Spin dynamics in Bi2Se3/ferromagnet heterostructures

Hyunsoo Yang

Electrical and Computer Engineering, National University of Singapore

eleyang@nus.edu.sg

Outline

• Spin-orbit torque (SOT) engineering

– Heusler alloy

– Oxygen manipulation of SOT

– SOT in Co/Pd and Co/Ni multilayers

– AHE & SOT in LaAlO3/SrTiO3 oxide heterostructures

– SOT in topological insulators (Bi2Se3/ferromagnet)

2

Charge electronics Spin electronics

Information transfer = electron transfer

Information processing = processing electron flow

Charge transfer and processing energy loss is huge All spin electroinics

http://imechanica.org/taxonomy/term/118?page=3

3

Spin waves

Spin transistor

MTJs

5

Miron et al. Nature 476, 189 (2011)

• Heavy metal/ferromagnetic material/oxide layer.• Current induced magnetization switching is observed (longitudinal field

needed).• Magnetization states depend on both current and field directions.• Possible mechanisms: Rashba effect & spin Hall effect (SHE).

Spin-orbit torques (SOT)

Liu et al. PRL 109, 096602 (2012)

Perpendicularly magnetized trilayer structures

Heavy Metal (HM)

Ferromagnet(FM)

OxideStrong Rashba field arises from asymmetric interfaces

Spin Hall effect arises from HM

M

In-plane currents can switch the magnetization

FM1

MgO

FM2

STT

SOT

7

Spin Hall vs. interfacial Rashba

-Sign of spin Hall angle changes across a transition thickness of SiO2 (t = 1.5 nm) -Cannot be understood by spin Hall physics suggest the role of interface

8Nat. Nanotech. 10, 333 (2015)

0SH

0SH

Reverse switching polarity by oxygen engineering

Spin-orbit torque switching currents

19

S FM HMc eff

SH

e M t AI H

-No damping term great flexibility for choosing FM, high speed

-No spin polarization term no need to use MgO

-Can use thick MgO eliminate MgO breakdown issue

-Large spin Hall angle (SH) or effective field (Heff) is the key

K.J.Lee, APL (2013)

Pinned FM

spacer

Free FM

I1

I2

SOT

STT

2 Sc eff

e M VI HP

STT

SOT

Heff = ħSH|je|/(2|e|MStF)

Large spin Hall angles from various materials

20

FePt/Au spin Hall angle (SH) = 0.1(Takanashi group)

Nat. Mater. 7, 125 (2008)

Appl. Phys. Lett. 101, 122404 (2012)

β-W SH = 0.3 (Cornell)

CuBi SH = -0.24 (Otani group)

Phys. Rev. Lett. 109, 156602 (2012)

Co/Pd multilayer SH = 4 (NUS)

Phys. Rev. Lett. 111, 246602 (2013)

Spin orbit torque from Heusler alloy

• Perpendicular anisotropy CFAS• Both HL and HT exist with a large SH

22

Pt/Co2FeAl0.5Si0.5 (0.8 nm)/MgO

Appl. Phys. Lett. 107, 022405 (2015)

30

Single magnetic layer vs. magnetic multilayer systems

Pt

cap

Substrate

MgO

• Co/Pd or Co/Ni interfaces contribute to PMA.

• Lower damping (~0.01)• Higher spin polarization (~80%)• Good thermal stability [=KuV/(kBT)]

• Perpendicular magnetic anisotropy (PMA) originates from Pt/FM interface.

• Low damping (~0.02)• Spin polarization (~50%)• Low thermal stability (not enough

volume)

PMA

A better choice for structural engineering

31

Spin orbit torques in Co/Pd multilayers

22 times

Ta (4nm)

Ru (20nm)

Pd (0.7nm)Co (0.2nm)

Pd (0.7nm)Co (0.2nm)Ta (4nm)

Phys. Rev. Lett. 111, 246602 (2013)

Structural asymmetry can be added up

-Two successive Co/Pd and Pd/Co interfaces are structurally dissimilar.-Lattice mismatch (9%) between Pd and Co

33

TEM data

Pd (0.7nm)

Co (0.2nm)

Pd (0.7nm)

Co (0.2nm)

Ta (4nm)

Maesaka, IEEE Trans. Magn. 38, 2676 (2002)

LAO/STO – 2DEG formation

34

LaAlO3

SrTiO3

SrTiO3 (Insulator 3.2 eV) LaAlO3 (Insulator 5.6 eV)

LAO (2 nm)2 DEGSTO

LaAlO3 grown on TiO2 terminated SrTiO3 (100)

2 DEG formed inside the STO side

Nat. Comm. 4:1838

STO

LAO (2 nm)2 DEG

Idc + Iac

LIA-1

LIA-2

I

0 90 180 270 360

-25

-20

-15

-10

-5

0

5 Data Fit

AM

R (

)

Angle (deg)

AMR measurements at H = 9 T , T = 4 K

Magnetism in LaAlO3/SrTiO3 heterostructures

In-plane angular measurements

RXX = a0+a1cos2(+) + a2cos4(+)

a0, a1, a2 are constants

36Appl. Phys. Lett. 105, 162405 (2014)

2 2 2 2 cosEFF A R A RH H H H H HR (+I) = 1.26 T HR (-I) = -1.48 T

HR HA

HEFF

αβ

Idc

Asymmetric spin-orbit fields

2 40 1 2H cos H cosXX EFF EFFR b b b

b0, b1, b2 are constants

HR, HA HEFF are Rashba, applied and effective fields

0 90 180 270 3602220

2250

2280

2310

Idc = +250 AIdc = -250 A

Angle (deg)

AM

R (

)

1635

1650

1665

1680

AM

R (

)

37Appl. Phys. Lett. 105, 162405 (2014)

Current induced spin-orbit fields in 2DEG

Assuming thickness of 2DEG t2DEG = 7 nm

current density = 7.14×108 A/m2

2.35 T @ 200 µA 32.9 Tesla/106 A/cm2

Nat. Mater. 7, 621 (2008)

0 100 200

0

1

2

3

HR (T

)

Idc (A)

H = +8 T H = -8 TIac = 20 A

The highest current induced torque reported in metallic system is only 0.5 T.PRL 111, 246602 (2013)

3 *2/R soeH m Hso = 1.48 T, αR = 12 meVÅ, spin splitting ∆ = 3 meV (~30 T)

cf. Co/Pd multilayer αR =360 meVÅ

38Appl. Phys. Lett. 105, 162405 (2014)

TiO2-terminated 001 STO substrates

Magnetic dipoles from LAO/STO

Nat. Phys. 7, 771 (2011)

10 u.c. LaAlO3

Magnetism imaging in LAO/STO by Squid

Squid magnetometry is sensitive to the stray field.

n-type similar to our samples

Squid data

MFM images

12 unit cell LAO films on TiO2-terminated (001) STO substrates

Nat. Comm. 5, 5019 (2014).

Ferromagnetism arises at lower gate voltages when the 2DEG is depleted.

Magnetism imaging in LAO/STO structures by MFM

Microscopic mechanism of the anomalous Hall effect

47

1. Intrinsic

2. Extrinsic

Berry Phase Electrons have an anomalous velocity perpendicular to the electric field related to their Berry’s phase curvature

Skew Asymmetric scattering due to the effective spin-orbit coupling of the electron or the impurity.

Side jump deflection by the oppositeelectric fields experienced upon approaching and leaving an impurity.

Topological insulators (TIs)

arXiv:1304.5693

ARPES spectra showing alinear band structure of thesurface states on a 3D TI

Physics 3, 62 (2010)

Nobel Symposium 2010, Shoucheng Zhang

Spin polarized surface currents Linear dispersion

66

J. Appl. Phys. 108, 113925 (2010)

Spin pumping

A schematic describing spin pumping

Enhancement of Gilbert damping

Spin pumping is a process in which a precessing magnetization induces spin currents into an adjacent magnetic layer

67

Tserkovnyak et al., Phys. Rev. Lett. 88, 117601 (2002)

Experimental setup

- Signal generator to excite magnetization dynamics in NiFe through a coplanar waveguide

- Voltmeter to measure spin pumping induced ISHE

- Vector network analyzer for FMR measurements

V

Magnetization oscillation provides high density spin currents into TI and a transverse voltage is detected in TI spin detector.

69Phys. Rev. B 90, 094403 (2014)

Characterization of Bi2Se3

Resistivity of 20 QL Bi2Se3 Carrier concentration of 20 QL Bi2Se3

Show a typical Bi2Se3 feature of saturation below 30 K.

Appl. Phys. Lett. 103, 213114 (2013)70

0 50 100 150 200 250 3008

9

10

11

12

13

T (K)n 2D

(1013

/cm

2 )

1 10 100450

500

550

600

650

700

750

T(K)

(c

m)

1 QL 1 nm

FMR measurements

-1 0 1 2

0.0

0.2

0.4

0.6

0.8

1.0

Bi2Se3/Py Py

FMR

sig

nal (

norm

.)

f - f0 (GHz)3 4 5 6 7

20

40

60

80Py linewidth linear fit for Py Bi2Se3/Py linewidth linear fit for Bi2Se3/Py

Line

wid

th (O

e)Frequency (GHz)

0 4H H f

rg = 1.5141019 m-2

Increase in linewidth is indicative of spin pumping.

4 Py Brg M d g

72Phys. Rev. B 90, 094403 (2014)

ISHE measurements

θsh = 0.01λsf = 6.2 nm

2 2 2 2 2

2 2 2 2

42 tanh28 4

r rf sfISHE BiSesh BiSe

BiSe sf

g h M MV dewdR dM

73Phys. Rev. B 90, 094403 (2014)

0.0 0.2 0.4 0.6-2

0

2

4

6

8

10 3 GHz 4 GHz 5 GHz 6 GHz fitting

VIS

HE(

V)

B (kOe)

0 5 10 15 20 25 30 35 40

0.10

0.15

0.20

0.25

0.30

Data Fitting

VIS

HE/R

(V

/)

Bi2Se3 thickness (nm)

(c)

VISHE ~

R is resistance of the filmJs is induced spin currentθSH is spin Hall angle

SHsJR

Weak anti-localization in Bi2Se3

2

22 2

1 ln4 2 4

eG B Bh eL B eL B

222 21 3 124 48 4 3q

so e so

e eh B B h B B

20c H G

c q

24so soB el

24e eB el

Taking le = 10 nm, spin orbit length lso was found to be 6.9 nm

lso ~ λsf suggest that spin-orbit coupling is dominant source of spin scattering

75Phys. Rev. B 90, 094403 (2014)

-8 -4 0 4 8-0.06

-0.04

-0.02

0.00Data HLN fitting

G (e

2 /h)

Magnetic field (T)

Temperature dependence

• Both spin Hall angle and spin diffusion length increase at low temperature

• θsh = 0.022 and λsf = 9.5 nm at 15 K

76Phys. Rev. B 90, 094403 (2014)

0 50 100 150 200 250 300

0.8

1.2

1.6

2.0

2.4

Temperature (K)

sh (%

)

5

6

7

8

9

10

sf (n

m)

Bulk spin relaxation time in Bi2Se3

79

0 1 2 3 4 5

0

5

10

15

20 +

-

Ker

r rot

atio

n (a

.u.)

Delay (ps)0 1 2 3 4 5

0

5

10

+ -- (a

.u.)

Delay (ps)

- Signal sensitive to bulk due to large penetration depth of light - Oscillation frequency is 2.13 THz from coherent vibrations of the A1g longitudinal optical phonons of Bi2Se3- Exponentially decay with a characteristic time of 1.3 ps

Time-resolved magneto optical Kerr effect

Jean Besbas et al. (under review)

No spin momentum locking

0 50 100 150 200 250 300

0.8

1.2

1.6

2.0

2.4

Surface (sh1) Bulk (sh2)

sh

(%)

Temperature (K)0 50 100 150 200 250 300

6

8

10

12

Temperature (K)

sf

(nm

)

(a) (b)

- Assumed spin Hall angle at opposite surfaces was taken to be of opposite signs.

- Spin Hall angle does not show any clear distinction between the surface and bulk value

- Momentum locking signature is not detected.

80Phys. Rev. B 90, 094403 (2014)

Comparison with other reports

Nature 511, 449 (2014) Nat. Mater. 13, 699 (2014)

Spin torque ferromagnetic resonance measurements θSH = 2.0 – 3.5

Magnetization switching by current induced spin orbit torque θSH = 140 – 425

In these experiments, a charge current flows through the TI material, unlike ours.

81

-200 0 200 400

-400

-200

0

200

400 CFB 5 nm CFB 4 nm CFB 3 nm CFB 2 nm CFB 1.5 nm

M/a

rea

(em

u/cm

2 )

H (Oe)

1 2 3 4 50100200300400

M

/are

a

CFB (nm)

MDL = 1.36 nm

1 10 100

200

300

400

T(K)

Rxx

(

)

Bi2Se3 20 QL

Properties of Bi2Se3 and Bi2Se3/Co40Fe40B20

20 QL Bi2Se3 films on Al2O3 (0001) by MBE.

A typical feature of resistivity saturation below 30 K for Bi2Se3.

The Co40Fe40B20 (CFB) dead layer ~1.36 nm.

83Wang et al., PRL 114, 257202 (2015)

-1000 0 1000-4

-2

0

2

4

6 50 K 20 K

f = 8 GHz

300 K 200 K 100 K

V (

V)

H (Oe)

ST-FMR measurement of Bi2Se3/CoFeB

ST-FMR measurements with a lock-in amplifier at H = 35. ST-FMR signal (Vmix) can be fitted by a sum of symmetric and

antisymmetric Lorentzian functions:

mix s sym ext a asym ext( ) ( )V V F H V F H Vs: in-plane torque || on CFBVa: total out-of-plane torque

PRL 114, 257202 (2015) 84

Two analysis methods1st method: from Vs/Va

2nd method: from only Vs and only Va separately

s s s/ /J E M t E s /

rfs H H sym ext( /4)( / ) (1/ ) ( )I cos dR dV F H

1 2rf H H Oe 0 eff ext asym ea xt( /4)( / )( ){[1 ( / )] / } ( )/I cos d d FV R M H H

s a1 2

0 s eff ext( )( / )[1+(4 / )]/ /e M td M HV V

85

s / s s s/ /J E M t E

Mellnik et al., Nature 511, 449 (2014)

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

Wang et al., Phys. Rev. Lett. 114, 257202 (2015)

Thickness of Bi2Se3

If only Oersted field induced out-of-plane torque (Oe) contributes to Va

In-plane spin-orbit torque ratio in Bi2Se3/CoFeB

0 50 100 150 200 250 3000.0

0.1

0.2

0.3

0.4 D 1 D 2 D 3

By Vs Only D 1 D 2 D 3

T (K)

||

By Vs/Va

|| (ǁ) increases steeply and nonlinearly to ~ 0.42 at low temperature andcould be almost 10 times larger than that at 300 K.

The polarization direction of || is consistent with spin-momentum-locked TSS. || by 1st and 2nd methods shows a significant difference below ~ 50 K, other

out-of-plane torque may contribution besides Oe.

87Wang et al., PRL 114, 257202 (2015)

1 10 100

200

300

400

T(K)

Rxx

(

)

Bi2Se3 20 QL

Pt

88

Wang et al., APL 105, 152412 (2014)

Niimi et al., PRL 106, 126601 (2011)

CuIr

Hao et al., PRB 91, 224413 (2015)

Ta

Spin Hall mechanism from Bi2Se3 bulk is not the main mechanism for the

nonlinear increase of || (ǁ) in Bi2Se3.

The direction of spin polarization is consistent with TSS of TIs.

In-plane spin-orbit torque (ratio) in Bi2Se3

0 50 100 150 200 250 300

0

50

100

150

|| (O

e)

T (K)

Pt

Wang et al., PRL 114, 257202 (2015)

0 50 100 150 200 250 3000

1

2

3

T (K)

(Oe)

D 1 D 2 D 3

0 50 100 150 200 250 3000.0

0.1

0.2

0.3

0.4

T (K)

D 1 D 2 D 3

() also increases at low temperature similar to || (ǁ). Rashba-split state in 2DEG of Bi2Se3 is not the main mechanism for . Hexagonal warping in the TSS of Bi2Se3 can account for ().

Out-of-plane spin-orbit torque ratio in Bi2Se3/CoFeB

90Wang et al., PRL 114, 257202 (2015)

91

Sci. Rep. 4, 4491 (2014)

T R / (z )ˆH k

PRB 77, 125344 (2008)

GaAs/AlGaAs InSb/InAlSb

Metal & 2DEG in semiconductor:

JPCM 23 035801 (2011)

Ta

Nat. Mater. 9 230 (2010)

Previous Rashba reports showed a smaller effect at low temperatures.

But, we observed larger effects ( ) at low temperatures.

Rashba effect might not be the main mechanism for and .

Is Rashba effect responsible for out-of-plane spin-orbit torque?

Out-of-plane torque in Bi2Se3

92

Wang et al., PRL 107, 207602 (2011)

Bi2Se3

Nomura et al., PRB 89, 045134 (2014)

-Recent reports showed there is substantial out-of-plane spin polarization due to

Hexagonal warping.

-Hexagonal warping in the TSS of Bi2Se3 can account for ().

Estimation of || from topological surface states (TSS)

0 50 100 150 200 250 300

0.6

0.9

1.2

0 50 100 150 200 250 3008

9

10

11

12

13

T (K)

n 2D (1

013/c

m2 )

Bi2Se3

I B

iSe/I C

FB

T (K)

By estimating ITSS:I2DEG:Ibulk, || from only TSS at low temperature is ~ 2.1 ±0.39 (with bulk contribution) ~ 1.62 ± 0.18 (without bulk contribution)

2D TSS 2DEG bulk= 2 + 2 +n n n n d

95Wang et al., PRL 114, 257202 (2015)

kF-TSS ~ 0.14 – 0.17 Å-1

kF-2DEG ~ 0.1 – 0.12 Å-1

If we assume TSS thickness ~ 1 nm, the 2D spin orbit torque efficiency SOT ~ 0.8-1.05 nm. IREE ~ 0.2-0.33 nm in Ag/Bi interface [Nat. Commun. 4, 2944 (2013)]

nTSS ~ 1.56 − 2.3×1013 cm-2

n2DEG ~ 1.59 − 2.3×1013 cm-2

nbulk ~ 1 − 3.1×1019 cm-3

(~ 1 − 3.1×1013 cm-2)kF-bulk ~ 0.066 – 0.097 Å-1

kF-bulk < kF-2DEG < kF-TSS and n2DEG < 2 nTSS

Exotic spin Hall angles from topological insulators

97

spin Hall angle (SH) = 2~3.5ST-FMR (Cornell)

Nature 511, 449 (2014)

PRB 90, 094403 (2014)

SH = 2 (low temp)ST-FMR (NUS)

SH = 140-425 (low temp)spin-orbit switching (UCLA)

Nat. Mater. 13, 699 (2014)

PRL 113, 196601 (2014)

SH = 0.01Spin-pumping (Tohoku)

SH = 0.01Spin-pumping (NUS)

PRL 114, 257202 (2015)

SH = 0.01-0.4Spin-pumping (Minnesota)

Nano Lett 15, 7126 (2015)

0.0 0.2 0.40

1

2

3

4

W

Ta

W

Pt

Ta

Pt

Spin

orb

it to

rque

(kO

e pe

r 108 A

/cm

2 )

Pd

30

20

10

0

Switc

hing

cur

rent

den

sity

(MA/

cm2 )

SH

BiSbTe (UCLA)

BiSbTe (UCLA)8.9E4 A/cm2

Co/Pd (NUS)

LAO/STO (NUS)

θSH

5 kOe

480~1460 T3000 T

4.4 140

Bi2Se3 (Cornell, NUS)

=2

2

113

Open questions

• Why is the spin Hall angle so different from spin pumping, ST-

FMR, and optical imaging measurements?

– Spin pumping, photovoltage – bulk dominant

– ST-FMR – surface dominant

• Are spin currents from TI big enough to switch 3d

ferromagnets?

• Is there any compensation of spin orbit torques from the

surface states and Rashba 2DEG?

• Can we realize a room temperature spin orbit torque TI

devices? 116

117

Coexistence of surface states and Rashba bands

Dr. Xuepeng Qiu Li Ming Loong

T. Venkatesan (NUS)Seah Oh (Rutgers)Aurelien Manchon (KAUST)K-J. Lee (Korea Univ.)

Dr. K. NarayanapillaiDr. Yi Wang Praveen Deorani Jiawei Yu

118