current research in current-driven magnetization dynamics
DESCRIPTION
Current research in current-driven magnetization dynamics. S. Zhang, University of Missouri-Columbia. Beijing, Feb. 14, 2006. Outlines. Magentoelectronics started from discovery of giant magnetoresistive (GMR) effect Spin-dependent transport in magnetic metal based nanostructures - PowerPoint PPT PresentationTRANSCRIPT
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Current research in current-driven magnetization dynamics
S. Zhang, University of Missouri-Columbia
Beijing, Feb. 14, 2006
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Outlines
Magentoelectronics started from discovery of giant magnetoresistive (GMR) effect
Spin-dependent transport in magnetic metal based nanostructures
Spin angular momemtum transfer: physics and potential technology
Perspectives and conclusions
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M.N. Baibich et al., Phys. Rev. Lett. 61, 2472 (1988).
400
110
H (kOe)-40 H // [ 0 11]
What is giant magnetoresistance?
R
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Origin of GMR—two current model
e e e e
EF
A ferromagnet Different numbers ofup and down electrons
R R
Up and down resistances
Low resistance High resistance
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GMR Reading head
Bit width
Bit length
Conductorlead
JM Spin
valve
Spin valve
OR MNMM
AF
“0”“1”
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Concert efforts: theorists, experiments and technologists on GMR Theorists: predict, explain, model and design GMR
effects and devices
Experimentalists: design, fabricate, characterize, and measure GMR devices
Technologists: produce, evaluate, pattern, integrate, and deliver GMR devices
It would be otherwise impossible to push the information storage so rapidly
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History of magentic tapes and hard disks
Now: 80Gbits/in2
5 years: 1 Terabits/in2
In 1988, giant Magnetoresistance (GMR) was discovered;in 1996, GMR reading heads were commercialized Since 2000: Virtually all writing heads are GMR heads
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GND
Magnetoelectronics: Magnetic Tunnel Junctions
High tunneling probabilityLow resistance
Low tunneling probabilityHigh resistance
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Al-O barrier
Cu (30)
IrMn
Co-Fe-B(4)
Ta (5)
IrMn (12)
Al-O (0.8)
Cu (20)
Ta (5)
Py (5)
Ta (5)
Co-Fe-B(4)
-1500 -1000 -500 0 500
0
20
40
60
80
100
0
10
20
30
40
50
60
T=4.2 KRp=23.4 RS=4.68 km2
TMR=95.4%
TM
R (
%)
H (Oe)
(b)
TMR curves measured at RT (a) and 4.2 K (b) for the Co-Fe-B/Al
2O
3/Co-Fe-B junction after annealing.
Annealed at 265 0CT=300 K
S=10 x 20 m2
Rp=22.3 RS=4.46 km2
TMR=58.5%
T
MR
(%
)
(a)
VSource: Dr. Xiufeng Han
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Brief History of TMJ
1974, M. Julliere (a graduate student) published an experiment article which claimed 14% TMR in Fe/Ge/Fe trilayers. A simple model was proposed (the paper became a sleeping giant).
1982, IBM reported 2% TMR on Ni/AlO/Ni. 1995, Moodera (MIT) and Miyazaki (Japan) reported
10% TMR for Co/AlO/Co. 1998, DARPA launched MRAM solicitation 1999, Motorola’s 128kB MRAM demo 2003, IBM, Motolora, 4Mb MRAM chip demo More than 10 startup MRAM companies formed. MRAM becomes internationally recognized future
technology
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Emerging non-volatile memory technologies
Flow
Spin
Quantity FRAM
PCRAM
MRAM
PFRAM SiC Bipolar
PMC
Molecular
Polymer Perovskite
NanoX’tal
3DROM
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Current-driven spin torques
GMR/TMR: magnetization states control spin transport (low-high resistance).
Adverse effect: spin transport (spin current) affects magnetization states?
Every action will have reaction—spin transfer
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T
spin angular momentum transfer? Momentum transfer—electromigration
Angular momentum transfer—magnetization dynamics
An impurity atom receives a force by absorbing a net momentum of electrons:electromigration is one of the major failure mechanisms in semiconductor devices.F
The atom receives a torque by absorbinga net spin angular momentum of electrons:the spin torque can be used for spintronics
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Interaction between spin polarized current and magnetization (J. Slonczewski, IBM)
m mout in
m e Bout
m e Bin P
dMJ J
dt
J PJ Me
J PJ Me
MpM
Spin torque on the magnetic layer M
( )
/
J P
eJ B
dMa M M M
dt
a PJ e
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0, 0
t t
Current torque on DW
(Magnetic field pressure on DW, )
0, 0t t
Massless motion!!
From Sadamichi Maekawa
Current induced Domain wall motion
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Magnetization dynamics: LLG equation (micromagnetics)
1;| | | | 1; 1
( )
( )
( )
J P
J J
eff
eff
p
eff
b
a m m m
m mm m c m
x x
dm dmm H m
dt dtdm dm
m H mdt dt
m m V
E mH
m
LLG+spin torque
Where
Spin valve
Domain wall
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Novelty of spin transfer torques
Manipulation of magnetization states by currents
Self-sustained steady state magnetization dynamics
Unusual thermal effects
Interesting domain wall dynamics
Dynamic phases: synchronization, modification and chaos
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First observation of current induced magnetic switching by Spin torques
Co1=2.5nmCo2=6.0nm
Katine et. al., PRL (2000).
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Self-sustained steady-states precession
2| | ( ) ( )eff J p p eff
dEm H a M m M m H
dt
The first term is always negative (damping), the second termcould be positive or negative (it even changes sign at different times).
Energy damping and pumping:
Limit cycle: the energy change is zero in an orbit
[ ( ) ( )] 0eff J pE
E dm m H a m m
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Calculated limit cycles2 2 2sin cos 2 sin cosE K H
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Kiselev et al., Nature (2003)
Experimental identification of limit cycles
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Unusual Thermal effects
Eb
PAP
Neel-Brown relaxation:
( , )exp( / )b Bf T E E k T
( , )f T Mwhere is algebraic dependent on T, E
Question: in the presence of the spin torque, how do we formulatethe relaxation time?
Thermal activation
Difficulty: the spin torque is not conservative: ( )J p ma m m F m
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LLG equation at finite temperatures
( ) ( ) ( )
0
( , ) ( ', ') 2 ( ') ( ')
eff J P
i jij
dm dmm H h m a m m m
dt dth
h r t h r t D r r t t
random field
( )
( )
eff m
eff
J p m
M
H E m
m H
a m m F m
Dm P
The magnetization receives following fields
Precessional conservative field
Non-conservative damping field
Non-conservative spin torque field
Diffusion field
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Solution of Fokker-Planck equation
( ) [ ( ) ( )] ( ) 0eff J p ME E
B
P E dm m H a m m dm D P m
D k T
is diffusion constant (dissipation-fluctuation relation)
The probability energy density is:
'
'
( ) exp
( )
' ' ( ')( )
eff
B
J pE
effE Eeff
E
EP E A
k T
a dm m m
E E dE E dE C Edm m H
where
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Experimental data interpretation
Telegraph noise
P
AP
P AP
P AP
H
P AP
J
P AP
J P AP
H+
J
R
Field alone Current alone
P AP
H
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H-I phase boundary of equal dwell times.
Comparison with experiments
Equal dwell timesfor P and AP states
P AP
By simultaneously changingH and J, one can always have
( )(1 )bc
IE H Const
I
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Synchronization, modification and chaos
Limit cycle
+ 1. Another oscillator2. AC external field3. AC external current
Linear oscillator
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Calculated limit cycles2 2 2sin cos 2 sin cosE K H
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Observation of synchronization by an AC current
Rippard et al, PRL (2005)
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Observation of mutual synchronization
Kaka et al., Nature (2005); Mancoff et al, Nature (2005)
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Observation of mutual synchronization
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Narrower spectrum width at synchronization
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Dynamic phases due to AC currents
M
M
M
M
20( )
0.02
200( )
0
aca Oe
H Oe
K
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Synchronization spectrax1
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Modification spectra (beating)
x2
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Synchronization and modification agree well with experiments
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Chaos spectrax3
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Theories of spin torques in ferromagnets
Me Berger, domain drag force, based an intuitive physics picture
Bazaliy, et al,
Waintal and Viret, a global pressure and a periodic torque
Tatara and Kohno, spin transfer torque and momentum transfer torque.
Zhang and Li, adiabatic torque and non-adiabatic torques
Barnas and Maekawa, non-adiabatic torque relates to the damping of the adiabatic torque
within a ballistic transport model for half-metallic materials
MM M
x
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Spin torques in a domain wall
1
ex s sf
m mJ m M
t M
�
Equation of motion for conduction electrons
( )e J Jff bM M
M H MM M
M M c Mx xt t
/ 0.01exJ J
sf
c b
where
Interaction between conduction electrons and magnetization:
ex
H m M
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( , , )xm m x v t y z / /xm t v m x
If the wall is in steady motion, the current driven wall velocity is independent wall structure and pinning potentials
extJ
x
WHcv
ext ext xH H e
Steady state wall motion
Steady state wall velocity is thus
xss ejj
eff J J
m m m mm H m b m m c m
t t x x
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Wall velocity for different materials in a perfect wire
Ms (A/m) P Wall velocity (m/s)
Co 14.46x105 0.35 1.41
Permalloy 8x105 0.7 5.1
Fe2O3 4.14x105 1.0 14.0
CrO2 3.98x105 1.0 14.6
27 /101 cmAjs
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Observed Domain wall motion in a nanowire
Yamagushi et al., PRL (2004)
Observed Wall velocity
8 2
3 /
1.2 10 /
v m s
j A cm
for
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Vortex domain wall motion driven by current
05.0,01.0
/108 28
cmAje
Wall transition: from vortex all to transverse wall
xv
yv
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Magnetic tunnel Junction
1 0
Goal: using a reasonable current to switch magnetization, ideally less than 106 A/cm2
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Conductorlead
J
Oscillation of M (GHz) by a DC current
Application 2: local AC magnetic field oscillators (generators)
Local AC field (1000 Oe) with spatial resolution 10nm!
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Application IV: concerns of CPP reading heads
Bit width
Bit length
Conductorlead
JM Spin
valve
“0”“1”
The large current density in CPPreading heads may produce unwanted switching!
Goal: eliminates current-inducedswitching for current densitylarger than 107A/cm2
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Acknowledgement
Students: Dr. Yu-nong Qi, Mr. Zhao-yang Yang, Mr. Jie-xuan He
Postdoctoral: Dr. Z. Li (Postdoctoral)
Collaborators: P. M. Levy (NYU)
A. Fert (Orsay-Paris) Funded by: NSF-DMR, NSF-ECS, DARPA, NSFC