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Hsin-wei Tseng
Ph.D. Candidate
School of Applied and Engineering Physics, Cornell University
Robert Buhrman Research Group
Spin-torque effect in MgO-based magnetic
tunnel junctions and
spin Hall effect magnetic switching
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Acknowledgement
• MgO MTJ growth and nanopillar fabricationJohn Read*, Patrick Braganca*, Yun Li**
*HGST, **Cornell University
• STEM and EELS characterizationPinshane Huang*, Judy Cha**, D.A. Muller*
*Cornell University, **Stanford University
• CoFeB MgO nanopillar deviceJ.A. Katine*, D. Mauri*
*HGST
• Other supportOukjae Lee*, Praveen Gowtham*, Luqiao Liu*, Chi-feng Pai, Jon Shu*, Junbo Park*, Jonathan Shaw*, Huanan Duan*
*Cornell University
•Funding
This work was made possible by a DOD ARO-MURI . Additional support
provided by the National Science Foundation MRSEC program through the
Cornell Center for Materials Research and through the NSF GRFP.
2
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Outline
• Introduction
– Non-volatile memory and logic
– Spin-torque effect
• Spin-torque effect in asymmetric MTJs
– Aberration-correction STEM EELS chemical map
– High TMR in asymmetric MTJs
– Electronic and spin-dependent transport
• Spin-torque effect under high current densities in MTJs
– Pulse-biased spin-torque microwave emission measurement
– Macromagnetic simulation
• Spin-Hall effect three terminal spin-torque device
– HSQ tri-layer lift-off nanopillar process
– 3 terminal spin-torque devices
– 3 terminal spin Hall effect magnetic switching device
• Conclusion
O B Fe
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Outline
• Introduction
– Non-volatile memory and logic
– Spin-torque effect
• Spin-torque effect in asymmetric MTJs
– Aberration-correction STEM EELS chemical map
– High TMR in asymmetric MTJs
– Electronic and spin-dependent transport
• Spin-torque effect under high current densities in MTJs
– Pulse-biased spin-torque microwave emission measurement
– Macromagnetic simulation
• Spin-Hall effect three terminal spin-torque device
– HSQ tri-layer lift-off nanopillar process
– 3 terminal spin-torque devices
– 3 terminal spin Hall effect magnetic switching device
• Conclusion
O B Fe
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Nonvolatile memory and logic
Why nonvolatile memory?
• Low standby power
• Avoid transfering data to slow
nonvolatile storage medias
• Improve scaling for CMOS
• Logic-in-memroy low power
and short interconnection delay
Why nonvolatile logic?
• Instant-on computation
• Low power consumption
5
Freescale MRAM product
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Spin-torque physics
Fixed layer
Free layer
Source line
MTJ
Word line
Bit line
Transistor
J.C. Slonczewski. JMMM 159, L1-L7(1996)J. A. Katine. et al. PRL 84, 4212-4215(2000)
Spin transfer torque induced switching– Fast operation speed (~ 1GHz)
– Switching depends on J rather than I high scalability
– Lower current density needed for field-induced-switching MRAM
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Spin-Transfer Torque in Magnetic Tunnel junction
CoFeB/MgO/CoFeB MTJ :
• Both in-plane and perpendicular torque
have been measured.
• Perpendicular torque is a stronger
effect than expected.
T ┴ – perpendicular torque
Td – damping torque
T|| – spin transfer torque
HeffT┴
Td
T||m
m
: magnetic moment
effH
: effective magnetic field
: Gilbert damping constant
: gyromagnetic ratio
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Outline
• Introduction
– Non-volatile memory and logic
– Spin-torque effect
• Spin-torque effect in asymmetric MTJs
– Aberration-correction STEM EELS chemical map
– High TMR in asymmetric MTJs
– Electronic and spin-dependent transport
• Spin-torque effect under high current densities in MTJs
– Pulse-biased spin-torque microwave emission measurement
– Macromagnetic simulation
• Spin-Hall effect three terminal spin-torque device
– HSQ tri-layer lift-off nanopillar process
– 3 terminal spin-torque devices
– 3 terminal spin Hall effect magnetic switching device
• Conclusion
O B Fe
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High TMR in asymmetric MTJs
J. C. Read, J. J. Cha, W. F. Egelhoff, H. W. Tseng, P. Y. Huang, Y. Li, D. a. Muller, and R. a. Buhrman, Applied Physics Letters 94
9
• High TMR in asymmetric FeCoB/Mg-B-O/Py80B20 MTJ.
• How about the spin-torque effect in the asymmetric MTJs?
Current in-plane tunneling TMR measurement
From : Pinshane Huang
Fe Ni Co
O B Fe
STEM EELS chemical map
From : Pinshane Huang
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Fe and Ni show phase segregation at the MgO
• Aberration-corrected STEM showed phase segregation in Fe-Ni alloy. 10
From : Pinshane Huang
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Abnormal switching in FeCoB/MgO/FeNiB asymmetric MTJ
• P-to-AP abnormal switching is observed with V<0. (V<0 prefer P)
11
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Switching Phase diagram in FeCoB / MgO / FeNiB MTJs
• Abnormal switching , back-hopping appear in asymmetric junctions
12
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Switching Phase diagram in FeCoB / MgO / FeCoB MTJs
• Symmetric junctions under similar fields and voltages exhibit normal switching behavior
13
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Switching Phase diagram in FeCoB / MgO / FeNiB MTJs
• Abnormal switching (P-AP) with V<0 is Fieldlike-torque dominated switching.• With Heff = 0 and Hc ~ 70Oe, we observe this abnormal switching, indicating
Fieldlike torque > 70Oe.
Abnormal switching
14
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Highly asymmetric bias dependence of TMR (High RA)
FeCoB/MgO/FeNiB
FeCoB/MgO/FeNiB
FeCoB/MgO/FeCoB
FeCoB/MgO/FeCoB
• Highly asymmetric bias dependence of TMR in both low and high RA asymmetric MTJ
15
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Highly asymmetric bias dependence of TMR (Low RA)
FeCoB/MgO/FeCoB
FeCoB/MgO/FeNiB
FeCoB/MgO/FeCoB
FeCoB/MgO/FeNiB
• Highly asymmetric bias dependence of TMR in both low and high RA asymmetric MTJ• The spin-dependent transport is highly asymmetric in FeCoB/MgO/FeNiB MTJs.
16
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-1 0 11
2
No
rma
lize
d d
I/d
V
Bias (V)-1 0 1
1
2
No
rma
lize
d d
I/d
V
Bias (V)
-0.5 0.0 0.51
2
Norm
aliz
ed d
I/dV
Bias voltage (V)
-0.5 0.0 0.51
2
No
rmaliz
ed d
I/d
V
Bias (V)
IrMn/Fe80B20/MgO/Fe40Co40B20IrMn/Fe40Co40B20/MgO/Fe80B20
As-grown
Annealed
Local minimum reverse asymmetry after annealing
•As-grown MgO MTJs - no local minima in tunnel spectroscopy, dI/dV• Annealed MgO MTJs – strong local minima in dI/dV• Electronic structure of the electrodes determines the dI/dV behavior.
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-1 0 11
2
No
rma
lize
d d
I/d
V
Bias (V)-1 0 1
1
2
No
rma
lize
d d
I/d
V
Bias (V)
-0.5 0.0 0.5
1.0
1.1
Norm
aliz
ed
dI/dV
Bias voltage (V)
-0.5 0.0 0.5
1.0
1.1
No
rmaliz
ed d
I/d
V
Bias (V)
IrMn/Fe80B20/MgO/Fe40Co40B20IrMn/Fe40Co40B20/MgO/Fe80B20
As-grown
Annealed
Local minimum reverse asymmetry after annealing
•As-grown MgO MTJs - no local minima in tunnel spectroscopy, dI/dV• Annealed MgO MTJs – strong local minima in dI/dV• Electronic structure of the electrodes determines the dI/dV behavior.
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-1.0 -0.5 0.0 0.5 1.0
0
10
20
Bias (V)
-1.0 -0.5 0.0 0.5 1.0
0
10
20
Bias (V)
-1.0 -0.5 0.0 0.5 1.030
60
90
Bias (V)
-1.0 -0.5 0.0 0.5 1.030
60
90
Bias (V)
TMR bias dependence reverse in annealed MTJs
TMR
(%
)
IrMn/Fe80B20/MgO/Fe40Co40B20IrMn/Fe40Co40B20/MgO/Fe80B20
TMR
(%
)
As-grown
Annealed TMR
(%
)
TMR
(%
)
•As-grown: asymmetric TMR bias dependence independent of Fe electrode location.•Annealed: the TMR asymmetry depends on Fe electrode location•Suggests the interfacial electronic structure has a major role in determining the TMR.
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• As-grown samples can exhibit bipolar STT switching in certain field bias ranges• Annealed samples exhibit unusual switching behavior:
•P to AP switching, heating seems to dominate over anti-damping STT•AP to P switching, “Hc” constant for V < 0.5 V and then goes to 0 with increasing V
-1 0 1-50
0
50
He
ff (
Oe
)
Bias (V)
IrMn/Fe80B20/MgO/Fe40Co40B20IrMn/Fe40Co40B20/MgO/Fe80B20
-1 0 1-100
0
100
Bias (V) H
eff (
Oe
)
-1 0 1-40
0
40
He
ff (
Oe
)
Bias (V)
STT in as-grown and annealed device
As-grown
Annealed
APAP
AP
P P P P
AP
P/AP
APAP
AP
P P P P
AP
P/AP
P/AP P/AP
-1 0 1-50
0
50
Heff (
Oe)
Bias (V)
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JC Sankey et al. PRL (2006)
ST-FMR measurement
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-0.6 0.0 0.6
-0.4
0.0
0.4 As-grown
d
FL/d
V
Bias (V)
Field-like torkance
-0.4 -0.2 0.0 0.2 0.40.00
0.05
0.10
F
L
Bias(V)
Field-like torque
STT in as-grown and annealed device CoFeB/MgO/CoFeB
• As-grown MTJs exhibit almost no field-like torque.
• Annealed MTJs show a stronger field-like torque.
Field-like torque is sensitive to the interfacial electronic structure.
As-grown : TMR ~20%, Spin polarization (P) ~ 30%Annealed : TMR ~ 90%, Spin polarization (P) ~ 56%
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-0.6 0.0 0.6
-0.4
0.0
0.4 As-grown
Annealed
d
FL/d
V
Bias (V)
Field-like torkance
-0.4 -0.2 0.0 0.2 0.40.00
0.05
0.10
F
L
Bias(V)
STT in as-grown and annealed device CoFeB/MgO/CoFeB
Field-like torque
• As-grown MTJs exhibit almost no field-like torque.
• Annealed MTJs show a stronger field-like torque.
Field-like torque is sensitive to the interfacial electronic structure.
As-grown : TMR ~20%, Spin polarization (P) ~ 30%Annealed : TMR ~ 90%, Spin polarization (P) ~ 56%
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Different electrode composition change ST-FMR
3 6
0
20
40
Vm
ix (V
)
Frequency (GHz)4 8
-15
0
15
Vm
ix (V
)
Frequency (GHz)
IrMn/Fe80B20/MgO/Fe40Co40B20IrMn/Fe40Co40B20/MgO/Fe80B20 IrMn/Fe80B20/MgO/Fe80B20
3.0 4.5 6.0 7.5
-30
0
30
Vm
ix (V
)
Frequency (GHz)
• Field-like torkance can be affected by electrode materials
•No-zero field-torkance at the zero bias suggests high asymmetric
field-like torque at high bias.
Positive field-like torkanceNegative field-like torkance
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Conclusion-1
1. Fieldlike torque can be the dominate
mechanism in magnetization switching.
2. Py80B20 exhibit phase segregation after
annealing.
3. Electronic structure in MTJs could affect spin-
torque effect in magnetic tunnel junction.
4. MgO MTJs exhibit significantly increase in the
filed-like torque after annealing.
5. Asymmetric MTJs has strong field-like
torkance at zero bias and can be altered and
controlled by electrode composition.
25
Fe Ni Co
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Outline
• Introduction
– Non-volatile memory and logic
– Spin-torque effect
• Spin-torque effect in asymmetric MTJs
– Aberration-correction STEM EELS chemical map
– High TMR in asymmetric MTJs
– Electronic and spin-dependent transport
• Spin-torque effect under high current densities in MTJs
– Pulse-biased spin-torque microwave emission measurement
– Macromagnetic simulation
• Spin-Hall effect three terminal spin-torque device
– HSQ tri-layer lift-off nanopillar process
– 3 terminal spin-torque devices
– 3 terminal spin Hall effect magnetic switching device
• Conclusion
O B Fe
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• Backhopping switching at high voltage?J. Z. Sun et al., JAP 105 (2009)
• Ultrafast (sub ps) spin-transfer torque under high voltage in MTJs?
2011 56 MMM: Nov 3rd, Thursday 9:18 AM GC-05
Motivation for understanding high voltage spin torque
AP-to-PP-to-APAP-to-P
27
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Various Spin-Transfer Torque Measurement
Microwave-based measurement
Sankey, J. C. et al. Nature Phys. 4,67 (2008)
STT-FMR based measurement
Switching-based measurement
Z. Li. et al. PRL 100, 246602(2008)
%30~T
-0.4V<V<0.4V
||T(prefer AP state)
Summary of Experiment Result
V~0.4V
V~1V ?
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Advantages:
• Enable measurement of
microwave emission
under high voltage single
pulse.
• Average FFT power
spectrum, instead of time-
domain signal, enhance
signal to noise ratio.
Limitations:
• Frequency resolution is
limited to sampling rate.
• Our linewidth resolution is
∆f=(sample rate/FFT
length)=40MHz.
• Typical MgO STNO
linewidth > 100MHz.
30dB
CoFe-based MgO MTJ layer structure:IrMn/CoFe/Ru/CoFeB (2nm)/MgO/CoFe (0.5)/CoFeB (3.4nm)/cap. (Sample from HGST, J.A. Katine)
29
High voltage pulse microwave measurement setup
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Microwave spectrum v.s. pulse voltage with Heff >HC
A
• Single peak feature with both current polarities.
• V<0 (P) and V>0 (AP) show similar microwave emission.
• V<0 : Stronger peak broadening than V>0.
(Noise on spectrum is due to instrumentation error and artifacts from FFT)
Heff = -90 Oe
V>0 (prefer AP )
Heff = 91 Oe
V<0 (prefer P )
F
e- APP
30
e-PAP
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• V>0, Coherent microwave emission indicate more coherent dynamics up to V=1.0.
• Above V~0.7, significant Broadband microwave emission.
• Hext ~ 400Oe fully suppress the Broadband microwave emission for V≤-1.0.
Heff = -124 Oe
V>0 (prefer AP )Heff = 145 Oe
V<0 (prefer P )
Microwave spectrum v.s. pulse voltage plot with Heff~2HC
31
F
e- APPP
A
e-AP
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• V>0 shows “No microwave emission”, as expected.
G
Heff = 0 Oe
V>0 (prefer AP )Heff = 0 Oe
V<0 (prefer P )
Microwave spectrum v.s. pulse voltage plot with Heff~0 Oe
V<0 exhibits broad and strong “Spin-torque excited FMR microwave emission”.
32
F
e-APAPP
A
e-P
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Power Phase Diagram and Microwave Emission
Broadband microwave emission
Spin-torque excited FMR microwave emission
No microwave emission
Coherent microwave oscillation
• Strong voltage dependence ?
• Why difference in “S” and “N” ?
• Why difference in “B” and “C” ?
33
Power Phase Diagram
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Power Phase Diagram
Strong voltage dependence due to HFL (Fieldlike torque)
• “B” and “C” show strong voltage dependence and shift ~ 40Oe ~ Hc.
• HFL (Fieldike torque) induce the shift and significantly affect the
dynamics behavior when Heff~0 Oe.
0 100
10
20
P (
dB
/GH
z)
Frequency (GHz)
V=+1.0
0 10
0
10
20
P (
dB
/GH
z)
V=-1.0
Frequency (GHz)
34
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Power Phase Diagram
• V=-1.0, HFL (AP) induced shift ~40Oe ~ HC and cause backhopping
switching in “S”.
• V=+1.0, HFL (AP) better confine nanomagnet and result in reliable
switching in “N”
Switching Phase Diagram
Backhopping related to strong microwave emission
Backhopping
35
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• Including the field-like torque term in the simulation reproduces
voltage dependence in the power phase diagram.
Macrospin simulation reproduce the twist of phase diagram
Field-like torque
36
Macrospin simulation
Power Phase Diagram
Experiment
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Macrospin simulation reproduce the twist of phase diagram
Field-like torqueNo Field-like torque
37
• Including the field-like torque term in the simulation reproduces
voltage dependence in the power phase diagram.
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1. Field-like torque effect at high voltage could
significantly affect dynamics and switching
behavior.
2. Joule heating, asymmetric anti-damping
torque, decrease of demagnetizing field do
not reproduce similar feature in the power-
phase diagram.
3. Field-like torque could induce back-hopping
switching with assisted of strong in-plane
torque under high voltage bias.
4. Field-like torque promotes reliable switching
in the P-to-AP direction.
Conclusion – (2)
38
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Outline
• Introduction
– Non-volatile memory and logic
– Spin-torque effect
• Spin-toruqe effect in asymmetric MTJs
– Aberration-correction STEM EELS chemical map
– High TMR in asymmetric MTJs
– Electronic and spin-dependent transport
• Spin-torque effect under high current densities in MTJs
– Motivation
– High voltage effect?
• Spin-Hall effect three terminal spin-torque device
– HSQ tri-layer lift-off nanopillar process
– Spin Hall effect(SHE)
– 3 terminal spin-torque devices
– 3 terminal spin Hall effect magnetic switching device
• Conclusion
O B Fe
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Nanopillar Process at Cornell
Carbon nanomask Open nanopillar
Process time~ 4 weeks
Lift-off nanomask Lift off ~ 40nm oxide
Process time~ 1 week
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Challenges and Solutions
41
Challenges:
Sputtering chamber target change
frequently
MgO sputtering rate varys sigificantly
(~10%)
Goal and requirement:
Require a fast and reliable nanopillar
fabrication
Achieve 50nm x 150nm shape
anisotropy
Reasonable yield rate >50%
Rapid Characterization
Large dot arrayOptical microscope
Lift-off yieldAFM
HSQ/PMMA lift-off process
Define lift-off nanomask
1. Spin omnicoat, PMMA(180nm), HSQ
(80nm). Each bake at 170oC for
1min.
2. Exposure dose for 5nmx100nm CAD
pattern is 15,000 uc/cm2.
3. Develop in 726MIF for 2min.
4. O2 plasma etch to transfer pattern
from HSQ to PMMA.
5. Ion milling and oxide evaporation.
6. Lift off oxide. (self-aligned top lead)
7. Photolith to open leads and grow top
lead.
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HSQ/PMMA nanomask lift-off yield is sensitive to e-beam dose
1μm 200nm
42
AFM images of HSQ/PMMA nanomask
1μm
200nmAFM images ofHSQ/PMMA
nanomask lift off 40nm SiO2
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Enable fast and complicated shape test
• Large nanopillar dot array enables microscope inspection enable rapid
characterization.
• Exposure only take a few seconds.
43
Dose = 15,000μC/cm2
CAD pattern Optical dark-field image
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Optical inspection to check dose
44
Dose = 15,000μC/cm2Dose = 1,000μC/cm2
• Large nanopillar dot array enables microscope inspection.
• Simple optical inspection to check dose and exposure results.
Optical dark-field imageOptical dark-field image
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HSQ/PMMA process could achieve ~ 40nm devices
• For 50x150nm feature yield is ~ 100%.
• For 40nm feature Yield is ~ 30%.
45
SEM image of dot array
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Three terminal spin-torque devices
• The separation of low-impedance
write process and high-impedance
read process.
Page 46
ST Written MRAM Cell
Cornell and HGST collaboration
Three terminal ST magnetic switch
P. M. Braganca, J. A. Katine, S. Member, N. C. Emley, D. Mauri, J. R. Childress, P. M. Rice, E. Delenia, D. C. Ralph, and R. A. Buhrman, IEEE Nanotechnology 8, 190-195 (2009).
J. Z. Sun, M. C. Gaidis, E. J. O’Sullivan, E. a. Joseph, G. Hu, D. W. Abraham, J. J. Nowak, P. L. Trouilloud, Y. Lu, S. L. Brown, D. C. Worledge, and W. J. Gallagher, Applied Physics Letters 95, 083506 (2009).
IBM and MagIC MRAM Alliance
• Resolve the issue of tunnel barrier degradation and still maintain MTJ
high signal output.
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Three-terminal spin Hall effect magnetic switching
• Spin Hall effect induced magnetization switching by β-Ta.
• Three-terminal structure separate read and write terminal to avoid high
current densities going through tunnel barriers.
47L. Liu, C.-F. Pai, Y. Li, H. W. Tseng, D. C. Ralph, and R. a. Buhrman, Science 336 (2012)
current
Protect short
SEM image
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Three-terminal spin Hall effect magnetic switching
48L. Liu, C.-F. Pai, Y. Li, H. W. Tseng, D. C. Ralph, and R. a. Buhrman, Science 336 (2012)
• Spin Hall effect induced magnetization switching by β-Ta.
• Three-terminal structure separate read and write terminal to avoid high
current densities going through tunnel barriers.
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Three-terminal spin Hall effect magnetic switching
49
• Spin Hall angle ~ 0.13 for β-Ta.
• Switching phase diagram shows very weak joule heating effect.
• Zero temperature critical current density Jco=3.7x107 A/cm2 .
• Spin-valve Py/Cu/Py ~ 5x107 A/cm2. Out-of-plane MTJ ~ 5x106 A/cm2
Switching phase diagram Ramp Rate measurement
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Conclusion
50
1. Electronic structure of electrode materials in MTJs
could significantly affect field-like torque and induce
unreliable switching.
2. High voltage pulse-biased measurement indicate field-
like torque could significantly change nanomagnet
microwave dynamics under high voltages.
3. HSQ/PMMA lift-off process enable complicated multi
terminal structure.
4. Spin Hall effect in thin film Ta can generate spin-
polarized currents in the top surface and can induce
magnetization switching with current ~ 107 A/cm2.
Acknowledgement:Yun Li, John Read, Patrick Braganca, J.A. Katine, D. Mauri, PinshaneHuang, Judy Cha, D.A. Muller, Oukjae Lee, Praveen Gowtham, Luqiao Liu, Chi-feng Pai, Jon Shu, Junbo Park, Jonathan Shaw, Huanan Duan. Edwin Kan, Daniel Ralph, and R.A. Buhrman
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Supplement Slides
51
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Ion milling reduce the aspect ration
HSQ nanomask after ion milling
HSQ nanomaskAs-developed
52
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θ
MBMT
T┴T||
Bottom
Electrode
(Fixed)
Top
Electrode
(Free)
V+ -
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AP-to-P single-shot P-to-AP single-shot
• AP-to-P switching show pre-oscillation
In-plane dominated switching
• P-to-AP switching show post-oscillation
Fieldlike-toque assisted switchingY. T. Cui et al., PRL104 (2010)
Devolder, T. et al. PRL100 (2008)
54
AP-to-P and P-to-AP switching is different
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Material experiment lab at Buhrman Grouup
55
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Optical images of HSQ tri-layer lift-off process
• Microscope images of HSQ-trillayer lift-off process before depositing top
lead.
• Left image: bright field. Right image : dark field.
56
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Annealed FeCoB/MgO/FeCoB exhibit non-uniformity in thin film
57
CoFe-based symmetric MTJs exhibit chemical non-uniformity in the thick fixed layers.
From : Pinshane Huang
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Fe and Ni show phase segregation at the MgO
58
Atomic-scale chemical imaging of FeCoB/MgO/ FeNiB by aberration-corrected STEM showed phase segregation in Fe-Ni alloy.
From : Pinshane Huang
Page /5059
Thermal activation model
Annealing• Enhances asymmetry of biasdependent in plane torque• Increases field like torque
P
AP
P/AP
P
P/AP
AP
Switching phase diagram
Page /5060
JC Sankey et al. PRL (2006)
ST-FMR measurement
Page /5061
As grown:TMR=12% P=24%
Annealed:TMR=85% P=55%
Annealing enhances
• Asymmetry inplane torkance
• Magnitude of the field like torkance
150nmX150nm
~ 70
ST-FMR measurement
Page /5062
Summary on experimental results
As-grown Annealed
top related