Spin Transport and Spin Torque

1. Spin Transport and Spin Torque Yuxuan Peng 2019.5.18

2. OUTLINE • What is spin current? • Generation of spin current • 1. Spin hall effect • 2. Spin pumping • 3. Spin Seebeck effect • Spin transport • Spin torques • 1. Spin transfer torque • 2. Spin orbit torque

3. OUTLINE • What is spin current? • Generation of spin current • 1. Spin hall effect • 2. Spin pumping • 3. Spin Seebeck effect • Spin transport • Spin torques • 1. Spin transfer torque • 2. Spin orbit torque

4. Spin Current Two types of spin currents • conduction-electron spin current • spin-wave spin current Nature464, 262–266 (2010)

5. OUTLINE • What is spin current? • Generation of spin current • 1. Spin hall effect • 2. Spin pumping • 3. Spin Seebeck effect • Spin transport • Spin torques • 1. Spin transfer torque • 2. Spin orbit torque

6. Family of spin-dependent Hall effects AHE charge current polarized transverse charge current SHE unpolarized charge current transverse pure spin current ISHE pure spin current transverse charge current. Rev. Mod. Phys.87, 1213 (2015)

7. Overview of SHE (conversion efficiency ) Spin Currents in Metallic Nanostructures. Ph.D. thesis (2011)

8. Spin Hall Magnetoresistance (SMR) YIG/Pt is zero is maximized Different from AMR AMR is governed by the angle of the applied current to the magnetization direction SMR depends on the angle of the spin accumulation with the magnetization PRL110, 206601 (2013)

9. OUTLINE • What is spin current? • Generation of spin current • 1. Spin hall effect • 2. Spin pumping • 3. Spin Seebeck effect • Spin transport • Spin torques • 1. Spin transfer torque • 2. Spin orbit torque

10. Ferromagnetic resonance (FMR) The time-dependent evolution of the magnetization orientation Landau–Lifshitz–Gilbert(LLG) equation Magnetization relaxation after the radio frequency field is turned off.

11. Spin pumping precessing magnetization in the FM generates a time-dependent spin current at the FM-NM interface that flows into the NM unit vector of the magnetization σ unit vector of the spin-current polarization its magnitude SP conductance of the particular sample Rev. Mod. Phys.87, 1213 (2015)

12. OUTLINE • What is spin current? • Generation of spin current • 1. Spin hall effect • 2. Spin pumping • 3. Spin Seebeck effect • Spin transport • Spin torques • 1. Spin transfer torque • 2. Spin orbit torque

13. Spin Seebeck effect Nat. Mater. 9, 894–897 (2010)

14. Spin Seebeck effect Seebeck effect electron Spin Seebeck effect spin will change sign from cold site to hot site is not relative to the polarization of heating current I (I is heating current) Nature455, 778–781 (2008)

15. Spin Seebeck effect Nat. Mater. 11, 391–399 (2012)

16. OUTLINE • What is spin current? • Generation of spin current • 1. Spin hall effect • 2. Spin pumping • 3. Spin Seebeck effect • Spin transport • Spin torques • 1. Spin transfer torque • 2. Spin orbit torque

17. Concepts of nonlocal spin transport The sign of VNL is determined by the relative magnetization orientation of FM1 and FM2. Int. J. Mod. Phy. B, 23, 2413-2438 (2009)

18. Nonlocal spin current detection Al2O3 tunnel barrier is used for spin-current CoFe (80 wt% Co) as FM Electrodes Large polarization when combined with Al2O3as a tunnelling barrier electrochemical potentials is the spin diffusion length The polarized spins are injected near x =0 and diffuse in both Al branches in opposite directions. The sign change in JS reflects the flow direction. Nature 442, 176–179 (2006)

19. Nonlocal spin current detection is the difference in the output voltage between parallel and antiparallel magnetization configurations of the FM electrodes at zero magnetic field Hanle effect associated with precessing spins Nature 442, 176–179 (2006)

20. Spin transport in YIG When the spin orientation of µs is parallel (antiparallel) to the average magnetization M, magnons are annihilated (excited), resulting in a non-equilibrium magnon population nm in the YIG Only the component of µs collinear to M contributes tomagnon injection/detection Nature Phys. 11, 1022–1026 (2015)

21. Spin transport in YIG Joule heating (for which T ∝ I2) will be detected in the second harmonic signal Another method Nature 561, 222–225 (2018)  Nature Phys. 11, 1022–1026 (2015)

22. Spin transport in YIG one-dimensional spin diffusion equation um um Magnon signal reaches far beyond the thermal gradient generated by the applied heating physics for electrical and thermal magnon generation is very different Magnon bosons Electron fermions Nature Phys. 11, 1022–1026 (2015)

23. Spin transport in 2D magnets MnPS3 No SHE Magnon relaxation length Thickness dependence Temperature dependence ？ Thermal generation SSE Phys. Rev. X 9, 011026 (2019)

24. OUTLINE • What is spin current? • Generation of spin current • 1. Spin hall effect • 2. Spin pumping • 3. Spin Seebeck effect • Spin transport • Spin torques • 1. Spin transfer torque • 2. Spin orbit torque

25. Spin transfer torques (STT) J. Magn. Magn. Mater. 320, 1190–1216 (1999).

26. Spin orbit torques (SOT) • SHE • Inverse spin galvanic effect (ISGE) (the Edelstein effect） Origins of spin-orbit torque ISGE PRL116, 096602 (2016) Appl. Phys. Lett. 5, 031107 (2018)

27. Spin orbit torques (SOT) STT-MRAM read and write current paths are coupled SOT-MARAM SOT scheme for writing TMR scheme for readout Science 336, 555 (2012) Appl. Phys. Lett. 5, 031107 (2018)

28. SOT in 2D magnets spin polarization direction of the spin current unit vector of the FGT magnetization Quantitative characterization of SOT second-harmonic (2w) Hall measurements Switching efficiency and SOT efficiency are higher than conventional SOT arXiv:1903.00571

29. Thank you !