A New Spin on Electronics-Spintronics-
Stuart WolfStuart WolfUniversity of VirginiaUniversity of Virginia
Presented atPresented atSPIN 08 October 11, 2008SPIN 08 October 11, 2008
Charlottesville, VACharlottesville, VA
SPIN 08 October 11, 2008
Beyond Conventional Electronics: Spintronics
Conventional Electronics Charge• Based on number of charges and their energy• Performance limited in speed and dissipation
Spintronics Spin• Based on direction of spin and spin coupling• Capable of much higher speed at very low power
SPIN 08 October 11, 2008
Outline of talk
Spin Transport Spintronic sensors for Magnetic Recording Magnetic Random Access Memory (MRAM) Spin Transfer Torque Random Access
Memory (STTRAM) Spin Torque Nano-Oscillators
SPIN 08 October 11, 2008
Spin Dependent Transport
- in all ferro and ferri-magnetic systems current is carried independently in two spin-channels
- conductivity in two channels can be very different
can be described by spin-dependent mean free paths or scattering times
current is spin-polarized
manipulate flow of spin polarized current useful sensors and memories
Energy
4s 3d
Co
0.35 0.35
3.35
Density of states
Spin-down
Spin-up
Neville Mott (1934)
SPIN 08 October 11, 2008
Two main types of digital data storage
Random access memory Hierarchy of memories SRAM- fast but expensive DRAM- less fast and less
expensive Highly reliable but volatile Flash: non-volatile, less
expensive, very slow, limited endurance
Hard disk drives Massive storage Non-volatile Very cheap Very slow Less reliable!
Digital data storage
ma s s s t o r a g e
SPIN 08 October 11, 2008
In a non-magnetic conductor, electrons scatter the same amount regardless of spin as current flows.
How much they scatter determines the resistance of the device.
Current in a metallic conductor
SPIN 08 October 11, 2008
In a Ferromagnetic conductor, however, electrons scatter differently depending on whether they are spin up or spin down.
In this case, the spin up electrons are scattered strongly while the spin down electrons are scattered only weakly.
Current in a ferromagnetic conductor
SPIN 08 October 11, 2008
If a non-magnetic conductor is sandwiched between two oppositely magnetized ferromagnetic layers, a number of electrons will scatter strongly when they try to cross between layers.
this gives higher resistance.
Spin-Dependent Scattering
If the ferromagnetic layers are magnetized in the same direction, far fewer electrons are strongly scattered and more current flows
This is measured as lower resistance
Useful for sensing magnetic fields or as a magnetic memory element
SPIN 08 October 11, 2008
To make a technologically useful device, a “pinning” layer is added to make it harder to change the magnetization of one layer than the other.
The pinning layer can be a simple layer of an antiferromagnetic material.
Spin-valve
Antiferromagnet
SPIN 08 October 11, 2008
400 H (Oe)-40
400
110
H (kOe)-40 H // [ 0 11]
spin-valve
multi-layer
Co95Fe5/Cu[110]
R/R~110% at RTField ~10,000 Oe
Py/Co/Cu/Co/Py
R/R~8-17% at RTField ~1 Oe NiFe + Co
nanolayer
NiFeCo nanolayerCuCo nanolayerNiFeFeMn
H(Oe)
H(kOe)[011]
10
MR(%)Giant Magnetoresistance (GMR)
NOBEL PRIZE !
Fert and Gruenberg
SPIN 08 October 11, 2008
Magnetic engineering at the atomic scale
Spin ValveGMR sensor
+ interface engineering
Spin ValveMagnetic Tunnel Junction
Ferromagnet
Ferromagnet
Spacer layerMetal or insulator
Anti- Ferromagnet
+ interface layer
+ interface layer
+ ArtificialAntiferromagnet
SPIN 08 October 11, 2008
Hard Disk Drive
N 5 8
3 m m
2 0 0 0 Å
C op p er
P erm allo y
A l O2 3
B ak ed p h o to -resist
W rite H ead
R ea d H ead
SPIN 08 October 11, 2008
Year
25% CAGR
~200%
60%
IBM Disk products
Lab demos
1st Thin Film Head
1st MR Head
1st GMR Head
IBM RAMAC (1st Hard Disk Drive)
100 Gb/in2
l
wtra ck w id th
b it len g th
d
A n iso tro p y k eep s th e m o m en t p o in tin g in th e d irec tio n o f
th e track
T h e tran s itio n w id th is a ffec ted b y b o th th e an iso tro p y
an d th e m ag n e tiza tio n
d
1 9 9 61 G b /in 2
1 9 9 86 G b /in 2
2 0 0 02 0 G b /in 2
.0 1 m
2 0 0 31 0 0 G b /in 2
l
wtra ck w id th
b it len g th
d
A n iso tro p y k eep s th e m o m en t p o in tin g in th e d irec tio n o f
th e track
T h e tran s itio n w id th is a ffec ted b y b o th th e an iso tro p y
an d th e m ag n e tiza tio n
d
1 9 9 61 G b /in 2
1 9 9 86 G b /in 2
2 0 0 02 0 G b /in 2
.0 1 m
2 0 0 31 0 0 G b /in 2
~30%
Hard Disk Drive areal density evolution
SPIN 08 October 11, 2008
SPIN 08 October 11, 2008 Seagate 2006
SPIN 08 October 11, 2008
Hard Disk Drive capacity shipped per year
100 Exabytesin ~2005
Year
Byte
s
Sh
ipp
ed
/ Y
ear
~100% CGR
SPIN 08 October 11, 2008
Spintronics Spin valve sensor Major impact on hard disk drive storage enabled >400x increase in storage capacity
since 1998 makes possible minaturization of hard disk
drives cell phones, PDA, MPEG players
makes possible access to all information
Spintronics Magnetic Tunnel Junction Major impact on random access memory? Just introduced to hard disk drive storage
SPIN 08 October 11, 2008
Spin Polarized Electron Tunneling: FM-I-FM
E E
NS NS
E F E F
E F E F
eV eV
FM FMFM FMI I
M MM M
Im a j Im a j
Im in Im in
1 2 1 2PI NN N N 21 1 2
AP N NI N N
Pinned FM
Free FM
1 2
1 2
2
1
with
AP P
P
R R PPMR
R PP
PN
N
N
N
Juliere (1975)
SPIN 08 October 11, 2008
CORRECTION: The legend in Figure 1(d) should read T = 295 K.
T=295 K
T=295 K
1975 1982
1995 1995
SPIN 08 October 11, 2008
SPIN 08 October 11, 2008
Exchange-Biased Magnetic Tunnel Junction (MTJ)
Non-Volatile Memory!Non-Volatile Memory!
FieldH=0
R/R
Ti, Ti/Pd or Ta/ Pt
Si, quartz, N58
Underlayer
Antiferromag
net
CoFe or NiFe/CoFe
Al2O3 Al2O3 CoFe/NiFe
Top lead
Substrate
Free ferromagnet
Pinned ferromagnet
Tunnel
barrier
Tunnel
barrier
Antiferromag
net Bottom electrode
MR
(%
)
Field (Oe)
-100-80 -60 -40 -20 0 20 40 60 80 1000
20
40
60
80
“”“”
“”“”
Magnetization
SPIN 08 October 11, 2008
History of development of MTJs
1994 1995 1996 1997 1998 1999 2000 2001 2002 2003 20040
50
100
150
200
250
T
unne
ling
Mag
neto
resi
stan
ce (
%)
Year
Al2O
3 results
MgO results in public domain IBM internal results with MgO
Record TMR –500%
SPIN 08 October 11, 2008
Conventional MRAM (1T-1MTJ)
u
B
K V
k T Thermal Stability Factor
SCALING PROBLEM Beyond 65 nm node!!!
Freescale
SPIN 08 October 11, 2008
Net change in S per e
Spin Torque Transfer Switching
STorque
t
Absorbed Angular Momentum Torque
Polarizing “fixed” layer (thick)
Active “free” layer (thin)
Spin polarized current generates torque on magnetization of free layer
2
S I
t e
2 2
IS N t t
e
Torque
MR ratio 0.5-5% Jc~107A/cm2
Katine et al, Phys. Rev. Lett. 84, (2000) 3149 .
SPIN 08 October 11, 2008
Switching current scales down with cell size
~ 6mA
~ 0.5mA
Albert et al, Appl Phys. Lett., 77 3809 (2000).
Grandis Inc
SPIN 08 October 11, 2008
“State of the Art” in STT-MTJ’s
Reductions in Jc ~ 9×105 A/cm2 and TMR ~ 73%
MgO increases
The improvement is over amorphous AlOx tunnel barriers that were initially studied and gave Jc ~ 8×106
A/cm2 and TMR ~ 42%J. Hayakawa, JJAP 41 (2005) L1267
Thermal Stability Factor Not Satisfied!!
SPIN 08 October 11, 2008
Current Scaling – MRAM vs STTRAM
SPIN 08 October 11, 2008
Challenges for STTRAM Switching Current Density Too High!
Small current needed to decrease size of MOSFET in series with MTJ cell (1T-1MTJ)
Small voltage across device needed to reduce probability of tunneling barrier breakdown
Need to reduce current density required to switch cell while achieving high MR%
Jc needs to be lowered to ~105 A/cm2
SPIN 08 October 11, 2008
New Materials Lower Switching Current Density
Spin Transfer Model
• Ms Saturation Magnetization Decrease• Gilbert damping parameter Decrease• Spin Transfer Efficiency Increase
0
2 2s F K sc
e M t H H MJ
[J.C. Slonczewski J. Magn Mater. 159 (1996) L1]
Also require: Anisotropy Energy / kT > 60 for 10 year retention
SPIN 08 October 11, 2008
New Materials Can we do better?
Co70Fe20B10P ~ 53 %~ 0.032
P measured using Superconducting Tunneling Spectroscopy (STS) with an AlOx tunnel barrier and was determined with FMR characterization post anneal
CrO2P ~ 94 %~ 0.0023
[C. Bilzer et al, JAP 100 (2006) 053903]
[P. Lubitz et al, JAP 89 (2001) 6695]
[Parker et al, PRL 88 (2002) 196601]
[P.V. Paluskar et al JAP 99 (2006) 08E503]
M1-xCrxO2 Newly Discovered RT Ferromagnetic Oxides! M=V and RuVO2 ×10 with charge injection Jc~ 104 A/cm2
SPIN 08 October 11, 2008
Excellent write selectivity ~ localized spin-injection within cell Highly Scalable ~ write current scales down with cell size Low power ~ low write current Simpler Architecture ~ no write lines, no bypass line and no cladding High Speed ~ Few nanoseconds
Key Advantages and Potential of STTRAM
SPIN 08 October 11, 2008
9.6 9.7 9.8 9.9 10.0
0.0
0.1
0.2
0.3
0.4
9 mA
8.5 mA 8 mA
7.5 mA
7 mA
6.5 mA
6 mA
5.5 mA
Pow
er (
pW)
Frequency (GHz)
Spin-Current Switched MRAM
Spin Transfer Nano-Oscillators
50 nm
1 m
0 50 100 150 200
-1.0
-0.5
0.0
0.5
1.0Switching in response to a 10 mA current pulse
Ea
sy A
xis
Ma
gn
etiz
atio
n
Time (ps)
Spin Torque Nano-Oscillators
Simulations: OOMMF math.nist.gov/oommf/
I
Tunnel junction
High-speed switching
Tunable High Q oscillator (2 GHz – 100 GHz)
Au
Cu
0.7 T, = 10o
CoFe
NiFe
I
simulation
data
SPIN 08 October 11, 2008
17.025 17.050 17.075
0
2000
4000
6000
8000
10000
Pow
er (
nV2 /
Hz)
Frequency (GHz)
f = 17.052 GHzf = 3.00 MHz
6 8 10
16
17
18
19
Fre
quen
cy (
GH
z)
Current (mA)
0.5 GHz/mA
Summary of Present Status Summary of Present Status
0.0 0.2 0.4 0.6 0.8 1.00
10
20
30
40
Field (T)
Freq
uenc
y (G
Hz)
Field TunableCurrent Tunable
Narrow Band
•Oscillators are tunable over a wide range of frequencies via applied field or current•Output is narrow band with Q values > 10,000•Voltage outputs in the mV regime
28 GHz/T
SPIN 08 October 11, 2008
Fundamental Frequency Limits
The gyromagnetic precession frequency of spins has no upper bound!
0 effH
For ultra-small contacts of diameter 3 nm < d < 8 nm, intralayer exchange dominates the energetics:
2
0
exeff
DH
d
SMT oscillators could fill the “THZ gap.”
“THz gap”
SPIN 08 October 11, 2008
0.1 0.2 0.3 0.4 0.5 0.6 0.7
-200
-100
0
100
200
VO
sc (V
)
Time (ns)
Idc= 7.4 mA
Idc= 7.6 mA
Idc= 7.8 mA
Idc= 7.85 mA
7.4 7.5 7.6 7.7 7.8
-100
-50
0
50
100
Rel
. Pha
se S
hift
(deg
rees
)Idc
(mA)
locked
Electronic Phase Control of Oscillations
The relative phase can be varied using the DC current!
W. H. Rippard et al, Phys. Rev. Lett. 95, 067203 (2005).
A
B
Locked
Phase Locking in Closely Spaced Spin Transfer Nano-Oscillators
SPIN 08 October 11, 2008
A biased at 11.5 mA; B swept 0 – 15 mA
nV)2/Hz
3.7
13.3
A
B
Locked
13.8 14.0 14.2 14.4 14.6 14.8
0
20
40
60
80
PS
D (
nV)2 /H
z
f (GHz)
13.8 14.0 14.2 14.4 14.6 14.8
0
2
4
6
8
PS
D
(nV
)2 /Hz
f (GHz)
13.8 14.0 14.2 14.4 14.6 14.8
0
10
20
30
PS
D (
nV)2 /H
z
f (GHz)
AB Locked
A
B
Phase Locking 500 nm Spaced ContactsPhase Locking 500 nm Spaced Contacts
Kaka et al, Nature, Sept. 2005
Spin valve
A
B500 nm
IB
IA
When phase locked power increases & linewidth decreases
)cos(2 BABAT PPPPP
0 90 180 2700
2
4
6
8
10
Po
we
r (p
W)
Phase Shift (deg)
SPIN 08 October 11, 2008
Applications of Spin Transfer Nano-OscillatorsApplications of Spin Transfer Nano-Oscillators
0 1 2 3 4
Frequency
Ampl
itude
Signal S(t) in
Inducedspin waves
Component signals out
Point contact STNOs
200 nm
i1i2i
3
)(~
1fS)(~
2fS)(~
3fS
STNO
I2
sin(t+2)
STNO
I3
sin(t+3)
STNO
I4
sin(t+4)
STNO
I1
sin(t+1)
SMT device/GMR sensor
Spin-transferoscillator
Near-field antenna
Chip-to-chip microwireless
Nanoscale Phased Array
High-speed parallel signal processing
Referenceoscillator
Not going to replace existing VCOs!
Target new applications requiring nanoscale high frequency
components!
Top Related