MSE 7025 Magnetic Materials (and Spintronics)cfpai/MSE7025/MSE7025_Lecture14_STT.pdfMSE 7025 Magnetic Materials (and Spintronics) Chi-Feng Pai [email protected] Lecture 14: Spin Transfer

MSE 7025 Magnetic Materials

(and Spintronics)

Chi-Feng Pai [email protected]

Lecture 14: Spin Transfer Torque And the future of spintronics research

mailto:[email protected]

Course Outline • Time Table

Week Date Lecture

1 Feb 24 Introduction

2 March 2 Magnetic units and basic E&M

3 March 9 Magnetization: From classical to quantum

4 March 16 No class (APS March Meeting, Baltimore)

5 March 23 Category of magnetism

6 March 30 From atom to atoms: Interactions I (oxides)

7 April 6 From atom to atoms: Interactions II (metals)

8 April 13 Magnetic anisotropy

9 April 20 Mid-term exam

10 April 27 Domain and domain walls

Course Outline • Time Table

Week Date Lecture

11 May 4 Magnetization process (SW or Kondorsky)

12 May 11 Characterization: VSM, MOKE

13 May 18 Characterization: FMR

14 May 25 Transport measurements in materials I: Hall effect

15 June 1 Transport measurements in materials II: MR

16 June 8 MRAM: TMR and spin transfer torque

17 June 15 Spin transfer torque

18 June 22 Final exam

From spin transfer torque, the spin Hall torque, to

spin-orbit torque: An Experimentalist’s Point

of View

April 11th, 2016 NTU-IAM Seminar Talk

Chi-Feng Pai

(modified from)

Spintronics: The beginning

• Giant magnetoresistance (GMR)

• Tunneling magnetoresistance (TMR)

>100%

~3%

( )

AP P

A P

R R

R


• Spin valve

S. Yuasa et al., Nature Mater. 3 868 (2004)

• Magnetic tunnel junction (MTJ)

TMR~180%

(top view)


• HDD read-head sensors

Spin transfer torque

• 4s (itinerant)-3d (localized) s-d interaction

L. Berger, Phys. Rev. B 54, 9353 (1996) Ya. B. Bazaliy, B.A. Jones, and S.C. Zhang, Phys. Rev. B 57, R3213 (1998)

e-

ˆsdJ s S r


• Landau-Lifshitz-Gilbert-Slonczewski (LLGS) eqn

L. Berger, Phys. Rev. B 54, 9353 (1996) J. Slonczewski, J. Magn. Magn. Mater. 159, L1 (1996) J. Z. Sun, Phys. Rev. B 62, 570 (2000)

0

( )2

eff

S

dm dm PIm H m m m

dt dt e M V


• Early experimental evidence

E. B. Myers et .al. Science 285, 867 (1999)



E. B. Myers et .al. Science 285, 867 (1999)



J. A. Katine et .al. PRL 84, 3419 (2000)



J. A. Katine et .al. PRL 84, 3419 (2000) Current citation: ~1,700 times

Spin torque switching

• Current induced ST switching

Spin valve MTJ

6 7 2

0 0

2( / 2) ~ 10 10 A/cmC S C eff

eJ M t H M

P

Spin torque switching

• Current induced field vs. torque switching

0CJ

Spin-torque Field

0CH J r

Spin torque microwave generation

• Current induced torque dynamics – Spin-torque nano-oscillator (STNO)

A. M. Deac et .al. Nat. Phys. 4, 803 (2008)

Spin torque devices

• Building blocks

Spin torque devices

• Building blocks

STT-MRAM and spin logic

• But then again, what industry cares about is the possible application in non-volatile memory (NVM)

• Or maybe all spin-logic (ASL)? Nat. Nanotech. 5, 266 (2010)

Spin Hall effect induced STT

• Can we use a pure spin current to generate spin-torque instead of using a spin-polarized current?

• LLGS equation with a spin-polarized current

• LLGS equation for the spin Hall effect induced spin current

Replace spin-polarization by the spin Hall angle!

eff

0

( )2 S

dm dm PIm H m m m

dt dt e M V

eff

0

( )2

SH

S

Idm dmm H m m m

dt dt e M V

Spin Hall effect induced STT

• Can we use a pure spin current to generate spin-torque instead of using a spin-polarized current?

• Critical switching current in a (in-plane-magnetized) F/I/F MTJ

• If we use the spin Hall effect…

0 0 eff

2( / 2)C S C

eJ M t H M

P

0 0 eff

2( / 2)C S C

SH

eJ M t H M

Replace spin-polarization by the spin Hall angle!

The spin Hall effect (revisit)

e-

e-

e-

e-

Js Je

J. E. Hirsch, Phys. Rev. Lett. 83 1834 (1999)

ˆS SH eJ J

M. I. Dyakonov and V. I. Perel, JETP 13 467 (1971)

Spin-orbit interaction

The spin Hall angle

/SH s eJ J

The SHE in transition metals

• The spin Hall conductivity calculated for 4d 5d elements

• ab initio calculation: θSH(Ta)<0 and θSH(Pt)>0

for highly resistive case, θSH(Ta) can be large

Tanaka, T. et al, Phys. Rev. B 77, 165117 (2008)

Spin Torque-Ferromagnetic Resonance

mix RF RFV I R

0 0 42

efff H H M

Spin current in plane torque τST symmetric peak

Oersted field perpendicular torque τH antisymmetric peak

0

( )2

eff e SH

S

dm dmm H m J m m

dt dt e M t

Spin Torque-Ferromagnetic Resonance

Spin current in plane torque τST symmetric peak

Oersted field perpendicular torque τH antisymmetric peak

/ / SS J RF eA H J

+

ˆ ˆ ˆm m m H

mix RF RFV I R

ST-FMR results of Ta and Pt

0 40 80 120 160-2

0

2

4

f = 9 GHz

CoFeB (4nm)/Ta (8nm)

Vm

ix (V

)

Bext

(mT)

0 40 80 120 160-20

-10

0

10

f = 9 GHz

CoFeB (3 nm)/ Pt (6 nm)

Vm

ix (V)

Bext

(mT)

CoFeB/β-Ta CoFeB/Pt

+ +

SS J SS JeA J eA J

ST-FMR results of Ta and Pt

0 40 80 120 160-20

-10

0

10

f = 9 GHz

CoFeB (3 nm)/ Pt (6 nm)

Vm

ix (V)

Bext

(mT)

+ +

SS J SS JeA J eA J

β-Ta 0.15SH Pt 0.07SH

0 40 80 120 160-2

0

2

4

f = 9 GHz

CoFeB (4nm)/Ta (8nm)

Vm

ix (V

)

Bext

(mT)

Three-terminal devices


External field induced switching

-15 -10 -5 0 5 10 15 2060

70

80

90

100

-20 0 2060

70

80

90

100

dV

/dI (k)

Bext

(mT)

dV

/dI

(k

)

Bext

(mT)

IDC

=0 mA

DC current induced SHE-ST switching

-1.5 -1.0 -0.5 0.0 0.5 1.0 1.5

60

80

100 B

ext = -3.5 mT

dV

/dI

(k

)

IDC

(mA)


-1.5 -1.0 -0.5 0.0 0.5 1.0 1.5

60

80

100 B

ext = -3.5 mT

dV

/dI

(k

)

IDC

(mA)-Ta 0.12 0.04SH

1E-3 0.01 0.1 1-1.5

-1.0

-0.5

0.0

0.5

1.0

1.5

IC AP to P

IC P to AP

Sw

itch

ing

Cu

rre

nt

(mA

)

Ramp Rate (mA/s)

DC current induced SHE-ST switching

0 0 eff

2( / 2)C S C

SH

eJ M t H M

5d transition metals

• Significant spin-orbit interactions

Ta W Re Os Ir Pt Au

Nb Mo Tc Ru Rh Pd Ag 4d

5d 73 74 75 76 77 78 79

41 42 43 44 45 46 47

Pt 0.068 0.005SH β-Ta 0.15SH

Liu et al., Science 336, 555 (2012)

(ST-FMR, ST-switchings) (ST-FMR, ST-switchings)

Liu et al., Phys. Rev. Lett. 106 036601 (2011)

Liu et al., Phys. Rev. Lett. 109 096602 (2012)

Tungsten (W)

• α-W

– BCC

– Conductive (20-40 μΩ·cm)

• β-W

– A15 cubic

– Resistive (150-350 μΩ·cm)

Sputtered W films

• Thickness-dependent resistivity and phase

4 8 12 16 200

50

100

150

200

250

300

(cm

)

W thickness (nm)

30 35 40 45 50

1000

1500

2000

2500

3000

3500

4000

-W (200)

-W (110)

-W (211)

-W (210)/-W (110)

Inte

nsity (

co

un

ts)

2(degree)

W(6nm)

W(8nm)

-W (200)

XRD

Pai et al, Appl. Phys. Lett. 101, 122404 (2012)

Sputtered W films

• Thickness-dependent resistivity and phase – STEM imaging

– EELS composition survey

Chi-Feng Pai et al (unpublished data)

W Fe

Hf

Mg

Ta

50 100 150

-160

-120

-80

-40

0

40

Py(5nm)/W(4nm)

Py(5nm)/W(10nm)

Vm

ix (V

)

Bext

(mT)

ST-FMR from W(4-20nm)/Py(5nm)

9 GHzf


4 8 12 16 20

0.05

0.10

0.15

0.20

0.25

0.30

|S

H|

W Thickness (nm)

ST-FMR from W(4-20nm)/Py(5nm)

-W ~ 0.3SH



-W 0.33 0.06SH

Pai et al, Appl. Phys. Lett. 101, 122404 (2012)

0 0 eff

2( / 2)C S C

SH

eJ M t H M


• 3-terminal devices with different W thicknesses

Thickness (nm) Resistivity (μΩ·cm) Phase |θSH|

5.2 260 β 0.33

6.2 80 α+β 0.18

15 21 α <0.07

In agreement with ST-FMR results

The spin Hall angle is phase/resistivity dependent

The ever-growing spin Hall angle

• Reported spin Hall angles or spin Hall efficiencies






• What’s next?

– Topological insulators?

– Bi2Se3 “effective” spin Hall angle ~ 80%

Liu et al, Phys. Rev. B 91, 235437 (2015)


• What’s next?


– Bi2Se3 “effective” spin Hall angle ~ 200-350%

Mellnik et al, Nature 511, 449 (2014)


• What’s next?


– (Bi0.5Sb0.5)2Te3

“effective” spin Hall angle ~ 14000%-42500% !!!

Mellnik et al, Nature 511, 449 (2014)


• What’s next?

– 2D materials? WTe2 “effective” spin Hall angle ??

MacNeill et al, arXiv (2016)

Documents

MSE 7025 Magnetic Materials (and Spintronics)cfpai/MSE7025/MSE7025_Lecture14_STT.pdfMSE 7025 Magnetic Materials (and Spintronics) Chi-Feng Pai [email protected] Lecture 14: Spin Transfer