Magnetic Race- Track Memory: Current Induced Domain...

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Metal Spintronics | Stuart Parkin © 2005 IBM Corporation

Magnetic Race-Track Memory:

Current Induced Domain Wall

Motion!

[email protected]

Stuart ParkinIBM Fellow

IBM Almaden Research CenterSan Jose, California

IBM Research


Two main types of digital data storage

Random access memoryHierarchy of memoriesSRAM- fast but expensiveDRAM- less fast and less expensiveHighly reliable but volatileFlash: non-volatile, less expensive, very slow, limited endurance

Hard disk drivesMassive storageNon-volatileVery cheap Very slowLess reliable!

Digital data storage

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MRAM (Magnetic Random Access Memory)

non-volatilehigh performancecheap compared to

other solid state memoriesexpensive compared to

hard disk drives

VAM2

JAMT

MA

M1

V1

CAPC

Challenge: can we build a solid state device with the same cost as a hard disk drive but the performance and reliability of solid state memory?

Magnetic tunnel junction storage elements

IBM-IFX 16 MbitMRAM chip

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Emerging memory technologies

Flow

Spin

Quantity FRAM

PCRAM

MRAM

PFRAM SiC Bipolar

PMC

Molecular

Polymer Perovskite

NanoX’tal

3DROM

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Storage-Class Memory: Domain-Wall Magnetic Shift RegisterPhilosophy

Want a solid-state memory with no moving parts which is very cheap and of moderate to high performanceMain approaches

Make extremely small cellsRequires significant engineering developmentsCurrent roadmaps suggest that f<30nm will be possible within 5 years, thus

making this approach extremely challengingAccess multiple bits from one set of logic

Similar philosophy used in conventional storage drives and in millipedeHowever we want a solid state memory with no moving parts

Recent developments in magnetic materials makes this approach viable and attractive by storing information in domain walls (spatially varying order parameter in homogeneous material)

Lots of new science: Spin currents and torque, domain wall fringing fields

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Vortex and transverse domain wall structures

Increasing width, thickness

Transverse wall

R.D. McMichael and M.J. Donahue, IEEE Trans. Magn. 33, 4167 (1997)

Vortex wall

Magnetostatic energy dominates:(shape anisotropy)

• magnetic moments along the wire• head-to-head domain walls

Micromagnetic simulationsLLG Micromagnetics SimulatorMike ScheinfeinCell size 5 x 5 x 5 nm3

width 250nm thickness 10 nm

width 140nm thickness 5 nm

Larger structures : more complex DW structures (double vortex,…)

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∂ ∂≠ =

∂ ∂0, 0t t

θ φCurrent torque on DW

(Magnetic field pressure on DW, )∂ ∂≠ ≠

∂ ∂0, 0t t

θ φ

Massless motion!!

Domain wall motion

From Sadamichi Maekawa

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Theory of current-driven domain wall motionAdiabatic vs non adiabatic spin torque

electron spins follows local magnetization (wide wall limit)

electron spins lags behind magnetization (narrow wall limit)

The adiabatic spin-torque is like a damping term (dissipative)the critical current in intrinsic (related to DW properties)no motion occurs below the critical currentturbulent motion occurs above the critical current – high velocity

The non-adiabatic spin-torque is like a magnetic field (precessional)DW velocity is non-zero for ideal wiresthe critical current is related to defects (roughness)

DW velocity vs current

damping α=0.01DW width 50 nmHp=1000 Oe

Spin torque amplitude : u (1 m/s ≡ J=106 A/cm2)Non-adiabatic contribution ~ β u

0

200

400

600

800

1000

0 200 400 600 800 1000

Vel

ocity

(m/s

)

u (m/s)

β=0β=α/5β=αβ=5α

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DW motion under field and current

Magnetic fieldH=-200 Oe

Negative currentPositive current

time

time

Field-driven dynamics Current-driven dynamics

1280 x 140 x 15 nm3

Strong damping a=1

Field-driven dynamics : neighboring walls move in opposite directionsCurrent driven dynamics : neighboring walls move in the same direction

H= 0

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Magnetic Race-track Memory

Current pulses move domains along “racetrack” shift registerTMR sensor to read bit patternSpecial current pulse-driven element to re-write a bit

A novel three-dimensional spintronic storage class memory The capacity of a hard disk drive butthe reliability and performance of solid state memory- a disruptive technology based on recent developments in spintronicmaterials and physics

Parkin, US patents 6834005, 6898132, 6920062

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Magnetic Shift Register Memory

Writing a bit – current pulse on special write element

Parkin, US patents 6834005, 6898132, 6920062

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Magnetic Race-Track Memory: Domain-Wall Magnetic Shift Register

Information stored as domain walls in vertical “race track”Reading and writing carried out along bottom of race trackElectronics built under race track using conventional CMOSDomains moved around track using nanosecond long pulses of current

- Data stored in the third dimension in tall columns of magnetic material

- Domains “race” around track for reading and writing

- 10 to 100 times the storage capacity of conventional solid state memory

- Could displace flash memory and hard disk drives for many applications

Alternating layers of two ferromagnetic

materials to pin domain walls

domain wall

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Domain wall positions in race-track pinned by “notches” in walls of magnetic columns

Magnetic Shift Register Concept

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Shift current pushes domains through stack

Write Element Read Element

Nanosecond long current pulses push domain walls around race-trackdue to a spin torque from transfer of spin angular momentum

Writing device

Reading device

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Magnetic race-tracks can be connected in seriesMany other configurations possible

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DRAM trench

Top

Mid

Bottom

DRAM trench: ~10 μm tall0.09 μm wide

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Fabrication of race-track- prototype race-tracks under development- trench with notches demonstrated- plating with magnetic material a major challenge

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Magnetic fringing field from moving domain wall writes bits

Domains in racetrack

form part of magnetic tunneling junction

Writing bits into race-track

Reading bits in race-track

Current moves domain wall in nearby wire

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Writing with Domain Wall Fringing Fields: Simulation

Scheme of the experiment: (a) Small ferromagnetic (Py) element (10 nm thick ellipse) is placed 10 nm above Co stripe (500 nm long, 100 nm wide, and 10 nm thick); (b) Bx component of the domain wall fringing field as a function of x 10 nm above the stripe; (c,d) Domain color maps (top view) illustrate that sweeping a domain wall across the stripe results in the reversal of the Py magnetization: (b) –before, and (c) – after the reversal. The magnetization direction is color coded to the color wheel.

(a)

(b)

(c)

(d)

-50

0

50

B x (m

T)

500 nm

100

nm

10 nm10 nm

(a)

(b)

(c)

(d)

-50

0

50

B x (m

T)

500 nm

100

nm

10 nm10 nm

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Micromagnetic Simulations of a Racetrack

Spacing between notches ~ 1.1 μmWire is smooth between the notches (no roughness)Notches are made from SEM images of real wires (slightly different from one another)

-30

-20

-10

0

10

20

30

0 5 10 15

I (m

A)

t (ns)

10.8 μm x 210 nm x 10nm

20mA = 109 A/cm2

+I

2.8ns pulse

4 domain walls, located next to one anotherbipolar pulses, 2.8 ns long

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© 2005 IBM CorporationMetal Spintronics | Stuart Parkin

Current induced domain wall motion in magnetic nanowires

Domain Wall (DW) race track memory

pin and depin DWs controllably

Current driven DW motion

Critical current to depin DW– - vs pinning strength– - vs DW structure

Current

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Fabrication of magnetic nano-wires

1) Sputter deposition (shadow mask)2) Coarse FIB

Py Py

SiOx

Focused Ion Beam (FIB)

3) Fine FIB

SiOx

Py

Py

Electron beam lithography

Pt

SiOxAu

Py

4 μm

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E-beam patterned magnetic nanowire to explore motion of DWs

Pointed end to preventDW nucleation

Pad for DW injectionNotches for DW trapping

gold contacts

4 μm

Wedged injector L-shaped structure

4 μm

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Probing DW structure using MFM

2.0µm

Vortex Domain Wall

Tranverse Domain Wall

2.0µm

AFM

MFM

AFM MFM M profile div(M)

Micromagnetic simulationsTopography

Domain Wall

300nm

Magnetic

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Structure of domain wall at a single notchStructure of DWs trapped at a notch

Micromagnetic simulations

300nm

AFM MFM M profile div(M)

Vortex wall

Transverse wall

Metastable states: Seven different structures at a given notch

MFM imaging micromag simulations (divM)

• The energetics of a domain wall trapped at a notch is complex

• Many metastable states are observed depending upon the history

• Magnetic states must be well controlled to ensure reproducibility

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Energy of metastable DW structures

1.4 10-9

1.6 10-9

-0.4 -0.3 -0.2 -0.1 0 0.1 0.2 0.3 0.4

Ener

gy (e

rg)

DW Position (μm)

vortex counter-clockwise

vortex clockwise vortex clockwise

vortex counter-clockwise

transverse

transverse

transverse

transverse

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Patterning permalloy nanowires

Pointed end to assist DW annihilation

Notches for DW trapping (pinning center)

- Ni81Fe19 (thickness 10 nm) blank film deposited on Si substrates- E-beam lithography- Gold/Rhodium contact pads

Au pads

w: 40-300nm

t: 10-40nm

4um

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Injecting DW into nanowire

1. Saturate magnetization of nanowire

M

Hsat

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2. Apply local magnetic field

- Local field created by passing current through contact wire

-1.5 -1.0 -0.5 0.0 0.5 1.0 1.5Position (um)

Local field (HI)

Total field

MI

HI

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M

IHI


2. Apply local and global magnetic field- If the total field is larger than the nucleation field

reversed domain created below the contact

-1.5 -1.0 -0.5 0.0 0.5 1.0 1.5Position (um)

Required nucleation field(~100 to 500 Oe) DW formed!

Hbias

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M


3. Apply global magnetic field

- global field (Hbias) applied by Helmholtz coil

reversed domain expands and DW propagates under Hbias

Hbias

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M


3. Apply global magnetic field

if Hbias is moderately small (i.e. smaller than the pinning field of the notch)DW pinned at the notch

Controlled nucleation and pinning of DW

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Probing the existence of DW in the nanowire

- Anisotropic Magneto-Resistance (AMR)

High resistance

Low resistance

R

- Resistance difference is proportional to the volume of transverse magnetizatoin

ΔR

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-0.35 -0.30 -0.25 -0.20 -0.15 -0.100

50

100

Cou

nts

ΔR (Ohms)

0 50 100 150-0.3

-0.2

-0.1

0.0

687.3

687.4

687.5

687.6

ΔR (O

hms)

Number Experiment

Resistance (O

hms)

Successive resistance measurement of DW states

Hsat

HbiasI

- meta-stable states at the notch

Hbias

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-0.30 -0.25 -0.20 -0.15

0

2

4

6

8

1042.9

31.3

19.8

8.4

6.1

5.0

0.8

3.9

2.8

Cou

nts

(a. u

.)

R(DW)-R(sat)

1.7

Meta-stable states at the notch

Hbias

Vortex wall

-0.23 ~ -0.25

Transverse wall

-0.16 ~ -0.21

t=10nm, w=200nm, notch depth=0.4w

DW at notch

DW in wire

Can create different DW states

R(DW)-R(Sat) calculated from simulation

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Depinning DW from a notch with magnetic field

M

Notch pinning potentialDW particle in a well

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Depinning from a notch with magnetic field

0.0

0.2

0.4

0.6

0.8

1.0

0 20 40 60 80

H (Oe)

Depinning P

robabilityH=0

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0.0

0.2

0.4

0.6

0.8

1.0

0 20 40 60 80

H (Oe)

Depinning P

robabilityH=20

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0.0

0.2

0.4

0.6

0.8

1.0

0 20 40 60 80

H (Oe)

Depinning P

robabilityH=40

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0.0

0.2

0.4

0.6

0.8

1.0

0 20 40 60 80

H (Oe)

Depinning P

robabilityH=43

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0.0

0.2

0.4

0.6

0.8

1.0

0 20 40 60 80

H (Oe)

Depinning P

robability

Hdepin=45 Oe

H=80

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0.0

0.2

0.4

0.6

0.8

1.0

0 20 40 60 80

H (Oe)

Depinning P

robabilityVortex wall

45 Oe

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0.0

0.2

0.4

0.6

0.8

1.0

0 20 40 60 80

H (Oe)

Depinning P

robability


Vortex wall

Transverse wall

45 Oe

62 Oe

Same notch

different pinning potential for different wall structures

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Depinning DW from a notch with current

Trap DW at the notch

(Apply magnetic field)

Pass voltage pulse

Study the presence of DW by AMR

R

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Pass voltage pulse


H

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Voltage pulse



Pass voltage pulse (~nanoseconds)


H

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Pass voltage pulse


R

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0 20 40 60 800

2

4

6

8S2245-w3c2 J83, 030805, 4ns, L2

J C (A

/cm

2 ) x10

8

Field (Oe)


Vortex wall

tpulse=4ns

- Fit to an analytical model

(LLG equation based)

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0 20 40 60 800

2

4

6

8S2245-w3c2 J83, 030805, 4ns, L2

J C (A

/cm

2 ) x10

8

Field (Oe)


Vortex wall

Transverse wall

5.2 x 108 (A/cm2)

tpulse=4ns

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Joule heating

Vin

oscillo

Vout

R

Time-resolved resistance measurement

0 20 40 60 80 1001.0

1.5

2.0

2.5

3.54V 3.15V 2.51V 1.99V 1.58V 1.00V 0.50V

050705

R(t)

/R(0

)

Time (ns)

S2245-w1c5 J41 short GSGB2

0 1 2 3 4 51.0

1.5

2.0

2.5 SiOx sub Si sub

R(V

)/R(0

)

Voltage (V)

S2245-w3 300nm/w4c3 300nm

@ 10ns

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Transverse wallVortex wall

0 20 40 60 800

2

4

6

8S2245-w3c2 J83, 030805, 4ns, L2

J C (A

/cm

2 ) x10

8

Field (Oe)

Without heating considered (Resistance at static level, low R high Jc)

5.2 x 108 (A/cm2)

tpulse=4ns

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0 20 40 60 800

2

4

6

8S2245-w3c2 J83, 030805, 4ns, L2

J C (A

/cm

2 ) x10

8

Field (Oe)



With heating considered (Resistance during pulse, high R low Jc)

tpulse=4ns

3.1 x 108 (A/cm2)

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0 20 40 60 800

2

4

6

8S2245-w3c2 J83, 030805, 4ns, L2

J C (A

/cm

2 ) x10

8

Field (Oe)



During heating

3.1 x 108

0 20 40 60 800

2

4

6

8S2245-w3c2 J83, 030805, 4ns, L2

J C (A

/cm

2 ) x10

8

Field (Oe)

5.2 x 108

Before heating (t<1ns)

JC for DW depinning before heating starts JC for DW depinning during or after heating

Same JC for the two states at low field

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Critical current vs pinning potential

0 20 40 60 80 100 120012345678

100nm 200nm 300nm

J C (A

/cm

2 ) x10

8

Pinning Field (Oe)

pulse duration 4ns

Theory1D model: Luc Thomas, Yaroslaw BazaliyG. Tatara et. al., Phys. Rev. Lett., 92, (2004) 086601-1

JC is independent of pinning potential depthas long as VPIN < 107 erg/cm3

0.0 0.1 0.2 0.3 0.4 0.5

0

20

40

60

80

100

120

0

2

4

6

8

wire width (nm) 300 200 100

Dep

inni

ng F

ield

(Oe)

Notch depth/wire width

Potential (erg/cm

3) x104

Atomic point contact type constraint

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Spintronics | Domain wall motion © 2005 IBM Corporation

x

yzΨ

q

Depinning of a domain wall : theoretical description

0

40

80

120

160

0 20 40 60 80

u c (m

/s)

H (Oe)

Calculatedα=0.01

One dimensional model for DW motion:

Discovery of two regimes for current-driven de-pinning - field-like behavior:

critical current depends upon field and pinning strength- current-driven depinning:

critical current essentially independent on field and pinning strengthExcellent agreement with experiments

Critical current vs magnetic field

Analytical:Field-like regime

Analytical:current-like regime

DW is described by two dynamic variables- position- momentum (tilt away from equilibrium direction)

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Seven consecutive identical current pulses in the MFM-Pulse: I=7mA J ~ 1 108 A/cm2 / 5 to 10 ms

e-

Domain Wall structure is modified by current pulses - both transverse and vortex walls are observed

Current induced change in DW structure

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Current induced Domain wall motion - summaryOscillatory depinning• DW motion driven by current pulses

Complex dependence upon pulse length and amplitudeOscillatory dependence upon pulse length Period between 3.5 and 7 ns

• ModelOscillatory motion of the DW within a pinning potential Oscillatory depinning reproduced - both with 1D model and micromagnetics

for both vortex and transverse wallsDepinning occurs when the pulse length in in sync with the DW oscillationsDW inertia : motion after the end of the pulse

Notch potential dependenceField driven DW depinning depends on DW statesCurrent driven DW depinning

weak dependence on DW statesweak dependence on pinning potential

important for applications

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Spintronics– new sensor and memory devices based on manipulating spin

polarized current spin-valve and MTJ devices

Hard disk drives>400 fold increase in storage capacity

Magnetic Random Access Memorypromises a solid state memory which is non-volatile, high

performance and cheap

Magnetic race-track memorypromises a novel data storage device with the capacity and

cost of a hard disk drive but with the performance and reliability of solid state memory

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Hype cycle for emerging technologies!

Source: Gerstner, August 2005

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SpinAps TeamMasamitsu Hayashi, Luc Thomas, Rai Moriya, Charles Rettner

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