hsinwei job talks

Page /50

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

Page /50

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

Page /503

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

Page /504

Outline

• Introduction














• Conclusion

O B Fe

Page /50

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

Page /506

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

Page /507

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

Page /508

Outline

• Introduction














• Conclusion

O B Fe

Page /50

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


Page /50

Fe and Ni show phase segregation at the MgO

• Aberration-corrected STEM showed phase segregation in Fe-Ni alloy. 10


Page /50

Abnormal switching in FeCoB/MgO/FeNiB asymmetric MTJ

• P-to-AP abnormal switching is observed with V<0. (V<0 prefer P)

11

Page /50

Switching Phase diagram in FeCoB / MgO / FeNiB MTJs

• Abnormal switching , back-hopping appear in asymmetric junctions

12

Page /50

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

Page /50

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

Page /50

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

Page /5017

-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.

Page /5018

-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)


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.

Page /5019

-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

(%

)


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.

Page /5020

• 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)


-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)

Page /5021

JC Sankey et al. PRL (2006)

ST-FMR measurement

Page /5022

-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%

Page /5023

-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%

Page /5024

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

Page /50

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

Page /5026

Outline

• Introduction














• Conclusion

O B Fe

Page /50

• 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

Page /50

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 ?

Page /50

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

Page /50

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

Page /50

• 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

Page /50

• 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

Page /50

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

Page /50

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

Page /50

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

Page /50

• 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

Page /50

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.

Page /50

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

Page /5039

Outline

• Introduction



• Spin-toruqe effect in asymmetric MTJs





– Motivation

– High voltage effect?



– Spin Hall effect(SHE)



• Conclusion

O B Fe

Page /5040

Nanopillar Process at Cornell

Carbon nanomask Open nanopillar

Process time~ 4 weeks

Lift-off nanomask Lift off ~ 40nm oxide

Process time~ 1 week

Page /50

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.

Page /50

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

Page /50

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

Page /50

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

Page /50

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

Page /50

Three terminal spin-torque devices

• The separation of low-impedance

write process and high-impedance

read process.

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.

Page /50

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

Page /50


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.

Page /50


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

Page /50

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

Page /50

Supplement Slides

51

Page /50

Ion milling reduce the aspect ration

HSQ nanomask after ion milling

HSQ nanomaskAs-developed

52

Page /50

θ

MBMT

T┴T||

Bottom

Electrode

(Fixed)

Top

Electrode

(Free)

V+ -

Page /50

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

Page /50

Material experiment lab at Buhrman Grouup

55

Page /50

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

Page /50

Annealed FeCoB/MgO/FeCoB exhibit non-uniformity in thin film

57

CoFe-based symmetric MTJs exhibit chemical non-uniformity in the thick fixed layers.


Page /50

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.


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

hsinwei job talks

Documents

spintransfer torque

terminal spintorque

stronger effect

nonvolatile logic

nonvolatile memory

asymmetric mtjs electronic

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