Pure Spin Currentsvia Non-Local Injection

and Spin Pumping

Axel Hoffmann

Materials Science Division and Center for Nanoscale MaterialsArgonne National Laboratory

10 nm

spin charge

2

Thanks toGoran Mihajlović, Oleksandr Mosendz, Yi Ji, John E. Pearson,

Frank Y. Fradin, J. Sam Jiang, and Sam D. Bader

Materials Science Division and Center for Nanoscale MaterialsArgonne National Laboratory

Miguel A. Garcia

Departamento Física de Materiales, Universidad Complutense de Madrid

Gerrit E. W. Bauer

Kavli Institute of NanoScience, Delft University of Technology

Peter Fischer and Mi-Young Im

Center for X-ray Optics, Lawrence Berkeley National Laboratory

$$$ Financial Support $$$DOE-BES

3

Outline

Why Pure Spin Currents?

Electrical Injection

Spin Hall Effect

Spin Pumping

Conclusions

10 nm

spin charge

1 m

I V

4

Spintronics

Putting

into Electronics

Novel DevicesNobel Prize New Physics

5

Charge vs. Spin CurrentsCharge Spin

J. Shi, et al., Phys. Rev. Lett. 96, 076604 (2006).

€

rj e = d

dt qr r ( )

€

rj e = q

r v

€

rj s = d

dt σr r ( )

€

rj s = σ

r v + ˙ σ

r r

Moving Spins

Spin Dynamics

No Need for Moving Spin: Potential for Low Power Dissipation!

Courtesy Claude Chappert, Université Paris Sud

7

New Goal:

8

Can we generate pure spin-currentsin paramagnetic materials?

• Non-local geometries

• Spin-dependent scattering (Spin-Hall)

• Spin pumping

9

Pure Spin Currents: The Johnson Transistor

F1 F2

N V

e-

L

F1 N F2

Emitter Base

or

Collector

M. Johnson,Science 260, 320 (1993)

M. Johnson and R. H. Silsbee,Phys. Rev. Lett. 55, 1790 (1985)

0

F2

F2

First Experimental Demonstrations

M. Johnson and R. H. Silsbee,Phys. Rev. Lett. 55, 1790 (1985)

Bulk Al: s = 450 m (4.2 K)

I+

I- V

Jedema et al., Nature 410, 345 (2001)

Cu film: s = 1 m (4.2 K)

10

Lateral Spin-Valve with Gold

Y. Ji, et al., Appl. Phys. Lett. 85, 6218 (2004)

a.c. current source

Lock-in detection

s = 63 15 nmIn gold at 10 K

11

Lateral Spin-Valve with Copper

500 nm

Shadow Evaporation SEM ImageFinished Device

Y. Ji, et al.,Appl. Phys. Lett. 88, 052509 (2006)

12

Spin Diffusion Length in Copper

T = 10 K

Y. Ji, et al.,Appl. Phys. Lett. 88, 052509 (2006)

P = 7%

13

Spin-Signal at Room Temperature

L = 300 nm, T = 10 K L = 350 nm, T = 300 K

Co/Cu Lateral Spin-Valve

s ≈ 110 nmat room temperature

14

Spin Hall Effect

Spin-dependent scattering gives rise to transverse spin imbalance

of charge currents

Direct observation in GaAswith optical detection

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

J. E. Hirsch, Phys. Rev. Lett. 83, 1834 (1999)M. I. Dyakonov and V. I. Perel, JETP Lett. 13, 467 (1971)

15

Spin-Skew Scattering

+

-

nucleus

electron

E

B- -

-

16

Spin Hall vs. Inverse Spin Hall

Spin Hall

Charge Current

Transverse

Spin Imbalance

Inverse Spin Hall

Spin Current

Transverse

Charge Imbalance

Spin Dependent Scattering

17

Spin Hall Angle

• understanding the effect of SO coupling on electron transport

• recognizing materials for spintronics applications

Importance:

c

SH

σσγ = spin Hall conductivity

charge conductivity

stronger spin orbit interaction larger 2γ

Goal:• experiments to quantify

2γ

18

Quantifying Spin Hall Angle in Metals

= 0.0001- 0.0003Al:

S. O. Valenzuela & M. Tinkham, Nature 442, 176 (2006)

T. Kimura et al., PRL 98, 156601 (2007)

T. Seki et al., Nature Mater. 7, 125 (2008)

Magnetotransport measurements:

Ferromagnetic resonance:

= 0.0037Pt:

= 0.113Au:

K. Ando et al., PRL 101, 036601 (2008)

= 0.08Pt:

• Large discrepancies in γ values !• Ferromagnets always used to generate/detect spin currents need to know spin polarization efficiency at injector/detector

E. Saitoh et al., APL 88, 182509 (2006)

How about Spin Hall effects without ferromagnets!

possible spurious signals: Hall, Anomalous Hall, MR

€

γ

€

γ

€

γ

€

γ

19

Charge Current Teleportation

Theoretical Idea: Use Spin Hall Effects Twice!

Direct Spin Hall Effect

Generate Pure Spin Current

Inverse Spin Hall Effect

Detect Pure Spin Current

L

D. A. Abanin et al., Phys. Rev. B 79, 035304 (2009)

J.E. Hirsch, Phys. Rev. Lett. 83, 1834 (1999) E. M. Hankiewicz et al., Phys. Rev. B 70, 241301(R) (2004)

M. I. Dyakonov, Phys. Rev. Lett. 99, 126601 (2007)

20

Gold Hall Bar Structures

1 µm

w = 110 nm t = 60 nm 5 μm

Spin Hall Angle in Gold: < 0.02

Too small to be practically useful!

Mihajlović et al., Phys. Rev. Lett. 103, 166601 (2009)

21

Unusual Application of Spin Dynamics

As found in: Queen Victoria Pub, Durham, U. K.

22

Spin Pumping• Ferromagnetic Resonance results

in time-dependent interfacial spin accumulation

• This spin accumulation diffuses away from the interface

• Results in net dc spin current perpendicular to interface

• Additional spin current gives rise to additional damping

• Quantify spin current from linewidth broadening

F N

IS

23

Combine Spin Pumping and Inverse Hall Effect

• Use Spin Pumping to Generate Pure Spin Current

• Quantify Spin Current from FMR

• Measured Voltage Directly Determines Spin Hall Conductivity

• Key Advantage: Signal Scales with Device Dimension

24

Determine Spin Hall Angle for Many Materials

Pt Au Mo

γ = 0.0120

±0.0001Technique easily adapted to any material!

γ = 0.0025

±0.0006

γ = -0.00096±0.00007

Can we Image Spin Accumulation Directly?

25

How about X-ray Dichroism?

Image at Cu L-edge Magnetic Difference Images

Mosendz et al., Phys. Rev. B 80, 104439 (2009)

Is There Any Hope for X-Rays?

26

s

d

E

N(E)

Ferromagnet(i.e., typical TM)

Contrast due to different density of states

at Fermi-level

s

d

E

N(E)

Spin Accumulation

Contrast due to spin-splitting?

Well below 1 meV!

27

Conclusions

Spin Currents behave differentcompared to Charge Currents– Possibility of Reduced Power Dissipation

Non-Local Electrical Injection– Generate Pure Spin Currents– Study Spin Relaxation

Spin Hall Effects– Generate and Detect Spin Currents

w/o Ferromagnets

Spin Pumping– Generate Spin Currents

w/o Electric Charge Currents

10 nm

spin charge

1 m

I V