Pure Spin Currents via Non-Local Injection and Spin Pumping Axel Hoffmann Materials Science Division and Center for Nanoscale Materials Argonne National Laboratory 10 nm spincharge

2 Thanks to Goran 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 Materials Argonne 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 spincharge 1  m IV

4 Spintronics Putting into Electronics Novel Devices Nobel Prize New Physics

5 Charge vs. Spin Currents Charge Spin J. Shi, et al., Phys. Rev. Lett. 96, (2006). 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-currents in paramagnetic materials? Non-local geometries Spin-dependent scattering (Spin-Hall) Spin pumping

9 Pure Spin Currents: The Johnson Transistor F1F1 F2F2 N V e- L F1N F2 EmitterBase or Collector M. Johnson, Science 260, 320 (1993) M. Johnson and R. H. Silsbee, Phys. Rev. Lett. 55, 1790 (1985) 0 F2 F2F2 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+ 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 nm In gold at 10 K

11 Lateral Spin-Valve with Copper 500 nm Shadow Evaporation SEM Image Finished Device Y. Ji, et al., Appl. Phys. Lett. 88, (2006)

12 Spin Diffusion Length in Copper T = 10 K Y. Ji, et al., Appl. Phys. Lett. 88, (2006) P = 7%

13 Spin-Signal at Room Temperature L = 300 nm, T = 10 KL = 350 nm, T = 300 K Co/Cu Lateral Spin-Valve s ≈ 110 nm at room temperature

14 Spin Hall Effect Spin-dependent scattering gives rise to transverse spin imbalance of charge currents Direct observation in GaAs with 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: spin Hall conductivity charge conductivity stronger spin orbit interaction larger Goal: experiments to quantify

18 Quantifying Spin Hall Angle in Metals = Al: S. O. Valenzuela & M. Tinkham, Nature 442, 176 (2006) T. Kimura et al., PRL 98, (2007) T. Seki et al., Nature Mater. 7, 125 (2008) Magnetotransport measurements: Ferromagnetic resonance: = Pt: = Au: K. Ando et al., PRL 101, (2008) = 0.08 Pt: Large discrepancies in  values ! Ferromagnets always used to generate/detect spin currents need to know spin polarization efficiency at injector/detector E. Saitoh et al., APL 88, (2006) How about Spin Hall effects without ferromagnets! possible spurious signals: Hall, Anomalous Hall, MR

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, (2009) J.E. Hirsch, Phys. Rev. Lett. 83, 1834 (1999) E. M. Hankiewicz et al., Phys. Rev. B 70, (R) (2004) M. I. Dyakonov, Phys. Rev. Lett. 99, (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, (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 FN ISIS

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 AuMo  = ± Technique easily adapted to any material!  = ±  = ±

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, (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 different compared 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 spincharge 1  m IV