Post on 06-Apr-2018
8/3/2019 Shang-Fan Lee et al- Spintronics ----Spin transfer torque in magnetic nanostructures and spin pumping effect
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Shang-Fan Lee ()
Y. D. Yao(), Y. Liou( )S. Y. Huang (), F. T. Yuan (), C. Yu ( ), T. W. Chiang (
), L. K. Lin (), L. J. Chang (), Faris B.
Y. L. Chen(), Y. C. Chiu (), Y. H. Chiu ()Institute of Physics, Academia Sinica
J. J. Liang () D. S. Hung()
Dept. of Physics, Fu Jen University Dept. of Info. Telecom. Eng.
Ming Chuan University
Financial support from National Science Council and Academia Sinica
----
Spintronics ---- Spin transfer torque in magnetic
nanostructures and spin pumping effect
8/3/2019 Shang-Fan Lee et al- Spintronics ----Spin transfer torque in magnetic nanostructures and spin pumping effect
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Spintronics :
Electronics with electron spin as an extra degree of freedomGenerate, inject, process, and detect spin currents
Generation: ferromagnetic materials, spin Hall effect, spin
pumping effect etc.
Injection: interfaces, heterogeneous structures, tunnel
junctions
Process: spin transfer torque
Detection: Giant Magnetoresistance, Tunneling MR 38, 898 (2007).
30, 116 (2008).
87, 82 (2009).
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Single Magnetic Domain Wall Resistance
Phase diagram of magnetization reversals
Edge roughness effect on domain wall mobility
Current driven magnetization reversals
Possible applications:
Magnetic sensors, Reading heads Magnetic RAM
Logic operation
Magnetic Nano-structures
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-400 -200 0 200 400
0.0
0.2
0.4
0.6
0.8
1.0
1.2
1.4
MR(%)
0.0 0.1 0.2 0.30.00
0.05
0.10
0.15
0.20
one - step
two - step
wid
th(m)
b/a
calculated one - step
calculated two - step
(a)P1
P3
-400 -200 0 200 400
0.0
0.2
0.4
0.6
0.8
1.0
1.2
1.4
H (Oe)
MR(%)
or
P2P2
Variation of magnetization reversal in pseudo-spin-valve elliptical rings
APL 94, 233103 (2009)
8/3/2019 Shang-Fan Lee et al- Spintronics ----Spin transfer torque in magnetic nanostructures and spin pumping effect
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Exchange bias in spin glass (Fe9.6 at.%Au)/NiFe thin films
APL 96, 162502 (2010)
We investigated the exchange bias in the (Fe9.6 at.%Au)/NiFe.
While the temperature was increased to a compensation
point, sign change in exchange bias was observed in the
thickness range of the FeAu layer from 5 to 100 nm. We
suggest that the inverse bias originates from the magnetic
relaxation of the SG layer induced by the reversible rotation
of interfacial FeAu spins coupled to the magnetic moments of
NiFe. The inverse bias was also found to decrease with the
increasing maximum field of a hysteresis loop
8/3/2019 Shang-Fan Lee et al- Spintronics ----Spin transfer torque in magnetic nanostructures and spin pumping effect
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(tranport of magnetization by an electrical curent)
- fundamentals
- switching of magnetization by spin transfer torque
applications (STT-RAM, reprogrammable devices)
- microwave oscillations by spin transfer and applications totelecommunications
Spin transfer Torque
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Compensation between magnetoresistance and switching
current in Co/Cu/Co spin valve pillar structure
MR ratio and current density for induced magnetization reversal showed compensation
behaviors. In order to achieve maximum efficiency (MR ratio) and minimum
consumption (critical current) in a practical device, the thickness of the injection layer
should be around the spin diffusion length for optimum performance.
APL 96, 093110 (2010)
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Transport geometry
CIP resistance can be measured easily, CPP resistance
needs special techniques.
From CPP resistance in metallic multilayers, one canmeasure interface resistances, spin diffusion lengths, andpolarization in ferromagnetic materials, etc.
lead
CIP geometry
~
CPP geometry
~
lead
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Magnetic switching Generation of microwave oscillations
SPIN TRANSFER
H
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Transverse
component
The transverse spin component is lost by the
conduction electrons, but is actually transferred to the
global SPIN of the layer rotation ofS S
S
F1 F2
0.1 m
S
Concept of spin transfer(Slonczewski 1996)
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Au
4 nm
10 nm
Free magn. layer
CuPolarizer
Trilayered pillar or tunnel junction
Metallic pillar 50x150 nm
x
1) Magnetization switching by spintransfer
2) Sustained precession of themagnetization of the free layerand generation of radio-frequency
oscillations
Two regimes of spin transfer
Applications: writing a memory, etc
Applications: spin transfer nano-oscillators (NSTOs) for
communications (telephone, radio,radar)
Zero or low field
Appl. field
H
Polarizermagnetization
Free layer magnetization
70 nm
Au
CoFeB
MgO
CoFeB
Tunnel junction 50x170 nm
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Experimental observations
A. Giant Magnetoresistance (Disc. 1988)
Change Magnetic Order Change Resistance (or Current)
F
N
F
P
AP
Read Heads; Sensors; MRAM (Tunneling)
B. Current-Driven Switching
F
F
N
J
First F Polarizes J. Polarized J exerts Torqueon second F. +Jc Flips to AP; -Jc Flips to P.
Write in MRAM? Write on MR Media?
Q: Physics; Minimize Jc
1.40
1.45
1.50
1.55
1.60
1.65
dV/dI()
AP
P295K
Py/Cu/Py
1.40
1.45
-0.2 0 0.2
dV/dI()
H (kOe)
P
AP
1.0
1.1
1.2
1.3
1.4
-6 -4 -2 0 2 4 6
dV/dI()
I (mA)
4.2KP
AP
1.0
1.1
-0.4 0 0.4d
V/dI()
H (kOe)
AP
P
P AP
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Compensation between magnetoresistance and switching
current in Co/Cu/Co spin valve pillar structure
MR ratio and current density for induced magnetization reversal showed compensation
behaviors. In order to achieve maximum efficiency (MR ratio) and minimum
consumption (critical current) in a practical device, the thickness of the injection layer
should be around the spin diffusion length for optimum performance.
APL 96, 093110 (2010)
)2(2
sksc MHtM
p
eJ
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The Definition of Spin Polarization
Spin polarization ():
NN
NNNP
normal metal
E
metallic ferromagnet
E
4s3d
half-metallicferromagnet
E
Uex
P = 0 P = 10 < P < 1
Spin polarization of current: Ballistic or diffusiveII
IIP
Mazin, PRL 87, 1427 (1999)
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How to Determine the Spin Polarization
NNNNP
E
D(E)
h
Spin-polarizedphotoemission
Point-contact
Andreev reflection
S tip
FV
~
Spin-LED
Substrate
FSemi-conductor spacerQW
~
Spin-polarizedtunneling
H
S
F
I
V
~
Efficiency of spin
injection. Effects ofthe interface and
spacer are included.
P is barrier
dependent and
junction dependent
ff
ff
NN
NN
P
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Andreev Reflection : A Probe of Spin Polarization
Andreev reflection:A conversion of normal current
to supercurrent occuring at ametallic N/S interface.
N SE
N(E)
D
DEF
eVE
N (E)N(E)
The suppression of Andreevreflection due to spinpolarization serves as aprobe of the degree ofspin polarization.
When N is ferromagnetic,
only part of the electronsare paired.
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Tip-Sample Approach: Differential Screw
differential
screw
The turning shaft
The tip
The sample
The sliding tank
net
movement
0.79375 mm
0.75mm
0.04375mm
Differential screw gives abetter control of theplacement of the tip of 40 mper revolution.
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Modified BTK Theory
Superconducting gapSpin polarizationPInterface barrierZ
Pdependence Z dependence T dependence
-4 -2 0 2 40.0
0.5
1.0
1.5
2.0
P=0.4
P=0
Norm
alizedconductance
V (mV)
T=1.5K
DmV
Z=0
P=1
-4 -2 0 2 40.0
0.5
1.0
1.5
2.0
Z=0.4
Z=0
V (mV)
T=1.5K
DmV
Z=0
Z=1
Three parameters :
G. E. Blonder et al., PRB 25, 4515 (1982).
Y. Ji, G. J. Strijkers et al., PRL 86, 5585 (2001).
-4.0 -2.0 0.0 2.0 4.0
1.0
1.5
2.0
T=5K
T=7K
V (mV)
T=3K
Note : ballistic transport assumed
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Domain wall
Another spin transfer effect:
displacement of a wall between magnetic domains
Magnetic film
Magnetization to the right Magnetization to the left
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Edge Roughness effect on the magnetization reversal
process of spin valve submicron wires
-40 -20 0 20 40
1.000
1.005
1.010
1.015
-40 -20 0 20 40
1.000
1.005
1.010
1.015
-40 -20 0 20 40
1.000
1.005
1.010
1.015
-40 -20 0 20 40
1.000
1.005
1.010
1.015
(d)(c)
(b)
MRRatio
no spike(a)
spike 26nm (pitch 200nm)
MRRatio
H(Oe)
spike 33nm (pitch 100nm)
H(Oe)
spike 51nm (pitch 200nm)
Submitted to APL
8/3/2019 Shang-Fan Lee et al- Spintronics ----Spin transfer torque in magnetic nanostructures and spin pumping effect
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Non-local measurement
Spin diffusion lengthHanle effect
Spin Hall effect & Inverse Spin Hall effect
Spin Pumping (spin battery)
Pure spin current
Spin Hall effect & Inverse Spin Hall effect
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The extrinsic SHE is due to asymmetry in electron scattering for up and
down spins. spin dependent probability difference in the electron
trajectories
The Intrinsic SHE is topological band structures,
8/3/2019 Shang-Fan Lee et al- Spintronics ----Spin transfer torque in magnetic nanostructures and spin pumping effect
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26
Pure Spin Currents: The Johnson Transistor
F1 F2
N V
e-
L
F1 N F2
Emitter Base
or
Collector
M. Johnson,
Science 260, 320 (1993)
M. Johnson and R. H. Silsbee,Phys. Rev. Lett. 55, 1790 (1985)
0
F2
F2
First Experimental Demonstrations
I+
I- V
Jedema et al., Nature 410, 345 (2001)
Cu film: s = 1 m (4.2 K)
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27
Spin Pumping
F N
Spin accumulation gives rise to spin current
in neighboring normal metal
IS
t
m
mgI rpump
S
4
In the FMR condition, thesteady magnetization
precession in a F is maintained
by balancing the absorption of
the applied microwave
and the dissipation of the spin
angular momentum --thetransfer of angular momentum
from the local spins to
conduction electrons, which
polarizes the conduction-
electron spins.
8/3/2019 Shang-Fan Lee et al- Spintronics ----Spin transfer torque in magnetic nanostructures and spin pumping effect
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tmmgI r
pumpS
4
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29
Direct Detection of Spin Pumping via Inverse Spin
Hall Effect
FMR
Spin Current
in adjacent
normal metal
Transverse
Charge Current
E. Saitoh, et al., Appl. Phys. Lett. 88, 182509 (2006).
The spin-orbit interaction bends
these two electrons in the
same direction and induces a
charge current transverse to Js,
The surface of the Py layer is of a
1*1 mm2 square shape. Two
electrodes are attached to both
ends of the Pt layer.
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The microwave mode with a frequency
off =9.45 GHz is exited in the cavity,
and The microwave power is 100 mW.
The FMR spectrum shows that the
magnetization in the Py layer resonates at
HFMR=130 mT.
A possible small discrepancy in the
sample position from the center of the cavity
may cause a microwave electric field at the
sample position and may generate dc AHE in
cooperation with FMR.
the ISHE contribution dominates the observed
V signal
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31
E. Saitoh, et al., Appl. Phys. Lett. 88, 182509 (2006).
8/3/2019 Shang-Fan Lee et al- Spintronics ----Spin transfer torque in magnetic nanostructures and spin pumping effect
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32
Spin Pumping
Ferromagnetic Resonance results in time-
dependent interfacial spin accumulation
This spin accumulation diffuses away from the
interface
Results in net dc spin current perpendicular to
interface
Additional spin current gives rise to additional
damping
Quantify spin current from
linewidth broadening
F N
IS
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33
Combine Spin Pumping and
Inverse Hall Effect
Use Spin Pumping to Generate Pure Spin Current
Quantify Spin Current from FMR
Measured Voltage Directly Determines Spin Hall Conductivity
Key Advantage: Signal Scales with Device Dimension
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34
Determine Spin Hall Angle for Many Materials
Pt Au Mo
= 0.01200.0001
= 0.00250.0006
= -0.000960.00007
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Enhancement of magnetic damping in NiFe thin
films by structural defects
H = H0 + 4f/
= g|e|/2mc
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H = H0 + 4f/ = g|e|/2mc
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Summary
GMR effect charge can be controlled
by magnetization (spin).
STT (spin transfer torque) effect
-- magnetization can be controlled by
spin polarized current.
-- New materials with high spin polarization,low saturation moments,
high Curie temperature are needed.
Pure spin current will it be realized in circuits?
8/3/2019 Shang-Fan Lee et al- Spintronics ----Spin transfer torque in magnetic nanostructures and spin pumping effect
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The Definition of Spin Polarization
Spin polarization ():
NN
NNNP
normal metal
E
metallic ferromagnet
E
4s3d
half-metallicferromagnet
E
Uex
P = 0 P = 10 < P < 1
Spin polarization of current: Ballistic or diffusiveII
IIP
Mazin, PRL 87, 1427 (1999)
8/3/2019 Shang-Fan Lee et al- Spintronics ----Spin transfer torque in magnetic nanostructures and spin pumping effect
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How to Determine the Spin Polarization
NNNNP
E
D(E)
h
Spin-polarizedphotoemission
Point-contactAndreev reflection
S tip
FV
~
Spin-LED
Substrate
FSemi-conductor spacerQW
~
Spin-polarizedtunneling
H
S
F
I
V
~
Efficiency of spin
injection. Effects ofthe interface and
spacer are included.
P is barrier
dependent and
junction dependent
ff
ff
NN
NNP
8/3/2019 Shang-Fan Lee et al- Spintronics ----Spin transfer torque in magnetic nanostructures and spin pumping effect
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Andreev Reflection : A Probe of Spin Polarization
Andreev reflection:A conversion of normal current
to supercurrent occuring at ametallic N/S interface.
N SE
N(E)
D
DEF
eV E
N (E)N(E)
The suppression of Andreevreflection due to spinpolarization serves as aprobe of the degree ofspin polarization.
When N is ferromagnetic,
only part of the electronsare paired.
Tip-Sample Approach: Differential Screw
8/3/2019 Shang-Fan Lee et al- Spintronics ----Spin transfer torque in magnetic nanostructures and spin pumping effect
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Tip-Sample Approach: Differential Screw
differential
screw
The turning shaft
The tip
The sample
The sliding tank
net
movement
0.79375 mm
0.75mm
0.04375mm
Differential screw gives abetter control of theplacement of the tip of 40 mper revolution.
M difi d BTK Th
8/3/2019 Shang-Fan Lee et al- Spintronics ----Spin transfer torque in magnetic nanostructures and spin pumping effect
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Modified BTK Theory
Superconducting gapSpin polarizationPInterface barrierZ
Pdependence Z dependence T dependence
-4 -2 0 2 40.0
0.5
1.0
1.5
2.0
P=0.4
P=0
Nor
malizedconductance
V (mV)
T=1.5KDmV
Z=0
P=1
-4 -2 0 2 40.0
0.5
1.0
1.5
2.0
Z=0.4
Z=0
V (mV)
T=1.5K
DmV
Z=0
Z=1
Three parameters :
G. E. Blonder et al., PRB 25, 4515 (1982).
Y. Ji, G. J. Strijkers et al., PRL 86, 5585 (2001).
-4.0 -2.0 0.0 2.0 4.0
1.0
1.5
2.0
T=5K
T=7K
V (mV)
T=3K
Note : ballistic transport assumed
Point contact Andreev reflection
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Point contact Andreev reflection
Superconducting tip -10 -5 0 5 10
1.00
1.05
1.10
MBTK:
T=4.20K
z=0.00
p=0.475
D=1.00 meV=0.030
3D BTK:
T=4.20K
z1=0.155
z2=1.05
z3=11.5
p=0.385
D=0.950 meV=0.030
Co
nductance
V(mV)
data
MBTK
3D BTK
Our new BTK model
D dEeVEfEZZZPFGG
NN
NS )(),,3,2,1,(
IIIII ddcczaazzz )**(3)*(21
II
I
III
N S
Modified BTK theory
D dEeVEfEZPFGG
NN
NS )(),,,(