spin pumping and brasília inverse edelstein effect rio in

Universidade Federal de Pernambuco Departamento de Física Recife, PE, Brazil [email protected]

Spin pumping and

inverse Edelstein effect

in YIG/graphene

Sergio M. Rezende

Spin Orbit Coupling and Topology in Low-D

Spetses, Greece

July 1st, 2016

Brasília

São Paulo

Rio

“Earlier” phenomena in magnetic multilayers (t<2000)

• Single films: Surface and induced anisotropies, magnetic damping

• FM/AF exchange bias effect

• Interlayer exchange coupling in FM/NM/FM (FM and AF coupling)

• Giant magnetoresistance-GMR

• Spin valve phenomena

• Spin transfer torque (free-layer/spacer/fixed-layer)

• Magnetic tunneling

Spintronics

“New areas” of Spintronics

• Spin current phenomena: Spin pumping and Spin Hall effects

• Spin Caloritronics: Spin Seebeck and spin Peltier effects

• Insulator-based spintronics

• Antiferromagnetic spintronics

• Spin orbitronics; 2D-Spintronics

SMR 2

Rapid development of Spintronics

Continuing discovery of new phenomena involving

charge, spin and heat currents has boosted the

development of SPINTRONICS, offering good

opportunities for basic research and for applications

Items published per year Citations per year

I. Spin current in metals Generation: Spin Hall Effects

Spin Pumping Effect

Spin Seebeck Effect

Detection: Inverse Spin Hall effect (ISHE)

II. Spin pumping experiments with YIG/metal

III. Spin to charge current conversion in YIG/Gr

Sample preparation and characterization

Spin pumping experiments

Interpretation with inverse Edelstein effect

Outline

SMR 4

Charge flux without spin flux

Metal layer

Charge and spin currents in metals

Spin current IS = I↑ - I↓ = 0

Charge current IC = I↑+I↓ ≠ 0

SMR 5 JC=charge/time.area

Charge current IC = I↑+I↓ = 0

Spin current IS = I↑ - I↓ ≠ 0

Spin flux without charge flux

Metal layer

Charge and spin currents in metals

JS=spin (angular momentum)/time.area SMR 6

I Charge current

SMR 7

Metal (NM) with strong spin-orbit coupling (Pt, Pd)

H

Spin Hall effect (SHE)

Theoretical prediction: Dyakonov & Perel 1971; Hirsch 1999

Intrinsic and extrinsic souces

Charge current spin current

Accumulation of spin-up

Accumulation of spin-down

Spin current JS

Charge current JC

Metal with strong spin-orbit coupling

(Pt, Pd)

Electrons with opposite spins in JC are deflected to opposite sides creating spin current

S-O US-OµL×S

H

Spin Hall effect (SHE)

spin polarization

spin Hall angle

05.0~SHin Pt Courtesy: Antonio Azevedo SMR 8

FM

)(tm

Js

Spin current

2002- A. Brataas, Y. Tserkovnyak,

G.E Bauer, B. I. Halperin

Precessing spins in FM layer pump

spin current (angular momentum)

into NM layer

NM

Spin pumping effect (SPE)

Spin current Spin precession SPE

t

MM

M

gJ r

S

24

Spin mixing conductance

STT

Inverse of the spin transfer torque by spin current SMR 9

SMR 10

Ferromagnetic Resonance (FMR)

Permalloy film (60 nm)

0.6 0.7 0.8 0.9 1.0

-0.6

-0.4

-0.2

0.0

0.2

0.4

0.6

Sig

na

l d

P/d

H (

arb

. u

nits)

Magnetic field H (kOe)

f = 8.6 GHz

8.2/ Bg

2/1

0 )]4([ effMHH

GHz/kOe

H

)(th

M

0

Microwave

cavity with

sample

Magnet

Detector Microwave generator

Processor M

b- Spin pumping: theory

Spin current density due to precessing magnetization at the interface

dt

MdMg

MJ S

)0()0(

4)0(

2

FM

NM

Effects of spin pumping: 1-Magnetic damping

Spin current = angular momentum/time= TORQUE

Enhanced damping

due to spin pumping

Flow of angular momentum out of the FM causes relaxation of M

SMR 11

SMR 12

Spin pumping damping in Permalloy/Pt

Py

0 10 20 30 40 50 60 70 800

10

20

30

40

50

60

70

80

Lin

ew

idth

H

(O

e)

Thickness tPy

(nm)

Spin pumping damping is known to be effective in very thin FM films

Observed by Misukami, Ando & Miyasaki (2002)

2-m scattering by surface roughness( ) Arias & Mills (1998)

SMR 13

Spin pumping damping in Permalloy/Pt

Py

Py/Pt (10 nm)

0 10 20 30 40 50 60 70 800

10

20

30

40

50

60

70

80

Lin

ew

idth

H

(O

e)

Thickness tPy

(nm)

Spin pumping damping is known to be effective in very thin FM films

Observed by Misukami, Ando & Miyasaki (2002)

ΔV

Did not explain the spin-to-charge current conversion

Electric detection of Ferromagnetic Resonance (FMR)

SMR 14

Effects of spin pumping: 2-Voltage generation

FMR signal

SMR 15

Explains the generation of VDC in terms of spin-pumping and the Inverse Spin Hall Effect (ISHE)

Effects of spin pumping: 2-Voltage generation

SMR 16

V ≠ 0

Charge current JC Spin current JS

Inverse Spin Hall effect (ISHE) [Hirsch 99, Saitoh 2006]

S-O

US-OµL×S

Courtesy: Antonio Azevedo

Spin current Charge current

(Onsager reciprocal of SHE)

Spin Hall effects: Good reviews

FMR-Spin pumping in FM/NM bilayer

SMR 18

Spin current generates charge current by ISHE

H M

V

JS

Pumped spin current

JC

x

y

z

Peak voltage

SMR 19

AC + DC voltage

FM (Ni81Fe19)

NM (Pt)

Si

Ag

- - - + + +

m(t)

Spin current generates charge current by ISHE

SJ

Contributions to DC voltage Spin pumping + ISHE Anisotropic magnetoresistance (classical induction)

FMR-Spin pumping in FM/NM bilayer

DC voltage generated by FMR

0

2

0 cos)(),( rfeffSHSP hHLgHV

00

2

0 sin2sinsin)('cos)(),( rfIIAMR hHLHLHV

SPAMR VVV

H

0

NM FM

)(th

0.70 0.75 0.80 0.85 0.900.0

0.5

1.0

1.5

2.0

Sig

na

l d

P/d

H (a

rb. u

nits)

Magnetic field H (kOe) 0.70 0.75 0.80 0.85 0.90

-0.6

-0.4

-0.2

0.0

0.2

0.4

0.6

Sig

na

l d

P/d

H (a

rb. u

nits)


Typical in-plane dependence of the dc voltage

DC voltage generated by FMR

0

3

6

9

12

V (

V)

0

3

6

9

12

0

3

6

9

12

V (

V)

0

3

6

9

12

0.6 0.7 0.8 0.9 1.0

-12

-9

-6

-3

0

V (

V)

Magnetic Field (kOe)

0.6 0.7 0.8 0.9 1.0

-12

-9

-6

-3

0


Symmetric for = 0o and 180o and changes polarity

0

Py (18.5 nm)/Pt (6.0 nm)

0.6 0.7 0.8 0.9 1.0-3

0

3

6

9

12

V (

V)


V

fit

Vsym

Vasym

How to separate VSP from VAMR

SMR 22

Spin pumping damping gives spin-mixing conductance

for Py/Pt

A. Azevedo et al, Phys. Rev. B 83, 144402 (2011)

Spin Hall angle

Spin diff. length

For Pt

SPE+ISHE in FM insulators/NM

YIG

SPE+ISHE

Yttrium Iron Garnet –YIG-(Y3Fe5O12)- ferrimagnetic insulator with very small magnetic losses.

SMR 23 Free from the AMR voltage

II. Spin pumping experiments in YIG/Metal

SMR 24

MW generator

MW Circulator

Power Supply

Lock-in

Sample in

the hole of

shorted

waveguide

Gaussmeter

Electromagnet

Detector

Modulation coils

(for FMR)

Nanovoltmeter

Microstrip assembly for broadband measurements

200 Oe

0.70 0.75 0.80 0.85 0.90

-0.6

-0.4

-0.2

0.0

0.2

0.4

0.6

Sig

na

l d

P/d

H (

arb

. u

nits)


Permalloy film (60 nm)

Ferromagnetic Resonance (FMR) in Py and YIG

0.70 0.75 0.80 0.85 0.900.0

0.5

1.0

1.5

2.0

Sig

na

l d

P/d

H (

arb

. u

nits)


dH

2.50 2.55 2.60 2.65 2.70

-60

-40

-20

0

20

40

60

Sig

na

l d

P/d

H (

arb

. u

nits)


H2

H 24 Oe

2.57 2.58 2.59 2.60

-60

-40

-20

0

20

40

60

Sig

na

l d

P/d

H (

arb

. u

nits)


H 0.5 Oe

30 Oe Yttrium Iron Garnet -YIG (Y3Fe5O12)- (28 µm)

f=9.4 GHz

SMR 25

standing SW mode

small damping

YIG

FMR damping by spin pumping in YIG/Pt Pt

M.B. Jungfleisch et al, Phys. Rev. B 91, 134407 (2015)

does not vanish for large dYIG

YIG (6 µm) f=9.4 GHz

SMR et al, Phys. Rev. B 88, 014404 (2013)

tcoh~ 300 – 500 nm

SP-line broadening in thick YIG films

Yttrium Iron Garnet –YIG (6 µm)

FMR f=9.4 GHz

SMR 27

FMR and spin pumping in YIG/Pt and YIG/Ta

Spin pumping free from AMR

P=32 mW

similar

Actually smaller than Pt

T=300 K

YIG (6 µm)/Pt (4 nm)

SMR 28

FMR and spin pumping in YIG/Pt and YIG/Ta

Knowing and for Pt, and for YIG/Pt, measurement of

versus microwave power gives for the YIG/Pt interface

SMR 29

SP-ISHE in metallic antiferromagnetic IrMn

Spin pumping by FMR

2/ HPgRV effSHNSP

T=300 K

SMR 30

Spin-Seebeck

Consistent with spin pumping

SP-ISHE in metallic antiferromagnetic IrMn

III. Spin to charge current conversion in YIG/Gr

Sample preparation and characterization

Spin pumping experiments

Interpretation with inverse Edelstein effect

2D-Spintronics

SMR 31

Preparation of YIG/graphene sample

1- Large area graphene grown by CVD on a Cu foil

2- Spun deposition of PMMA (120 nm)

3- Soaking in (NH4)2S2O8 to remove Cu foil

4- Washed with

deionized water

5- YIG/substrate is lifted from underneath PMMA/Graphene and Gr bonds to the YIG film

7- Cleaning in isopropanol and drying in nitrogen flow

6- PMMA removed

with acetone

YIG (6 μm) grown by LPE on (111) GGG. Cut with 1.5 x 3.0 mm

500 1000 1500 2000 2500

Graphene on YIG/GGG

2DG

Graphene on SiO2/Si

2D

Inte

nsity(a

.u.)

G

YIG/GGG

Raman Shift(cm-1)

Characterization of YIG/SLG structure

Evidence of a single-layer graphene on YIG

Features of SLG

SMR 33

Characterization of YIG/SLG structure

STM images

40 nm 0.5 nm 0.5 nm

10 20 30 40 50 60 70 80 90

101

102

103

104

105

106

107

50.8 51.0 51.2 51.410

2

104

106

108

GGG/YIG

Inte

nsity(c

ou

nts

/se

c)

2(deg)

GGG(444)YIG(444)

X-ray diffraction

0 1 2 3 4 5 6 7

Graphene on GGG/YIG

Yttriu

mY

ttriu

m

Iro

n

Ca

rbo

nO

xyg

en

Percentage

C: 10,06%

O: 45,32%

Fe: 27.75%

Y: 16.87%

Iro

n

Inte

nsity(a

.u.)

Energy (keV)

EDS

No traces of

impurities

SMR 34

I-V curve shows Ohmic contacts between the electrodes and SLG

SMR 35

Resistance measurements

-1.0 -0.5 0.0 0.5 1.0

-0.1

0.0

0.1 R=9 kOhms

Cu

rre

nt

(mA

)

Voltage (V)

Magnetoresistance measured with I=300 µA, static field H in the plane + field modulation

for lock-in detection

Magnetoresistance of YIG/SLG structure

Conclusion: Proximity effect induces magnetization and spin-orbit coupling in SLG

Khoeler’s rule

-80 -40 0 40 80

-1.0

-0.5

0.0

0.5

1.0 H up

H down

M/M

s

H (Oe)

YIG

-80 -40 0 40 80

M x

dM

/dH

H up

H down

H (Oe)

-80 -40 0 40 803.82

3.83

3.84

H up

H down

dR

/dH

(

/Oe

)

H (Oe)

MR measurement with field modulation

Ferromagnetic resonance in YIG/SLG-9.4 GHz

2.50 2.52 2.54

-0.1

0.0

0.1

FM

R d

P/d

H (

arb

. u

nits)

H (kOe)

YIG/SLG

2.52 2.54 2.56

-0.2

0.0

0.2

FM

R d

P/d

H (

arb

. u

nits)

H (kOe)

YIG bare

2.50 2.52 2.54

-0.1

0.0

0.1

FM

R d

P/d

H (

arb

. u

nits)

H (kOe)

YIG/Pt

Microwave driving f= 9.4 GHz P= 80 mW

(shorted waveguide)

Spin pumping experiments with YIG/SLG

2.48 2.50 2.52 2.54 2.56-40

-20

0

20

40

0

90º

180º

R=9 k

YIG/SLG

VS

P (V

)

H (kOe)

Sample B

SMR 38

T=300 K

Frequency dependence of spin-pumping voltage


0.4 0.8 1.2 1.6-40

-20

0

20

40

6 GHz5 GHz4 GHz

VS

P (V

)

Magnetic field H (Oe)

3 GHz

Nonlinear 3-m process inhibits

FMR

Linear variation of with frequency is a signature of the

spin pumping process

SMR 40

µW power dependence of spin-pumping voltage


2 .5 0

2.52

2.54

2.56

0

10

20

30

40

50

60

70

0

3 0

6 0

9 0

1 2 0

VS

P (

V)

Powe

r (mW)

H (kOe)

Linear variation of with µW power (h2)is another signature of

the spin pumping process

Interface dependence of spin-pumping voltage


Sample A

2.4 2.5 2.6 2.7-200

-100

0

100

200

= 0

= 90o

= 180o

VS

P (V

)

H (kOe)

YIG/SLG (15.9 k)

Sample C

Smaller linewidth

Smaller

Smaller

Larger linewidth

Larger

Larger

SMR 41 J. B. S. Mendes et al, PRL. 115, 226601 (2015).

Spin-charge conversion in YIG/SLG

Spin-pumping+ ISHE?

PROBLEMS 1- Fit of this equation to data leads to unphysical parameters. 2- Single layer graphene does not support 3D spin current.

Mechanism of spin-charge conversion in YIG/SLG

SMR 43

2D electron gas with spin orbit coupling (SOC)

Rashba energy

SOC

Rashba field

SMR 44

Defines direction of spin quantization

Fermi circles

Spin-momentum locking


Edelstein effect: Charge current induces spin polarization

Zero net spin polarization Charge current driven by electric field produces

net spin polarization Rojas-Sánchez et al. Otani’s talk on Monday


Inverse Edelstein effect (IEE)- Onsager reciprocal of EE: Non-equilibrium spin polarization generates a charge current

surface charge current spin current

IEE parameter (dimension of length)

SMR 46 momentum relaxation time

0- Isolated single-layer graphene has weak SOC


1- Contact with YIG enhances SOC of the carriers in SLG

2- SOC and symmetry breaking creates Rashba field

-x

z

y

3- Microwave driving excites FMR which pumps spin current into SLG with polarization in the direction of the field H (y)

SMR 47


4- Non-equilibrium spin polarization creates charge current in the –x direction by IEE, that produces a voltage

SMR 48

-x

z

y


SMR 49

0 30 60 90 1200

20

40

60

YIG/SLGV

IEE

-pe

ak(

V)

Microwave power (mW)

All quantities are known. So we obtain

Comparison with other systems

Rashba parameter

The Rashba parameter in YIG/SLG is quite smaller than in other 2D systems.

Not surprising since graphene has very weak SOC.

estimated from measured RN

S-de Haas oscillations

ARPES

Spin pumping experiment is a powerful tool to

study the conversion of spin currents into charge

current.

SMR 51

Summary

In contact with YIG, graphene has magneto-

resistance typical of magnetic systems and

exhibits enhanced spin-orbit coupling.

Microwave driven FMR in YIG/SLG generates

spin currents in graphene that are converted into

charge currents by the inverse Edelstein effect,

made possible by the Rashba field.

Thanks to all co-authors

Universidade Federal de Pernambuco Departamento de Física Recife, PE, Brazil Sergio M. Rezende [email protected]

Thanks to all co-authors

Universidade Federal de Pernambuco Departamento de Física Recife, PE, Brazil Sergio M. Rezende [email protected]

SUPPORT

MANY THANKS FOR YOUR ATTENTION

RECIFE, Pernambuco

Fernando Machado Antonio

Azevedo

Roberto Rodríguez

Joaquim Mendes

spin pumping and brasília inverse edelstein effect rio in

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