4/30/2014 1

1

Charge, Spin, and Heat Transport in the Proximity of Metal/Ferromagnet Interface

Ssu-Yen Huang

National Taiwan University Johns Hopkins University

1 2 2

 Introduction

 1G and 2G Spintronic devices

 Spin current

 Spin Hall effect

 Spin Seebeck Effect (SSE)

• Entangled with anomalous Nernst effect (ANE)

• Intrinsic spin-dependent thermal transport

• Entangled with magnetic proximity effect (MPE)

• Intrinsic Spin Seebeck effect

 New MR by MPE (or Spin Hall MR)

 Summary

Outline

3

G-kW-h

5% of total electrical power

Power Consumption of Information Technology

Refreshing in “off” state

Monumental problem METI / Green IT Promotion Council (2008) E. Pop, Nano Res 3, 147 (2010)

20%

4

IC Power density approaches that of nuclear reactor S. Borkar, Intel

Can spin provide a solution ?

1. High efficiency devices 2. Reduction of heat

dissipation

5

Electronics

In the beginning, there was only electronics……..

Charge

Spin

6

Giant Magnetoresistance (GMR)

Tunnel Magnetoresistance (TMR)

Spin Transfer Torque (STT)

Grünberg/Fert

*2007 Nobel

Areal Density Spintronics

GMR AMR

Spin-valve read-head

Three important discoveries in Spintronics

10⁺⁹ increase in density 10^-8 reduction in cost 10⁺¹² bits/in²

7 metal

FM1 FM2

Field (1G) Devices

“0” “1”

Reference Storage

Spin-dependent scattering

Spin-selective tunneling

P AP

Low R High R

FM1 FM2 insulator

GMR TMR

Field Sensing & Non-Volatile Storage free

fixed

Spintronic GMR and TMR Devices

8 word / sense lines high R

low R Magnetic Tunnel Junction (MTJ)

Read Write

“1”

“0”

Advantages:

Non-volatile memory Short access time Low power consumption

Key Challenges: High density Eliminate field writing

Universal memory: speed as SRAM, density as DRAM, rewritability as flash Non-Volatile Storage: Magnetic Random Access Memory

(MRAM)

9

electrical current affects magnetic configurations

I

e^- Incident electron

M

transmitted

reflected

Large M: spin polarizer Small M: M can be rotated



torque  sin

without a magnetic field

Slonczewski, JMMM 159, L1 (1996)

Berger, PR B 54, 9353 (1996), JAP 57, 1266 (1984), JAP 49, 2156 (1978) Waintal et al., PRB 62, 12317 (2000)

Spin transfer torque

10

(1G) Field Devices

“0” “1”

Reference Storage

P AP

Low R High R

(2G) Current (STT )Devices

I > I_C

Field Sensing

& Non-Volatile Storage

Requires very large jc > 10⁶ A/cm² !!

What are new Spintronic Effects for 3G devices?

1G and 2G Spintronic Devices

11

Integer quantum Hall effect (von Klitzing, Nobel 1985) Fractional quantum Hall effect

(Stormer, Tsui, Laughlin, Nobel 1998) Spin Hall effect

Inverse spin Hall effect Magnon Hall effect Topological Hall effect maybe more…

Ordinary Hall effect (E. H. Hall, 1879)

Anomalous Hall effect (E. H. Hall, 1880)

Various Hall effects

x z y

Edwin Hall (1879, 1880) A student of Henry Rowland @ JHU

 V

:

j

j

B ( or M

)

V_y

V_y

Charge, Spin, Thermal Transport in thin films

 T

:

E VB

 T

 T

12 12

13

Spin-Orbit Coupling Lorentz Force

1879 1880 2004

Only Charge Charge + Spin Only Spin

Detect by voltage Detect by voltage Why? Detect by what ?

Definite Sign q(v  B) Definite Axis but Not Definite Sign AHE can be either sign SHE can be either sign

(Nagaosa et al.,)

Hall effect Anomalous Hall effect Spin Hall effect

F=q (E+ VB)

14

+ nucleus

E B

electron -

Electron frame

“sees” B field with gradient

The mechanism of SHE

Spin-Orbit Coupling

15 15

Direct Spin Hall

Charge Current

 Transverse Spin Imbalance (measured by what ?)

Spin Dependent Scattering

ISHE in Pt detects pure spin current Inverse Spin Hall

Spin Current

 Transverse Charge Imbalance (measured by side voltage)

Direct Spin Hall vs. Inverse Spin Hall effects

How to detect ? ¹⁶

Charge current  pure spin accumulation

Optical observation SHE in semiconductors

(Optical) Observation of Spin Hall effect

Kato et. al. Science 306, 1910 (2004)

17

Spin Caloritronics

Electronics Charge

Spin Heat

Spin Calortronics

Spin Seebeck effect

18

Spin Seebeck Effect

T S

V 

 

18

T S Vspin spin

 

K. Uchida et al., Nature, 455, 778, (2008).

) )(

( S S T

j j

j_s _ _ _{ }_{ } 

How to detect JS ? v

T J_c=0

Metals, insulator, or semiconductors

T J_c=0 J_s0

Ferromagnetic metals

up down

19

Detection of Spin Current by Inverse Spin Hall Effect ISHE in Pt (spin–orbit scattering) converts a spin current into an electromotive force ESHE







 _{SHE ISHE S}

y E D J

E

K. Uchida et al., Nature, 455, 778, (2008). 19

Asymmetric in H Sign change

Proportional to T Cold side Hot side

20

8 mm 4 mm

K. Uchida et al., Nature 455, 778 (2008); Nature Mater. 9,894 (2010); Kajiwara et al., Nature 464, 262 (2010)

Long transmission of Spin Current

6 mm 4 mm

Mystery 2: spin current (mm’s >> spin diffusion length) without dissipation ?

Mystery 1:

Conduction-electron spin current

Spin-wave spin current

20

Sign change Asymmetric in H

FM insulators FM metals

?

C. M. Jaworski et al., Nature Materials, 9, 898 (2010) 21

Spin Seebeck effect in broken FM semiconductors

21

Revision

2 : magnon-phonon drag through substrate

Adachi et al., APL 97, 252506 (2010) Jaworski et al., PRL 106, 186601 (2011) GaMnAs/GaAs Transmission of spin currents ?

  )

tanh(2 ) ( )

(

sd sd SH p m th

t t t

T T G t

E 

 







22 22

j

V

FM insulator

Pt

m

SSE in FM Insulator SSE in FM Metal, Insulator

intentional vertical _zT x

z y

_xT FM metal

intentional in-plane _xT

j

m V

FM Metal

Transverse configuration

Longitudinal configuration

Transverse (_xT)and Longitudinal (_zT) Spin Seebeck

23

Pt

Pt

Uchida et al., Nature 455, 778 (2008)

Uchida et al., Nat. Mater 9, 894 (2010)

Jaworski et al., Nat. Mater 9, 898 (2010)

Pt strip and in-plane temperature gradient _xT indicated

Pt strip detects j

In-plane



24

FM

T in-plane Huang, Wang, Lee, Kwo, and Chien,

“Intrinsic spin-dependent thermal transport,” PRL 107, 216604 (2011)

.

Intrinsic Caloritronic effect (not substrate dominated) ?

 H

Intrinsic spin Seebeck effect ?

Pt

v

Intrinsic spin-dependent thermal transport ?

v

24

25

Create in-plane gradient _xT

v v

Hot Cold

v

Higher T Lower T

Heat flow θ

H

θ H 1 2 3 4 5

1 2 3 4 5 Py

26

V  sin

Asymmetric in H

Consistent, Robust, but Strange





H=2000 Oe

26

H Py H

Py

xT _xT

e.g., opposite signals at = 90° and = 270°.

Py/Si

But this is physically impossible !

27



_xT _xT

_xT

H=2000 Oe H=2000 Oe

Reversed





28

This is anomalous Nernst effect with perpendicular



Sign change No sign change



m

(Top view) Transverse geometry, Vy

Out-of-plane



Only



Uniform Heating from substrate

29

Same ANE sign and value everywhere

In the transverse configuration (_xT) : where does _zT come from?

_zT

30

Thin film on substrate: in-plane and out-of-plane gradient

_xT

_zT due to substrate FM

intentional in-plane _xT

m

Anomalous Nernst effect: sensitive detector of zand zT E_ANE  _zT

 m

substrate

30

31

What causes out-of-plane gradient zT ?

Thermal conduction through substrate overwhelms!

Resistivity (Ωcm) > 1 >1 5x10-6 10^-6

Thermal conductivity 125 56 30 80 (W/m-K)

32

Substrate (10⁴ x thicker)

Substrate (10⁴ x thicker) Electrically Insulating Not thermally Insulating

V^- Electrical

V⁺ Electrical Current exclusively in-plane

T⁺ T^- Thermal Heat Current NOT exclusively in-plane Electric Current vs. Heat Current

33

Entanglement of ANE (due to _zT) and SSE (due to _xT)

Both along y

VANE and (VSSE )Pt

additive

m (or H)

(E_SSE )_Pt 

j

 m

_xT

Pt

v j

Spin Seebeck Effect (SSE)

FM

m

x z y

E_ANE  _zT

 m

FM

v

Anomalous Nernst Effect (ANE)

m

In transverse configuration: SSE and ANE are entangled

33 S. Y. Huang et. al, Phys. Rev. Lett. 107, 216604 (2011)

_xT

sensitive detector of _zand _zT

10⁴ x thicker !!

34

substrate FM

m

_xT

v

v

Thermal AMR (Longitudinal)

Substrate-Free sample (

)

T)

35 Longitudinal voltage:

thermal AMR

Intrinsic spin transport properties with in-plane _xT

Symmetric in H by using a substrate free sample

Transverse voltage:

Planar Nernst effect sin2M

Necessary Signatures of FM film with in-plane xT ! Vth = Vth + (Vth -Vth||)cos²M

cos²_M

sin2_M

36 36

j

V

FM insulator

Pt

m

SSE in FM Insulator SSE in FM Metal

intentional vertical _zT x

z y

_xT FM metal

intentional in-plane _xT

j

m V

FM Metal

Transverse configuration SSE + ANE

Longitudinal configuration SSE

Spin Seebec effects with in-plane _xTand out-of-plane zT

PRB 83, 224401 (2011), PRL 109, 196602 (2012), PRL 111, 187201 (2013), PRB 88, 064410 (2013), PRB 88 214304 (2013), PRB 88, 184425 (2013), and etc.

Substrate-free limit No strong evidence of SSE

Metals (Cu, Py, Pt) on Insulators (Si, YIG)

Ferromagnetic Insulator:

YIG (Y₃Fe₅O₁₂) Hall

Line

YIG -2000 -1000 0 1000 2000

-1.0 -0.5 0.0 0.5 1.0

H_T H_ll

M/MS

H(Oe) H_

Huang et al., Phys. Rev. Lett., 109, 107204 (2012) shape anisotropy

37

H_

HT

H_

Pt/Si Non-magnetic

Pt: Magnetic Proximity effects

Cu/YIG Non-magnetic

Pt/YIG AMR ! Ferromagnetic !

cos²

-1000 -500 0 500 1000

411.215 411.220 411.225 425.89 425.90 425.91

_

H (Oe) _II

Pt (10nm)/Si

R Cu (10nm)/YIG

M/Ms

Pt/YIG vs. Pt/Si

38

Thickness dependence of AMR in Py/YIG & Pt/YIG

Pt/YIG and Py has opposite t dependence

1 10

0 1 2 3

20

    



t (nm)

Py(t)/YIG

Pt(t)/YIG

YIG

39 39

QuickTime?and a decompressor are needed to see this picture.

New MR  0 at large t New MR increases at small t

QuickTime?and a decompressor are needed to see this picture.

AMR ≈ constant at large t AMR  0 at small t

All moments contribute Magnetic Proximity effect Moments near interface contribute

AMR vs. New MR

Anomalous Hall Effect in Pt/YIG

-100 -50 0 50 100

-0.002 -0.001 0.000 0.001 0.002

H (kOe) 100K50K 250K 300K 5K

RAHE()

2K

0 50 100 150 200 250 300

-0.001 0.000 0.001 0.002

R

(  )

T(K)

Pt (10nm)/YIG

_xy = ROB VH

I BZ

MZ

x z y

+ RS4M

41

FM

41

-100 -50 0 50 100

-0.02 -0.01 0.00 0.01 0.02

Pt (10nm)/YIG

RH()

300 K 250 K 100 K 50 K 5 K 2 K

H (kOe)

Thermal voltages in Pt/YIG and Pt/Si (Hall samples)

Thermal voltage New MR Share H dependence

T = 11 K

42

Pt/YIG

756.935 756.940 756.945 756.950 756.955

-0.05 0.00 0.05

-1000 -500 0 500 1000

-0.05 0.00 0.05 0.0 -1.0

0.2V

0.5V

R ()

Cu (10nm)/YIG Pt (15nm)/Si

Pt (10nm)/YIG

H(Oe) (V₁₂)_th (V₃₆)_th

0.0V 1.0

0.0 1.0

Vth(a.u.) (V12)

th

Vth(V)

_

_II (V₃₆)_th

0.0V 756.935 756.940 756.945 756.950 756.955

-0.05 0.00 0.05

-1000 -500 0 500 1000

-0.05 0.00 0.05 0.0 -1.0

0.2V

0.5V

R ()

Cu (10nm)/YIG Pt (15nm)/Si

Pt (10nm)/YIG

H(Oe) (V₁₂)_th (V₃₆)_th

0.0V 1.0

0.0 1.0

Vth(a.u.) (V12)

th

Vth(V) (V₃₆)_th

0.0V

42

Thicknesses dependence of thermal voltage

ANE only

(up to 6 µV/K) Pt/YIG Py/YIG similarly large

10 0

20 40 60

30

VthV)

t (nm) Pt/YIG

Py/Si

2 10

0 20 40 60

30

VthV)

t (nm) Pt/YIG

Py/Si

Py/YIG

2

43

-1000 -500 0 500 1000

833.8 834.0 834.2 834.4 834.6 834.8 835.0 835.2 835.4

Vth (V)

(V₁₂)_th

_II

_

4V

R ()

1V Py(10nm)/YIG

(V36)_th

H (Oe)

(E_ANE)_Py  _zT

 m

Py/YIG

43

AMR Thermal

44

Pt/Ni 0.29 μ_B

Induced magnetic moments in Pt/Ni, Pt/Co, & Pt/Fe

PRL 85, 413 (2000)

Pt/Co 0.68 μB

PRB 60, 12913 (1999)

Pt/Fe 0.5 μ_B

Phys. Status Solidi A 196, 33 (2003)

X-ray Magnetic Circular Dichroism (XMCD)

44

Magnetic proximity effect in Pt/YIG

45 45

6Å YIG

Four layers Au or Pt

All four Pt layers are significantly polarized.

Au layers are essentially unpolarized Pt(1.5nm)/YIG

300K 0.054B

20K 0.076B

Phys. Rev. Lett. 110, 147207 (2013)

X ray Circular dichroism (XMCD) Spin density

Induced magnetic moments in Pt/YIG

spin current detector: Au ? Phys. Rev. Lett. 110, 067206 (2013) 7 Pt layers

Assuming all Pt have same moment

Pt/YIG vs. Au/YIG

New MR Yes No Anomalous Hall Yes No Moment (Theory) Yes No

Moment (XMCD) Yes Not observed Spin Seebeck 50x larger

Comparison of Pt/YIG and Au/YIG

Intrinsic SSE

New Magnetoresistance

Magnetic Proximity Effect

47

Ɵ



in the plane



M/Ms

y x (I)

z



V M

y x (I)

z



V

M y

x (I) z



M

V

New MR !!

(z) ≈ ||(x)

_xy scan = _yz scan

xz scan= constant

Anisotropic MR vs. New MR

48

0 90 180 270 360

314 315 316 317 318 319

R() Py(10nm)/Si

,, (degree)

xy

AMR I(x) & MPy

New MR

(z) ≈ _T(y)

xy scan = xz scan

_yz scan = constant

_||(x) > _T(y) same xy scan ||(x) > T(y) same _xy scan

0 90 180 270 360

1728.8 1729.0 1729.2



,, (degree)

R()



yz

0 90 180 270 360

314 315 316 317 318

319 Py(10nm)/Si

xy



yz

0 90 180 270 360

1728.8 1729.0 1729.2



,, (degree)

R()

PHYSICAL REVIEW B 87, 220409(R) (2013)

48

49

Spin Hall MR in Pt/YIG: charge/spin current conversion (Nakayama et al.,) SOC metals/NO magnetic moment

Spin Hall Magnetoresistance (SMR)

The reflection J_s depends on STT ρ_||(x)> ρ_T(y) ; ρ(z) = ρ_||(x)

SMR:

 SOC metals on YIG

 No magnetic moment

 Spin current

_Pt||y axis (independent of H) x

y z

0 90 180 270 360

1728.8 1729.0 1729.2



,, (degree)

R()



yz

Nakayama et al. Phys. Rev. Lett. 110, 206601 (2013)

j

j

j

SHE ISHE

Pt/Py vs. Au/Py

50

0 90 180 270 360

37.6 37.8 38.0

xy

,,(degree)

Sheet resistance ()

Pt(3nm)/Py(5nm)/Pt(1.5nm)



yz

0 90 180 270 360

37.7 37.8 37.9 38.0

Sheet resistance ()

,,(degree) Au(3nm)/Py(5nm)/Au(1.5nm)







Magnetic Proximity can be detected in FM metal from XMCD and NEW MR

New MR AMR AMR

MR_xy=MR_xz+MR_yz

50

51

Thicknesses dependence

0 2 4 6 8 10

0 4 8 12

(10-3)

t_Py (nm) Au(3 nm)/Py(tPy)/Au(1.5 nm) H = 40 kOe

_xy

_xz

_yz

0 90 180 270 360

37.7 37.8 37.9 38.0

Sheet resistance ()

,, (degree) Au(3nm)/Py(5nm)/Au(1.5nm)

xy

xz

yz

Py(t

)

0 2 4 6 8 10

0 3 6 9 12 15

t_Py(nm)

(10-3)

xy

Pt(3 nm)/Py(t_Py)/Pt(1.5 nm) H = 40 kOe

xz

_yz

0 90 180 270 360

37.6 37.8 38.0

xy

,, (degree)

Sheet resistance ()

Pt(3nm)/Py(5nm)/Pt(1.5nm)

xz

yz

Pt/Py(t

)/Pt

0 1 2 3 4 5

0.0 0.8 1.6 2.4 3.2

t_Pt(nm)

(10-4)

xy

Pt(t_Pt)/YIG

H = 15 kOe xz

yz

0 90 180 270 360

1728.8 1729.0 1729.2

xy Pt(2.5nm)/YIG

,, (degree)

R() xz

yz

Pt(t

)/YIG

AMR AMR

New MR New MR

AMR(own Moments) + New MR (induced Moments) New MR vs. Spin Hall MR

Pt/YIG New MR Yes Yes Pt/Py AMR + New MR Yes ?

Au/Py AMR No ?

Au/YIG No new MR No ? Experimental

observation

Induced Moment ? (AHE, XMCD)

Spin Hall MR Prediction ?

Magnetic proximity effect accounts for all cases Pt/YIG_BB New MR Yes ?

52

New MR observed in cases with induced moments

53

Entanglement with anomalous Nernst (_zT)

 Transverse Spin Seebeck (_xT) (metals, semiconductors, insulators):

 Longitudinal Spin Seebeck Effect (ferromagnetic insulators):

Summary

Complicated Magnetic proximity effects in Pt Entanglement of SSE and ANE

 Pt is not an ideal spin current detector (magnetic proximity effects):

Au is better spin current detector

 NewMR in FM metals and Insulator

53

 new MR in Pt/YIG, Py/YIG, Pt/YIG_BB, and Pt/Py

 No new MR in Au/YIG and Au/Py

New MR by magnetic proximity effect or Spin Hall MR ? Intrinsic spin-dependent thermal transport on substrate free sample

54

US NSF

Taiwan NSC Acknowledgement

• Johns Hopkins University: Prof. Chia-Ling Chien, Danru Qu, Bingfeng Miao

• University of Arizona: Prof. Weigang Wang

• National Tsing Hua University: Prof. J. Raynien Kwo

• Academia Sinica: Dr. Shang-Fan Lee

• University of Delaware: Prof. John Q. Xiao

• Arizona State University: Prof. Tingyong Chen

• University of California, irvine: Prof. Ruaiqn Wu

• Chinese Academy of Science: Prof. Jianwang Cai

54