.61

0.62

-20 -10 0 10 20

0

3

6

9

12

15

18

21 Ra

Rp

MR= 18.7%T= 300KIn-Situ

(b)

MR

rat

io (%

)

Applied Magnetic Field µ0H (mT)

Page 66: Development of Ferromagnetic/Insulator/Ferromagnetic ...

65

Figure 49: I-V curves (left) and the MR ratio as a function of applied magnetic field (right) for different oxidationmethods. At (a) natural oxidation method (ex-situ), the junction has area of 10x10 µm and structure of NiFe20nm/Al2O3 1.3nm/Co 15nm. At (b) natural oxidation method (in-situ), the junction has area of 2x4 µm and structureof FeMn 12nm/CoFe 6nm/Al2O3 1.3/ CoFe 2nm/ NiFe 8nm. At (c) UV oxidation method, the junction has area of10x4 µm and structure of NiFe20nm/AlOx1.3nm/Co15nm.

IV.4.6. Electrical and magnetic properties of the TMR devices at different temperatures

Fig. 50-a shows the temperature effect on the I-V curves within temperature range 290K-4K. At

(a) the junction has a size of 180µm2 and a structure of NiFe 20nm/Al2O3 1.3nm/Co 15nm, the

sample was prepared by ultraviolet oxidation for 1hour. The I-V curves were measured by

applying sweeped voltage ±300 mV on the junction, which is sufficient enough to observe the

nonlinearity of the tunnel curve. With decreasing the temperature, the tunnel current decreased.

This result is in agreement with Strattons theory [91], who explains the relation between the

conductance and the temperature. The tunneling current at constant voltage can be expressed as

I(T)= I (0) (1+1/6π2C2K2B T2) (9)

where I (T) and I(0) are the tunneling currents at temperature T and 0 K, respectively. KB is

Boltzmann‘s constant and C is a constant which depends on the barrier height and thickness.

-20 -10 0 10 20-5

0

5

10

15

20 Ra

Rp

MR= 20%T= 300KUV oxid.

(c)

MR

rat

io (

%)

Applied Magnetic Field µ0H (mT)

-0.4 -0.2 0.0 0.2 0.4-3

-2

-1

0

1

2

3(c)

dI/d

V (

mA

/V)

Tun

nel C

urre

nt (

mA

)

Applied Voltage (V)

4

5

6

7

8

Page 67: Development of Ferromagnetic/Insulator/Ferromagnetic ...

66

Figure 50: I-V curves at different temperatures at (a) and I (T)/I (0) vs. T2 at (b).

Fig. 50-b shows that the I (T)/I (0) values of the junction is roughly proportional with T2 in the

whole temperature range measured which is the same behavior as explained before by Strattons

theory. Fig. 51-a shows the MR ratio curves as a function of the external magnetic field at

different temperatures for a junction with a size of 180µm2 . The structure was NiFe 20nm/Al2O3

1.3nm/Co 15nm. The sample was prepared by ultraviolet oxidation for 1hour. The MR ratio was

12% at room temperature, which increased to 19.9 % with decreasing the temperature down to

77K. When the temperature was decreased to 4K, no changes in the MR ratio were observed. Fig.

51-b shows the magnetoresistance ratio as a function of the external magnetic field at different

temperatures. The junction has 24µm2 area and a structure of FeMn 12nm/CoFe 6nm/Al2O3

1.3nm/CoFe 2nm/NiFe 8nm, which was prepared by natural oxidation (in-situ) in oxygen under

pressure 130mbar for 12 hours. The junction shows the temperature effect on the MR ratio, which

is the same behavior as explained before. Fig. 52 summarized the MR ratio as a function of

different temperatures for samples prepared by (a) UV oxidation method and (b) natural

oxidation method (in-situ) for the above two samples. There are two reasons for the dependence

of the MR ratio on temperature; the first one was explained by Zhang et al. [92] who put forward

a theory which explains the dependence of the conductivity for spin up and spin down and

therefore the magnetoresistance effect on temperature with magnon generation and absorption in

the electrodes. The other explanation is due to the traps in the barrier, which offer sites located in

the barrier with energies below the barrier height. Electrons which tunnel in two steps through the

barrier via the trap have their spin information randomized. With increasing temperature, traps

become more important in conduction; their activation energy is lower than for thermionic

-0.6 -0.4 -0.2 0.0 0.2 0.4 0.6-6

-4

-2

0

2

4

6

4k25k110K270K

(a)

290 k

Tun

nel C

urre

nt (

mA

)

Applied Voltage (V)0 20000 40000 60000 80000

0.0

0.2

0.4

0.6

0.8

1.0 (b)

I(T

) / I

(0)

T2 (K2)

Page 68: Development of Ferromagnetic/Insulator/Ferromagnetic ...

67

emission. At low temperature the traps will be filled with electrons and the electron will tunnel

from the first electrode to the second electrode through the barrier as direct tunneling in one step

and not through the traps which leads to high MR ratio.

.

Figure 51:Magnetoresistance curves at different temperature for junction oxidized by UV oxidation method and hasa structure of NiFe 20nm/Al2O3/Co 15nm at (a), and natural oxidation method has a structure of FeMn 12nm/CoFe6nm/Al2O3/ CoFe 2nm/NiFe 8nm at (b).

-20 -15 -10 -5 0 5 10 15 20

0

3

6

9

12

15

18

99K

167K225K

300K

MR

rat

io (%

)

Applied Magnetic Field µ0H (mT)

-20 -10 0 10 20

0

5

10

15

20

25

30

(b)

300K 200K 171K 91K

MR

rat

io (

%)

Applied Magnetic Field µ0H (mT)

Page 69: Development of Ferromagnetic/Insulator/Ferromagnetic ...

68

Figure 52: shows the MR ratio as a function of the temperature for a junction oxidized by UV oxidation method hasa structure of NiFe 20nm/Al2O3/Co 15nm at (a) and natural oxidation method (in-situ) has a structure of FeMn12nm/CoFe 6nm/Al2O3/ CoFe 2nm/NiFe 8nm at (b).

IV.4.7. Further Parameters which effect on the tunnel magnetoresistance devices

There are different parameter, which effect on the tunnel magnetoresistance devices like bias,

temperature, magnetization direction and current direction, were investigated in this work.

(a)- For bias dependence, the tunnel magnetoresistance effect was measured as a function of the

applied voltage. With studying several junctions, it was found that the MR ratio dropped by a

factor two at applied voltage in the order of 200-500mV. The bias dependence of tunnel

magnetoresistance is not yet understood completely. Fig. 53 shows at (a1 and b1) the MR ratios

as a function of the applied voltage. At a1 the junction has a structure of NiFe 20nm/Al2O3

1.3nm/ Co 15nm and area of 10x3 µm which was prepared by UV oxidation for 1 hour, the MR

shows 10.1 % at 10mV and 4.81% at 600mV. At b1 the junction has a structure of FeMn

12nm/CoFe 6nm/Al2O3 1.3/ CoFe 2nm/ NiFe 8nm and area of 2x3µm which prepared by natural

oxidation method (in-situ) for 12 hours, the MR shows 16.7% at 10mv while at 400mv shows

7.2%. Fig. 53 shows at (a2 and b2) similar results of bias dependence for the same junctions a1

and b1 at 91K. The bias dependence effect has been explained in the literature by different

phenomenon: like excitations of magnons [93,94] or phonons [93]. Inelastic tunneling process via

defect [93,95], electron interactions at the electrode-barrier interface [96,97,98]. The bias

dependence of the tunnel magnetoresistance devices can change from barrier to other depending

on the quality of the barrier, which is explained with phenomenological model [95]. This model

suggests that, there are localized defect states in the barrier. Excitation of electrons from these

50 100 150 200 250 300

12

14

16

18

20 (a)

MR

rat

io (

%)

Temperature (K)

50 100 150 200 250 300

16

18

20

22

24

26

28

(b)

MR

rat

io (

%)

Temperature (K)

Page 70: Development of Ferromagnetic/Insulator/Ferromagnetic ...

69

defect states either thermally or by hot electron impact, creates states available for a two-steps

tunneling channel, as shown in Fig. 54. Therefore there are two tunneling processes, direct

tunneling through the barrier and indirect tunneling (two tunneling channel) through the defects.

In these two tunneling channels the electrons tunnel from the first electrode to one of the

localized defects while the second tunneling channel, the electron tunnel from the localized

defect to the second electrode. The tunneling in the two-step is spin independent because the

electrons lose its spin memory during the tunneling, which leads to a decrease in the MR ratio.

The MR of this spin independent current can be expressed as:

MR = ((Ipara –Ianti )/(Ianti))/(1+I2/Ianti ) (10)

Where I2 is the current carried by the two-step tunneling. Ipara and Ianti are spin dependent currents

carried out by direct tunneling when magnetic moments are parallel and antiparallel, respectively.

It is clear that the ratio of the spin-dependent and spin-independent currents determines the MR

ratio. With increasing the bias, the probability of the electrons which can tunnel through the

defects in two-step channel is increased. This leads to a decrease in the MR ratio and explains the

bias dependence of tunnel magnetoresistance devices.

-20 -10 0 10 20-2

0

2

4

6

8

10

12

-20 -10 0 10 20

0

3

6

9

12

15

18

600 mV300 mV100 mV

(a1) 10 mV

MR

rat

io (

%)

Applied Magnetic Field µ0H (mT)

400 mV300 mV100 mV

(b1)

10 mV

MR

rat

io (

%)

Applied Magnetic Field µ0H (mT)

Page 71: Development of Ferromagnetic/Insulator/Ferromagnetic ...

70

Figure 53: The MR ratio as a function of an applied magnetic field at (a1and b1) for different applied voltage atroom temperature and low temperatures at (a2 and b2).

Figure 54: Schematic view of the two-step tunneling via defect states.

(b)- Temperature dependence has explained before at IV.4.6.

(c)-Magnetization direction of the junction as a function of the external magnetic field direction

is one of the parameters, which effect on the tunnel magnetoresistance devices. Fig. 55-a shows

the MR ratio at easy axis (i.e. the magnetization direction is parallel to the external magnetic

field) and hard axis (i.e. the magnetization direction perpendicular to the external magnetic field)

of a junction prepared by UV oxidation method that has a structure of NiFe 20nm/Al2O3 1.3nm/

Co 15nm and area of 10x3 µm. The MR ratio shows 10.1% at easy axis and 8.7% at hard axis.

The magnetization at easy axis rotates first at the electrode which has low coercivity while the

Barrier

Metal 1 Metal 2

Step 1 Step 2

Conduction band

Trap States

-20 -10 0 10 20

0

4

8

12

16

20

24

-20 -10 0 10 20-2

0

2

4

6

8

10

12

14

16

400 mV300 mV100 mV

(b2)

T= 91K

10 mV

MR

rat

io (

%)

Applied Magnetic Field µ0H (mT)

400 mV300 mV100 mV

T= 91K(a2) 10 mV

MR

rat

io (

%)

Applied Magnetic Field µ0H (mT)

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71

magnetization at the second electrode still has the opposite direction till the magnetization in the

two electrodes are antiparallel. This leads to maximum MR ratio. The magnetization at hard axis

is difficult to rotate completely and the magnetization in the two electrodes is not completely

antiparallel which leads to a decrease in the MR ratio. Fig. 55-b shows the MR ratio of a junction

prepared by natural oxidation method (in-situ) for 12 hours under oxygen pressure of 130 mbar.

The junction has a structure of FeMn 12nm/CoFe 6nm/Al2O3 1.3/ CoFe 2nm/ NiFe 8nm and the

junction area was 2x3µm. The MR ratio shows 16.8% at easy axis and 3.8% at hard axis.

Because all layers are sputtered in the same direction, therefore the magnetization in all layers are

parallel. At easy axis the magnetization of the soft layer will rotate while at the pinned layer not

and the magnetization of the two layers will be antiparallel which leads to maximum MR. At hard

axis the soft layer NiFe which is the top electrode will rotate and the second layer CoFe will

rotate while the bottom electrode will stay in the same direction because the antiferromagnetic

pinned the magnetization in the other direction. Therefore the magnetization will not be

antiparallel at easy axis which leads to decrease the MR ratio.

Figure 55: MR ratio as a function of applied magnetic field at easy and hard axis for two junctions. At (a) preparedwith UV oxidation and has a structure of NiFe 20nm/Al2O3 1.3nm/ Co 15nm. At (b) prepared with natural oxidation(in-situ) and has a structure of FeMn 12nm/CoFe 6nm/Al2O3 1.3/ CoFe 2nm/ NiFe 8nm.

(d)- One of the parameter, which effects on the tunnel magnetoresistance devices, which was

observed during the experimental work, is the current direction. Fig. 56-a shows a junction

prepared by UV oxidation method and has a structure of NiFe 20nm/Al2O3 1.3nm/Co 15nm and

-20 -10 0 10 20

0

2

4

6

8

10

12

-20 -10 0 10 20

0

2

4

6

8

10

12

14

16

18

hard axis(a) easy axis

MR

rat

io (

%)

Applied Magnetic Field µ0H (mT)

easy axis

(b)

hard axis

MR

rat

io (

%)

Applied Magnetic Field µ0H (mT)

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72

junction area of 30µm2, in which the current direction changed the MR ratio at positive current

the MR ratio is 10.1% while at negative current is 9.8%. For the other junction at (b) the structure

was FeMn 12nm/CoFe 6nm/Al2O3 1.3/ CoFe 2nm/ NiFe 8nm and has a junction area of 2x3µm,

the MR as a function of applied voltage at room temperature and at low temperature shows bias

dependence direction. The difference at MR ratio as a function of the current direction is

consistence with the band-structure effect as explained in literature [99]. Since the tunneling

current comes mostly from electrons near to the Fermi level of the positive electrode, which are

closest in energy to the barrier top, variation in their tunneling probability as a function of bias is

a mapping of the negative electrode‘s density of states above its Fermi level. Difference in the

electrode/barrier interface cannot be ruled out as another possible cause of this correlation [3].

Till now the MR ratio as a function of the current direction is not completely understood and

needs more research to define which parameters plays a role in this effect (i.e. band structure or

the quality of the barrier).

Figure 56: MR ratio as a function of applied magnetic field with different current direction for junction preparedwith UV oxidation and has a structure of NiFe 20nm/Al2O3 1.3nm/ Co 15nm at (a). MR ratio as a function of appliedvoltage for junction prepared with natural oxidation (in-situ) at different temperatures and the junction has astructure of FeMn 12nm/CoFe 6nm/Al2O3 1.3/ CoFe 2nm/ NiFe 8nm at (b).

-20 -10 0 10 20-2

0

2

4

6

8

10

12

-450 -300 -150 0 150 300 450

9

12

15

18

21

24

negative current(a) positive current

MR

rat

io (

%)

Applied Magnetic Field (mT)

i

jj = 91k

(b) i = 300K

MR

rat

io (

%)

Applied Voltage (mV)

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73

IV.4.8. Realibility and breakdown of TMR devices

In the breakdown measurements different junction area varied between 4-600 µm² were

investigated. All junctions have the same electrode thickness of Ni80Fe20 20nm/Al2O3 1.3nm/ Co

15nm, which was prepared by UV oxidation for 1 hour. Fig. 57 a and b shows the I-V curves for

a single junction of 60µm² and 45µm² respectively. The breakdown occurs during a voltage ramp

measurement. At (a) the breakdown occurs at 1.83V while, at (b) the breakdown occurs at 2.1V.

Biasing the junction at a slightly lower voltage (1 V) leads to a strong increase in the lifetime of

the device, which means several days or more [100]. After breakdown, the I-V characteristic is

found to be nearly ohmic. The resistance of the junctions after breakdown is typically in order of

20-130 Ω, which depends on the junction area. At (c) the junction area as a function of the

breakdown voltage shows that with increasing the junction area the breakdown voltage decrease.

The breakdown in the barrier leads to a decrease in the magnetoresistance effect from 14% to

0.8% as shown in Fig. 57-d the junction has a structure of NiFe 20nm/Al2O3 1.3nm/Co 15nm and

area of 3x10µm. The resistance also decreased after breakdown from 1964 Ω to 18 Ω at low bias.

The breakdown as a function of the junction area can be explained by the probability of finding a

weak point in the oxide. In the small junction area the probability to find a weak point in the

oxide is less than in the larger area. This is explained in the schematic diagram of Fig. 58 which

shows the defects in the barrier. At smaller junction like (B) junction there are no defects, while

at larger junctions like (A) and junction (C) there are many defects depending on the junction

area. Therefore, by decreasing the junction size the probability of the junction breakdown will

decrease. This observation has a great importance in the MRAM technology application, where

the ultimate goals design a large array of reliable sub-micron sized junctions.

Page 75: Development of Ferromagnetic/Insulator/Ferromagnetic ...

74

Figure 57: I–V curves for two-junction areas at (a) 60µm2 and at (b) 45µm2. Breakdown voltages of series ofNiFe/Al2O3/Co tunnel junctions as a function of junction area at (c). MR ratio as a function of applied magneticfield before and after breakdown at (d).

0 50 100 150 200

0.8

1.0

1.2

1.4

1.6

1.8

2.0

2.2

0.0 0.3 0.6 0.9 1.2 1.5 1.8 2.1

0.00

0.02

0.04

0.06

0.08

-25 -20 -15 -10 -5 0 5 10 15 20 25

0

3

6

9

12

15

0.0 0.3 0.6 0.9 1.2 1.5 1.8 2.1 2.4

0.000

0.001

0.002

0.003

0.004

0.005

(c)Bre

akdo

wn

Vol

tage

(V

)

Junction area (µm2)

(a)

Breakdown

Tun

nel C

urre

nt (

A )

Applied Voltage ( V )

(d)

before breakdown

after breakdown

MR

rat

io (

%)

Applied Magnetic Field µ0H (mT)

(b)

Breakdown

Tun

nel C

urre

nt (

A)

Applied Voltage (V)

Page 76: Development of Ferromagnetic/Insulator/Ferromagnetic ...

75

Figure 58: Schematic diagram for the defects in the barrier.

IV.4.9. MRAM array of tunnel magnetoresistance elements characterization

The interest in magnetic random access memory (MRAM) devices has been reached recently

with a possibility to use spin tunnel junctions as the memory cell element. The MRAM cells,

which used as a memory array, are needed to be selectable by the diode. Each tunnel junction can

be connected with one diode with which the current will flow only through the single junction.

By turning the diode on or off, the bit is selected for reading. Writing information into the

MRAM cells is quite different from reading them. The cell is written by a magnetic field, which

is generated by current flows through a word line. These words line are densely packed in the

core of the memory with each word line covering many different bits [101].

Figure 59: Mask for MRAM array.

Junction

Bottom electrode

Top electrode

Second ElectrodeFirst Electrode Barrier

A

B

C

Defects

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76

Figure 60: Shows several memory elements, selected out of the array, also the bit line current and the word linecurrent for each memory element.

In the magnetic tunnel junction structure, the direction of polarization of the hard layer is used to

store the information. The resistance of the memory tunnel junction is either low or high

dependent on the relative polarization, parallel or antiparallel, of the free layer with respect to the

hard layer. Fig. 59 shows the mask for TMR junctions of the top and bottom electrode. The

presented junction has a structure of NiFe 20nm/Al2O3 1.3nm/Co15 nm with different areas. The

junctions were prepared by ultraviolet oxidation for an hour under pressure 100mbar. Fig. 60

shows the picture for MRAM mask in which the bit lines was the bottom electrode of 25µm wide

and 20 nm thick. The word line on top was separated from the junction by a SiO2 passivation

layer ≈ 200nm thick. The thickness of the Au word line is 150nm of 14 µm width.

Figure 61: Resistance as a function of applied magnetic field for a tunnel magnetic junction.

Word line current

Bit line current

Junction

-20 -10 0 10 20

1250

1300

1350

1400

Res

ista

nce

)

Applied Magnetic Field µ0H (mT)

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77

Fig. 61 shows the resistance as a function of the applied magnetic field for a junction of MRAM

cells with area 4 x 10µm, the MR ratio shows 13% at room temperature and the resistance was

1.25kΩ. The MRAM memory circuit consists of magnetic tunnel junctions, which are integrated

with 0.25µm CMOS technology [102]. The magnetic tunnel junction is added after preparing the

CMOS circuitry. In this work two different ways are used to characterize the MRAM cell, which

is integrated with a diode. The first one is sputter depositing the magnetic tunnel junction on the

top of the diode. The second , which is used in this work, is connecting the magnetic tunnel

junction in series (by bonding) with the diode, which was GaAs diode. The GaAs p-n diode has a

doping level of 10-17 cm-3 for both p and n regions. The diode dimension is 320µm2 and the

magnetic tunnel junctions dimension varies between 1 x 3µm and 4-6 x 10µm. Fig. 62 shows the

I-V curves for the MTJ, diode, diode + resistance and the device (MTJ + diode). The MTJ is

connected in series with a diode, to separate the diode characteristics from the MTJ

characteristics; the diode was connected with a resistor, which is the bottom electrode in order to

define the I-V of the junction after connection with the diode. For GaAs the voltage needed to run

the diode is 1 V, at this value a lot of current starts to flow through the GaAs diode. The I-V

characteristics of this compound device were measured at zero field.

Figure 62: I-V curves for the MTJ and the diode.

0.0 0.4 0.8 1.2 1.6 2.0-2.0x10-4

0.0

2.0x10-4

4.0x10-4

6.0x10-4

8.0x10-4

1.0x10-3

MTJ diode diode+R diode+MTJ

Tun

nel

Cur

rent

(A

)

Applied Voltage (V)

Page 79: Development of Ferromagnetic/Insulator/Ferromagnetic ...

78

For the writing process there are two possibilities, the first one by applying a large current

through the word line and top electrode to generate a magnetic field sufficient to switch the hard

layer which is chosen to store the information. The second possibility is applying a sufficient

large external magnetic field to switch the hard layer. For the writing an external magnetic field

was applied of 5mT which was enough to switch the hard layer (Co=15nm).

Figure 63: Shows the expected writing and reading timing diagram.

Fig. 63 shows the writing processing in which, if a certain current was applied to write “1” or

“0”, it is expected to appears across the memory element storage/sense line to read them by the

sense output voltage Vs [29]. The Figs. 64 and 65 show the reading signal after writing, in these

figures two different currents were applied. The first one through the word line in order to

generate small magnetic field sufficient to switch the soft layer and the second one through the

junction in order to measure the changes in the resistance through the junction after and before

switched the soft layer which was in two states at low state “1” and high state “0”. The input

signal was in two states the first one from +50mA to –50mA and the second one from 0 to –

50mA as shown in Figs. 64-a and 65-a, while Figs. 64-b and 65-b show the output signal in the

two-directions (±). With comparing the top and the bottom Figs. (i.e. the input and the output) it

is shown clear that with changing the input signal, a strong response at the pulse sense output is

appeared across the memory element storage / sense line.

Read ProcessWrite Process

1

0

1

0

Word currentIw

Sense OutputVs

Page 80: Development of Ferromagnetic/Insulator/Ferromagnetic ...

79

The output signal is shown in Fig. 65-b is measured at 100 mV which shows a very strong output

response compared with the output Fig. 64-b which measured at 1.3 mV. From these two curves

it is concluded that at higher voltage 100mV show more reliable readout than at 1.3mV because

at 100mV produces more response signal than the lower voltage 1.3 mV. The input signal is

changed compared with Fig. 64 to define the memory cell ability to read at any signal and with

different direction word current. The memory cell for reading and writing was tested at input

word current from few mA-100mA for different times and shows reliability for readout process

in both directions positive and negative. As comparing the previous Fig. 63 with the results in

Figs. 64 and 65 show a good agreement.

(a)

(b)

Figure 64: Input and output signals for readout process at +50>-50mA (left) and at 0> -50mA (right).

2 4 6 8 10

12

14

16

18

20

22

24

26

-40

-20

0

20

40

60

Cur

ren

t (m

A)

Time (arb.units)

2 4 6 8 10 12 14 16 18 20 22 24 261.20

1.25

1.30

1.35

1.40

Vol

tage

(m

V)

Time (arb.units)

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80

(a)

(b)

Figure 65: Input and output signals for readout process.

In the next Fig. 66 shows the read curves after writing in the two directions positive and negative

writing. The figure shows the output voltage from the memory cell as a function of the current

through the bit line. The cell can read from a few mA and become more saturated from 25mA till

100mA. The difference between the positive and negative changes is due to the changes in the

magnetic single at the MR loop, which is shown in Fig. 61. This is a disadvantage for the sample,

which has non-homogeneity in the MR loop. This is due to the anisotropy shape, which plays a

very important role in the MR loop. The MR loop needed to be sharp in the switching which is

determined by (a combination of) (a) the shape anisotropy, (b) include exchange coupling.

2 4 6 8 10

12

14

16

18

20

22

24

26

-40

-20

0

20

40C

urre

nt (

mA

)

Time (arb.units)

5 10 15 20 25102

103

104

105

106

107

Vol

tage

(m

V)

Time (arbit.units)

Page 82: Development of Ferromagnetic/Insulator/Ferromagnetic ...

81

Figure 66: Read curves after writing in two directions positive and negative.

Finally it is concluded that MRAM arrays can quite well be used in non-volatile memory

applications, which can read and write based on Si technology, but it needs a lot more time for

research before being ready for large-scale fabrication.

-100 -50 0 50 100

1.28

1.30

1.32

1.34

1.36

1.38

Pos. Write Neg. Write

Vol

tage

(m

V)

Current (mA)

Page 83: Development of Ferromagnetic/Insulator/Ferromagnetic ...

82

Summary

The aim of this work is developing the ferromagnetic/insulator/ferromagnetic (TMR)

devices for magnetic data storage and magnetic field sensors. Therefore the switching field of the

element as a function of the size which is one of the important parameters for designing arrays of

MRAM was studied. In addition the characterization of the magnetic materials used were

investigated, the oxidation methods which were used to prepare the TMR devices, the barrier

characterization, the electrical and magnetic properties at different oxidation methods and

different temperatures, realiability and breakdown properties of the TMR devices, and MRAM

array characterization based on TMR elements were investigated.

From the switching characteristics and magnetization behavior of the magnetic materials

used it is shown that:

[a]- For Co single layer of 15nm thickness a trapped magnetization vortices appear in

structures with low aspect ratios (length/width) of 1.5 and 2 which disappeared at aspect ratio >

3. It was found that the magnetization vortices of these patterned elements are also strongly

dependent on the width of the element. With narrower line-width the presence of magnetization

vortices is more pronounced. The switching field and magnetic moment of the arrays which were

measured by (SQUID) magnetometer at different temperatures (4, 300 and 400 K) show

significant temperature dependence. The switching field of the elements increased as the

temperature decreased while at the same temperature it increased with increasing the aspect ratio.

The micromagnetic simulation results show an agreement with the experimental results.

[b]- For NiFe single layer the switching characteristics of the NiFe as a function of the

film thickness (2, 3 and 4 nm) showed that with increasing film thickness the switching field

increases as the total magnetic moment increases. The switching field increased with increasing

the aspect ratio for the same thickness.

[c]- The switching behavior of magnetic-tunnel junctions consisting of trilayers NiFe

20nm/ Al2O3 0.8nm/ Co 15nm shows a relation with the size of the elements. The switching field

for the soft layer and the hard layer increases with increasing the aspect ratio, this is due to the

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83

dipole-dipole interaction which is very strong at small aspect ratio and also due to the

magnetization vortex which appear at small aspect ratio for Co single layer and disappeared at

large aspect ratio.

The barrier thickness plays an important role in the preparation of the tunnel

magnetoresistance devices. For optimal reproducibility on wafer level, the barrier thickness must

be controlled on an atomic level. Important barrier properties such as the thickness and the

potential height can be obtained fairly well from the results of the electric transport

measurements using the theoretical models of the spin dependent transport in the magnetic tunnel

junctions.

The barrier characterization results were:

[a] The XPS results showed that, 1nm thin aluminum film is sufficient to cover the bottom

electrode, which then leads to large magnetoresistance.

[b] The MOKE results showed a weak coupling between the two electrodes at 1.3nm

Al2O3 which indicate that the barrier is pinhole free.

The tunnel magnetoresistive junctions which were prepared with different oxidation

methods like natural oxidation in air (ex-situ) or natural oxidation in pure oxygen (in-situ) or

ultraviolet radiation assisted oxidation in oxygen has the following characterization:

[a] The oxidation times for UV are conveniently short, less than 20 min is sufficient to

completely oxidize a thin film of 1.3 nm Al.

[b] The resistance values of the junctions change from one oxidation method to the other for

the same barrier thickness and the same junction area.

[c] The largest MR ratio, which was obtained with the UV radiation assisted oxidation

method, is 20% at room temperature.

[d] The UV assisted process is much more reliable than all the others. At UV radiation

assisted oxidation in oxygen 90% from the TMR junctions show MR values at least 10% at

300 K, which is much higher than for natural oxidation in-situ and ex-situ.

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84

[e] No changes of the resistance or the MR ratios were observed even after two years with

junctions prepared with UV radiation assisted oxidation in oxygen while the junctions

prepared with natural oxidation show intrinsic breakdown i.e. short circuit after few weeks.

The tunnel magnetoresistance showed a bias and temperature dependence irrespective of

the applied oxidation method. The reasons for temperature and bias dependence is partly given by

Zhang et al. [92] who put forward a theory which explains the dependence of the conductivities

for spin up and spin down and thus the magnetoresistance effect as function of temperature and

bias with magnon generation absorption in the electrodes.

The other explanation that is due to the presence of the trap states in the barrier at which the

electron doesn’t tunnel directly from electrode to the other in one step but the tunneling occurs in

two or even more steps. In the first step the electron tunnels from the first electrode to one of the

trap states in the barrier and in the second step the electron tunnels from the trap state to the

second electrode. In this mechanism the electrons lose some of their spin information. The

number of such trap states depends on the quality of the barrier, which changed from one

oxidation method to the other i.e. i.e. with controlling the quality of the barrier the bias and

temperature dependence can be neglected.

The dielectric breakdown of patterned magnetic tunnel junctions was also studied. For

junctions with a barrier of 1.3 nm and thinner, an almost immediate breakdown was observed

when the applied voltage approached 2.1 V. For junctions, which have large areas the probability

to breakdown is much higher than for junctions with smaller areas. Our conclusion for this is the

probability to find a weak point in the oxide is much higher at large area than at smaller areas. At

the moment of breakdown a single short circuit is created in the barrier, leading to a low

resistance path in the barrier and a strong decrease in the magnetoresistance ratio. The time

dependent breakdown probability is voltage dependent. These investigations indicate that ultra

small junctions have the most promising features for applications of TMR as storage elements.

From MRAM characterization it is concluded that the MRAM arrays based on tunnel magnetic

junctions can quite be used as nonvolatile memory cells, which can read and write the digital

information depending on Si technology, but it needs long time for research before starting to use

in a fabrication scale.

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85

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Zusammenfassung

In der Wissenschaft und der Technologie, sowie im täglichen Leben spielt der

Magnetismus eine wichtige Rolle. Die Anwendungen des Magnetismus in der gegenwärtigen

Technologiegesellschaft kann in der Fähigkeit der magnetischen

Materialien gesehen werden, Informationen zu speichern. Dieses umfaßt allgemein zu nutzende

und verwendete Geräte, z.B.: Floppy-Disketten, Festplatten, Optische Speicherelemente und

magnetische Streifen auf Kreditkarten. Neue Fortschritte in diesem Bereich sind in vielerlei

Hinsicht durch die Verbesserungen in den magnetischen Materialien voran getrieben worden.

Eine spezielle Kategorie vielversprechender künstlich gebildeter Materialien sind die

sogenannten magnetischen Multilayers oder Superlattices, bestehend abwechselnd aus

magnetischen Schichten mit unterschiedlichen Eigenschaften und einigen nicht magnetischen

metallischen Schichten (Fe/Cr) mit typischen Dicken in Nanometerbereich. Es wurde festgestellt,

daß große Änderungen des Widerstandes eintraten, wenn ein magnetisches Feld angelegt wurde.

Dieser Effekt ist das Resultat der gegenseitigen Ausrichtung der Magnetisierungs- Richtungen

der magnetischen Schichten als Funktion des angelegten Feldes. Da die Elektronenstreuung in

solchen Systemen spinabhängig ist, entsteht der beobachtete Effekt des Magnetowiderstandes.

Die Entdeckung dieses Effektes führte zu einer enormen Zunahme der Bemühung der

wissenschaftlichen Forschung an überlagerten magnetischen Filmen und damit zur Entdeckung

des Riesenmagnetowiderstand (GMR) Effektes bei vielen weiteren Kombinationen abwechselnd

magnetischer/nicht magnetischer Schichtsysteme (z.B. Co/Cu/NiFe). Wesentlich für den

Riesenmagnetowiderstandseffekt ist die Fähigkeit von zwei ferromagnetischen Schichten, ihre

Magnetisierungsrichtung separat zu drehen. Dieser Effekt existiert ungeachtet der Richtung in der

ein Strom durch das Schichtsytem fließt. Es kann zwischen die zwei Konfigurationen

unterschieden werden: Einerseits kann der Strom parallel zur des Schichtfläche fließen (CIP),

anderseits senkrecht zu des Schichtfläche (CPP), wobei die Elektronen durch alle Schichten der

mehrschichtigen Struktur fließen. Der gefundene Wert für die CPP Konfiguration war sogar

größer als der in der CIP Konfiguration. Vor kurzem richtete sich die Entwicklung der

Magnetowiderstands-Bauelemente auf die Tunnelkontakte, die aus zwei ferromagnetischen

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Metallschichten bestehen, welche durch eine sehr dünne Isolierungschicht getrennet sind. Der

Elektronentransport erfolget senkrecht zur Fläche der Schichten und wird gemessen, indem an

den Elektroden über die Isolatorschicht eine Spannung anlegt wird. Der Magnetowiderstand

basiert auf einer spinabhängigen Tunnelwahrscheinlichkeit, bedingt durch eine energetische

Aufspaltung der Energiebänder, verursacht durch den Spin der Elektronen. Dieser

Magnetwiderstand ist direkt proportional zu den Polarisationsstärken der Elektronen an den zwei

Isolator / Metallgrenzflächen, die zu der Polarisation im Inneren der zwei magnetischen Filme

unterschiedlich sein können.

Gegenwärtig bekommt der Tunnelmagnet-Widerstand (TMR) eine größere Bedeutung, da

er ihn in Mikro- und Submikro- Dimensionen strukturiert werden kann. Dies wird in der

Hochtechnologieindustrie bei zwei Hauptanwendungen, einerseits als Mittel der Speicherung von

Information und anderseits als Sensor genutzt, um magnetisch gespeicherte Informationen

auslesen zu können.

In dieser Arbeit werden die Resultate der Experimente zum spinabhängigen Transport in

den strukturierten mehrschichtigen Systemen dargestellt.

Der erste Teil der Arbeit behandelt die Charakterisierung der magnetischen Materialien mit

unterschiedlichen Techniken. Im zweiten Teil werden Auswirkungen auf die Herstellung und die

Entwicklung der Tunnelmagnetwiderstands- Einheiten mit unterschiedlichen

Oxidationsmethoden dargestellt. Diese sind die natürliche Oxidation, die thermische Oxidation

und die von ultravioletter Strahlung unterstützte Oxidation in Sauerstoff. In dieser Arbeit werden

die Resultate der ultravioletten Oxidationsmethode ausgiebig diskutiert, weil sie weltweit neu

sind.

Der dritte Teil, behandelt die zukünftige Anwendbarkeit der Magnetowiderstandkontakte

als RAMS (MRAM). Bevor jedoch diese Strukturen in den kommerziellen Bereich gelangen

können, ist ein gutes fundamentales Verständnis ihrer Eigenschaften und ihrer Grenzen

erforderlich. Das stellt das Ziel der Forschung dar, die in dieser Arbeit beschrieben wird.

Die Tunnelwahrscheinlichkeit eines Elektrons durch eine Isolatorbarriere ist abhängig von der

Elektronendichte. Die Dichte am Fermi- Niveau ist ungleich für “Spin-oben“ und “Spin-unten“

Elektronen. Dies ist das Resultat der Austauschwechselwirkung in den magnetischen Schichten.

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Die Tunnelwahrscheinlichkeit der Spin-oben und Spin-unten Elektronen ist mithin verschieden.

Das Resultat ist ein spinpolarisiertes Tunneln, vorausgesetzt, es gibt keine Spinflips. Die

wirksame Dichte der Zustände für die tunnelnden Elektronen mit Spin-oben und Spin-unten in

beiden Elektroden, D1 oben, D1 unten und D2 oben, D2 unten, werden in Abbildung (1) gezeigt.

Der Tunnelstrom für die parallele und antiparallele Magnetisierung kann ausgedrückt werden

durch:

Ip α D1↑ D2 ↑ +D1 ↓ D2 ↓

Ia α D1↑ D2 ↓ + D1 ↓ D2 ↑

Die wirksamen D↑ und die D↓ sind nicht die realen Dichten der Zustände in den Ferromagneten,

weil die tunnelnden Elektronen durch die Grenzfläche zwischen Elektrode und Isolator

beeinflußt werden. Die Asymmetrie in der wirksamen Dichte der Zustände der Auf- und Ab-

Elektronen wird durch die Polarisation P an den Grenzflächen beschrieben:

P= (D↑ – D↓)/ (D↑ + D↓)

Das Magnetwiderstands-Verhältnis ist damit:

∆R/R= 2P1P2/ (1-P1P2)

P1 und P2 sind die Polarisation der tunnelnden Elektronen an der ersten und zweiten Elektrode.

Das TMR-Verhältnis kann prinzipiell unendlich groß werden, wenn beide Materialien eine 100 %

vollständige Polarisation aufweisen.

Abbildung (1): Schema des spinabhängigen Tunnels: Die ähnliche Dichte der Zustände für eine paralleleAusrichtung der Magnetisierungs- Richtung der ferromagnetischen Schichten (obere Abbildung) führt zu einergrößeren Leitfähigkeit, verglichen mit der antiparallelen Ausrichtung.

Eine große Anzahl der Proben wurde mit elektrischen und magnetischen Messtechniken

vorbereitet und characterisiert. Die benutzten Substrate sind thermisch oxidierte

e-EF

M1 M2

EF

M1 M2

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93

Silizium- Wafer (100), da diese (vor der Oxidation) eine geringe Oberflächenrauhigkeit besitzen.

Die typische Oxiddicke schwankt zwischen 50 und 150 nm. Diese Oxidschicht ist für die

elektrische Isolation zwischen den Kontakten auf dem gleichen Wafer erforderlich. Für diese

Oxidschichten erhöht sich die Oberflächenrauheit etwas. Sie ist zur Oxiddicke proportional. Die

untere magnetische Schicht ist aus unterschiedlichen Gründen die kritischste:

(a) Die Oberflächen-rauigkeit dieser Schicht muß klein sein.

(b) Für die Strukturierung ist es erforderlich, daß die Dicke dieser Schicht mindestens 10 nm

beträgt.

(c) Das aufgebrachte Aluminium sollte eine benetzende Eigenschaft auf der Oberfläche haben.

Für die Aufbringung der drei Schichten von Co/Al2O3/NiFe wurde die Methode des Magnetron-

Sputterns verwendet. Mit den drei Targets: Kobalt, Aluminium und Permalloy nacheinander

wurden die Schichten gesputtert, ohne das Vakuum zu brechen. Der Basisdruck betrug 2 x 10-6

mbar. Während der kurzen Sputterzeit muß das Substrat mit einem wassergekühlten

Substrathalter auf 22 ° C gehalten werden. Nach dem Sputtern des Co-Films von 10-20 nm folgt

ein dünner

Aluminum-Film von 0,8-2 nm. Die Aluminiumschicht wurde entweder an Luft bei

Raumtemperatur (ex-situ), d.h. außerhalb der Sputteranlage oder in-situ, d. h. innerhalb der

Sputteranlage in Sauerstoff, ebenfalls bei Raumtemperatur oxidiert, auch durch thermische

Oxidation (in-situ), bei einer Temperatur von 60-90° C oder durch Plasmaoxidation (in-situ), und

durch ultraviolette Strahlung unterstützte Oxidation in einer Sauerstoffatmosphäre (in-situ)

oxidiert.

Schließlich wird ein Dünnfilm von NiFe aufgesputtert. Die weitere Bearbeitung der

Wafer mit diesen drei Schichten erfolgt in einem Reinraumlabor.

Mit optischer Lithographie werden die Querschnitte der Kontakte definiert und

ausschließend geätzt, indem eine Ionenstrahlätzung (IBE) durchführt wird. Eine Argonenergie

von 250 eV wurde verwendet, um die Seitenwandbeschädigung so klein wie möglich zu halten.

Die Ätzung wird gestoppt, sobald die Co-Schicht erscheint. Nach einem weiteren

lithographischen Prozeß wird die Struktur definiert und mit Ionenstrahl geätzt. Dann wird der

strukturierte Wafer mit einer 150 nm dicken SiO2- Schicht bedeckt, welche die Zuleitungen

isoliert. Danach erfolgt eine 300 nm dicke Gold-Aufdampfung auf der Oberseite nach einem

weiteren lithographischen Prozeß, womit die Zuleitungen erzeugt werden. Die elektrischen

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Messungen erfolgten dann an vier Punkten zwischen Goldkontaktauflagen und der

Grundelektrode. Systematisch wurden die Kontaktquerschnitte innerhalb der Grenzen von 1 bis

600 µm2 verändert.

Zur Vorbereitung der Tunnelmagnetwiderstands-Elemente wurden die Materialien der

zwei Elektroden und der Barriere mit unterschiedlichen Techniken untersucht. Die

Schaltungseigenschaften für die verwendeten Materialien (NiFe, Co und Co/Al2O3/NiFe) sind

wichtig, wenn man Reihen von MRAMs und Sensoren des magnetischen Feldes entwirft. Diese

Parameter wurden mit verschiedenen Techniken, wie alternativem Gradientmagnetometer (AGM)

bei Raumtemperatur und SQUID bei unterschiedlichen Temperaturen untersucht. Die Proben für

AGM und SQUID wurden mittels Elektronenstrahllithographie vorbereitet (Substrat 3.5 mm x

3.5 mm) und in Arrays von 107 Elementen mit Elementgrößen von 100 x 150 nm bis 6000 x 2400

nm strukturiert.

Der Abstand zwischen den Elementen schwankt zwischen 0,5 und 5 µm, abhängig von

der Geometrie der Elemente und der Dicke des Materials. Die Morphologiedes Strukturen wurde

mit dem RasterKraftmikroskop (AFM) und das Magnetisierungsverhalten mit dem

Magnetokraftmikroskopie (MFM) untersucht. Die Abmessungen der Proben sind die gleichen

wie bei AGM und SQUID. Ein Problem ist die Schaltcharakteristik, die zwischen der

magnetischen Kopplung der Elektroden besteht, welche mit dem Magnetooptischen Kerreffekt

(MOKE) untersucht wurde. Die Größe der Proben waren 10 mm x 10 mm. Sie wurden mit

MOKE nach dem Sputtern und vor der Lithographie gemessen.

Hochwertige dünne Barrieren haben keine Löcher, die zu Kurzschlüssen führen können.

Solche Löcher können mit der Röntgenstrahl-Photoemissions-Spektroskopie (XPS) ermittelt

werden. Nach dem Sputtern der Kobalt-Grundelektrode wurde eine sehr dünne

Aluminiumschicht auf gesputtert und mit XPS untersucht, ob sich während der Oxidation des

Aluminiums ein Oxid der Grundelektrode gebildet hat.

Die Schalteigenschaften und die Magnetisierungsstrukturen von 15 nm dicken Kobalt-

Einzelschichten, bestehend aus Arrays unterschiedlicher Breiten (100 nm, 200 nm und 600 nm)

wurden untersucht. Diese Arrays wurden durch Elektronenstrahllithographie mit

unterschiedlichem Längenverhältnis (Länge / Breite = 1.5, 2, 3 und 4) geschrieben. Der Effekt

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der Breite und das Längen-zu-Breite-Verhältnis wurden systematisch mit AGM, AFM, MFM

und mit der mikromagnetischen Simulation, die auf der Lösung der Landau-Lifshitz-Gilbert-

Gleichung basiert, studiert. AGM und MFM zeigen, daß aufgefangene Magnetisierungs-

Turbulenzen in den Strukturen mit niedrigen Längenverhältnissen L/W = 1.5 und 2, aber nicht in

den Strukturen mit hohem Längenverhältnis L/W>3 erscheinen. Es wurde gefunden, daß die

Magnetisierungs- Turbulenzen dieser Elemente stärker von der Breite des Elements abhängig

sind. Bei kleinerer Linienbreite ist das Vorhandensein der Magnetisierungs-Turbulenzen offenbar

schon vorhanden. Das Schaltfeld und das magnetische Moment der Arrays wurden mit einem

DC-SQUID-Magnetometer bei unterschiedlichen Temperaturen gemessen (4K, 300K, 400K).

Die Schaltfelder zeigen eine Temperaturabhängigkeit. Dieses Feld der Elemente wird erhöht mit

dem gleichen Längenverhältnis (Länge / Breite), während die Temperatur verringert wird. Ein

magnetisches Kraftmikroskop des Typs Digital Instruments 3000 (MFM) wurde benutzt, um

magnetische Bilder der remanenten Zustände bei Raumtemperatur zu erhalten. Die

mikromagnetischen Simulationsresultate stimmen mit den experimentellen Resultaten gut

überein.

Die Schalteigenschaften und des Magnetisierungsverhalten von NiFe wurden als

Funktion der Filmschichtdicke studiert. Sie zeigen, daß sich bei Zunahme der Filmdicke das

Schaltfeld erhöht, da sich das gesamtmagnetische Moment erhöht und es schwer ist, die

Magnetisierung zu drehen, wenn die (Dimensionen) Abmaße eines Elements sehr klein werden.

Das Schaltverhalten von magnetischen Tunnelkontakten, bestehend aus Trilayern NiFe 20 nm /

AlOx 0.8nm / Co 15nm verschiedener Längenverhältnisse (Länge zu Breite)=1.5, 2, 3, 4 und bei

unterschiedlichen Breiten von 100 nm, 200 nm und 600 nm wurde untersucht. Die beobachteten

Resultate zeigen die Relation zwischen Schaltfeld und Größe der Elemente. Das Schaltfeld für

die weiche magnetische Schicht erhöht sich bei der Zunahme des Längenverhältnisses, während

das Schaltungsfeld der harten magnetischen Schicht bei Zunahme des Längen-Breiten-

Verhältnisses verringert wird. Mit dem Verringern des Verhältnisses (L/W) wird die

magnetostatische Kopplung zwischen den zwei ferromagnetischen Schichten erhöht.

Die Barrierendicke spielt eine wichtige Rolle für die Vorbereitung der

Tunnelmagnetwiderstands-Einheiten. Für optimale Reproduzierbarkeit auf dem Wafer muß die

Barrierendicke bis auf eine Atomlage genau gesteuert werden. Wichtige Barriereneigenschaften,

wie die effektive Dicke und Höhe, können aus den elektrischen Transportmessung mit den

theoretischen Modellen zum Tunneln ermittelt werden.

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96

Durch die XPS-Messung wurde festgestellt, daß ein 1 nm dicker Aluminiumfilm genügt,

um die Grundelektrode zu bedecken.

Die Tunnel-Magnetwiderstände werden mit unterschiedlichen Oxidationsmethoden,

natürliche Oxidation in Luft (ex-situ), natürliche Oxidation im reinen Sauerstoff (in-situ) oder

UV unterstützte Oxidation in Sauerstoff hergestellt.

Diese haben folgende Eigenschaften:

(A) - Die Oxidationszeiten bei des UV Methode sind üblicherweise kurz. Bereits weniger als 20

Minuten vermutlichgenügen um einen Film von 1.3 nm Aluminium vollständig zu oxidieren.

(B) - Die Widerstandswerte der Kontakte unterscheiden sich von einer Oxidationsmethode zur

anderen bei gleicher Barrierendicke und gleicher Kontaktgeometrie.

(C) - Der Widerstand der Kontakte ist umgekehrt proportional zur Kontaktfläche. Dieses zeigt

die Homogenität des Prozesses.

(D) - Mit den präparierten Proben wurde der größte Magnetowiderstand (MR) von 20% mit der

UV-Oxidations-Methode bei Raumtemperatur erreicht.

Der UV unterstützte Prozeß ist viel zuverlässiger. Ein MR- Wert von mindestens 10% bei 300

Kelvin wurde erreicht, und dies mit einer Ausbeute von 90%, die wesentlich höher ist als bei

natürlicher Oxidation, in-situ oder ex-situ. Es wurden selbst nach zwei Jahren keine Änderungen

des Widerstandes oder des Magnet-Widerstands-Wertes mit dieser UV-Oxidation beobachtet.

Dagegen zeigen die Kontakte, mit natürlicher Oxidation hergestellt, nach wenigen Wochen ein

intrinsisches “Breakdown”.

Der Tunnelmagnetwiderstand zeigte eine Vorspannungs- und Temperaturabhängigkeit,

die bei unterschiedlichen Oxidationsmethoden verschieden groß war.

Dafür gibt es mindestens zwei Gründe. Nach Zhang[92] ist die inelastische magnetische

Elektronenstreuung nicht zu vernachlässigen. Die Erzeugung und Vernichtung von Magnonen ist

mit einem Spinflip verbunden. Durch solche Prozesse verlieren die tunnelnden Elektronen ihr

Spingedächtnis, d. h. es gibt einen Elektronentransport durch die Barriere, der nicht mehr von der

gegenseitigen Richtung der Magnetisierungen abhängt. Da dieser Beitrag temperaturabhängig ist

und auch spannungsabhängig sein wird, kann damit die Beobachtung zumindest qualitativ erklärt

werden.

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Für nicht perfekte Barrieren, d. h. für Barrieren mit Gitterdefekten oder größeren Baufehlern

öffnet sich ein weiterer Kanal für den Elektronentransport von Defekt zu Defekt. Solche

Hopping- und Mikrotunnelprozesse sind mit erhöhter Wahrscheinlichkeit mit

Spinrichtungsänderungen gekoppelt und können dadurch den Magnetwiderstandseffekt

verringern. Die Verringerung ist um so größer, je mehr Transportkanäle mit Spinflip es gibt. Die

Experimente zeigen, daß Defekte in den Isolatorschicht die Eigenschaften des magnetischen

Tunnelkontakts erheblich beeinflussen.

Der elektrische Durchschlag von gesputterten magnetischen Tunnelkontakten, die UV

unterstützt in O2 oxidiert wurden, war Gegenstand weiterer Untersuchungen. Solche Werte

stellen interessante technologische Spezifikationen für eine Realisierung von MRAMs und von

magnetischen Leseköpfen dar.

In den Kontakten mit einer Barriere von 1.3 nm und kleiner wird fast ein sofortiger

Zusammenbruch beobachtet, wenn die angelegte Spannung annähernd 2.1 V beträgt. Bei

Kontakten mit größerer Fläche ist die Wahrscheinlichkeit zum Durchbruch größer als bei

kleineren. Die Wahrscheinlichkeit, einen schwachen Punkt im Oxid zu finden, ist größer bei

großen als bei kleinen Querschnitten. Ein Durchbruch erzeugt einen Defekt in der Barriere, führt

zu einem niedrigen Widerstand in der Barriere und zu einer starken Abnahme des

Tunnelwiderstands-Verhältnisses. Die Wahrscheinlichkeit für einen Durchbruch ist von der

angelegten Spannung abhängig.

Für zukünftige Anwendungen werden extrem kleine Kontakte als MRAMs gefordert.

Es wird festgestellt, daß für die Anwendung der Tunnelmagnetkontakte die bisher

benutzte MRAM-Matrix als Permanentspeicher zum Lesen und Schreiben genutzt werden kann.

Allerdings ist noch viel Entwicklungsarbeit nötig, bevor eine Massenfertigung beginnen und eine

Markteinführung stattfinden kann.

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Acknowledgment

I am very grateful to Prof. J. Schelten, Institute for Thin Film and Ion Technology,

Forschungszentrum Juelich, Germany, for proposing the point of research and his supervision of

the present work. His continuous guidance, advice and valuable comments throughout the

discussion of the results are much appreciated.

The author would like to thank Prof. P. Grünberg, for his helpful discussion, IFF,

Forschungszentrum Juelich, Germany,

The author wishes to express his deep gratitude to H. Boeve, Dr. J.De Boek and Prof. G.

Borghs, IMEC, Leaven, Belgium for their cooperation during the measurements of the MRAM

carried out there.

My deep gratitude to Dr. S. Taherani, Dr. H. Gronkin, Dr. J. Shi and J. Jansesk, MRAM

group, Motorola labs. Physical Science Research Lab., AZ, USA, for their cooperation and kind

support during my stay at Motorola. Their help to finish the research work “Characterization and

micromagnetic modeling of different magnetic materials patterned at nonmeter-scale arrays” are

much appreciated.

The author is very grateful to the scientific and technical staff of the lithography

department: Dr. R. Lehmann, Dr. M. Pabst, Dr. A. van der Hart, F. Schroteler, J. Zilikens, A.

Pracht, M. Noon, P. Bochum, U. Kurz, Dr. M. Siegel, A. Steffen, Institute for Thin Film and Ion

Technology, Forschungszentrum Juelich, Germany, for their kind help throughout this work.

The author would like to thank Dr. W. Oepts and Prof. R. Coehoorn, Philips Research

Labs., Eindhoven, Netherlands for their helpful discussion of the breakdown measurements.

Finally, I am thankful to the Egyptian Government, “Edaret El-Behsat” for offering me

the chance to have one of its fellowships to finish the present work.