1. Spin-Orbit Torques from Interfacial Rashba-Edelstein Effects
Axel Hoffmann Materials Science Division Argonne National Laboratory
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2. Chiral Symmetry breaking with Permalloy/Pt
Outline
“Spin-Hall” ST-FMR
Chiral Symmetry breaking with Permalloy/Pt
Permalloy/MoS2
Ag/Bi
Conclusions
Axel Hoffmann, MSD, Argonne National Laboratory
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3. Spin Hall Effects are Key Enabling Phenomena
Charge + Spin Currents
Spin
Dynamics
&
Nonmagnetic Metal
Ferromagnet
Spin Hall Effect
Spin Transfer Torque
Spin Pumping
inverse
Spin Hall Effect
Axel Hoffmann, MSD, Argonne National Laboratory

4. Spin Hall Effect Driven Magnetization Dynamics
Landau-Lifshitz-Gilbert equation:
damping-like
field-like
How can we create and detect spin currents?
Axel Hoffmann, MSD, Argonne National Laboratory

5. Spin-Torque Ferromagnetic Resonance
Excitation of spin dynamics by Oersted field and SHE torques.
Mixing of anisotropic magnetoresistance (AMR) in Py with microwaves lead
to dc rectification.
Lineshape analysis can quantify spin Hall angle.
L. Liu et al., Phys. Rev. Lett. 106, (2011)
Axel Hoffmann, MSD, Argonne National Laboratory

6. Spin-Torque Ferromagnetic Resonance
Landau–Lifshitz–Gilbert equation (LLG) τF
τSTT
L. Liu et al., Phys. Rev. Lett. 106, (2011)

7. Chiral Symmetry Breaking in ST-FMR
Axel Hoffmann, MSD, Argonne National Laboratory

8. Permalloy/Pt: Bulk vs. Interface
Spin Hall Angle:
Interface:
87%
Bulk: 3%
L. Wang et al., Phys. Rev. Lett. 116, (2016)
Axel Hoffmann, MSD, Argonne National Laboratory

9. due to shape anisotropy
Coordinate system for ST-FMR Measurements with arbitrary magnetic field direction
In general ψ lags θ
due to shape anisotropy
Axel Hoffmann, MSD, Argonne National Laboratory

10. Out-of-plane FMR of Permalloy
Static equilibrium condition for FMR
FMR field vs. field angle θ
Axel Hoffmann, MSD, Argonne National Laboratory

11. ST-FMR Permalloy/Pt in-Plane Angular Dependence
Axel Hoffmann, MSD, Argonne National Laboratory

12. ST-FMR Permalloy/Pt in-Plane Angular Dependence
Axel Hoffmann, MSD, Argonne National Laboratory

13. ST-FMR Dependence on Out-of-Plane Magnetic Field
Measured at 5.5 GHz, φ = 45°
Axel Hoffmann, MSD, Argonne National Laboratory

14. ST-FMR Dependence on Out-of-Plane Magnetic Field
Measured at 5.5 GHz, φ = 45°
Axel Hoffmann, MSD, Argonne National Laboratory

15. ST-FMR Dependence on Out-of-Plane Magnetic Field
Measured at 5.5 GHz, φ = 45°
Axel Hoffmann, MSD, Argonne National Laboratory

16. ST-FMR Dependence on Out-of-Plane Magnetic Field
Measured at 5.5 GHz, φ = 45°
Axel Hoffmann, MSD, Argonne National Laboratory

17. Spin Hall Effect Driven Magnetization Dynamics
Landau-Lifshitz-Gilbert equation:
damping-like
field-like
Even in field
Odd in field
Axel Hoffmann, MSD, Argonne National Laboratory

18. Opposite Chirality of Torques for Opposite Magnetic Fields
Normal:
τF+τD rotates towards original direction of original damping–like torque
Reversed:
τF+τD rotates towards original direction of original field–like torque
Axel Hoffmann, MSD, Argonne National Laboratory

19. Dynamic Reciprocity with Pure Field-like Torque
Field reversal only results in unchanged dynamics
with π-phase shift
Axel Hoffmann, MSD, Argonne National Laboratory

20. Dynamic Reciprocity with Pure Damping-like Torque
Field reversal results in unchanged dynamic
without phase shift
Axel Hoffmann, MSD, Argonne National Laboratory

21. Dynamic Non-Recirpocity with Both Torques
Axel Hoffmann, MSD, Argonne National Laboratory

22. Dynamic Amplitude Non-Recirpocity
Axel Hoffmann, MSD, Argonne National Laboratory

23. Angular Dependence of Symmetric Voltage for “Normal” Field Configuration
Axel Hoffmann, MSD, Argonne National Laboratory

24. Angular Dependence of Symmetric Voltage for “Reversed” Field Configuration
A similar result is found for the antisymmetric voltage
Axel Hoffmann, MSD, Argonne National Laboratory

25. Comparison Theory and Experiment “Normal”
Axel Hoffmann, MSD, Argonne National Laboratory

26. Comparison Theory and Experiment “Reversed”
Axel Hoffmann, MSD, Argonne National Laboratory

27. Spin Transfer Torque with 2D Materials
Axel Hoffmann, MSD, Argonne National Laboratory

28. 2D Semiconductors with Strong Spin-Orbit Coupling
Dichalcogenides: MoS2, MoSe2, WS2, WSe2
Rashba Spin-orbit splitting
Electric field induced Zeeman splitting
Are the spin-orbit torques?
D. Xiao et al., Phys. Rev. Lett. 108, (2012).
H. Yuan et al., Nature Phys. 9, (2013).

29. Large Area monolayer MoS2 on SiO2/Si with CVD
W. Zhang et al., APL Mater. 4, (2016)
Axel Hoffmann, Materials Science Division, Argonne National Laboratory

30. Both are indicative of single monolayer MoS2
Optical Spectra
Both are indicative of single monolayer MoS2
W. Zhang et al., APL Mater. 4, (2016)
Axel Hoffmann, Materials Science Division, Argonne National Laboratory

31. Spin Transfer Torque Measurements Py/MoS2
Any torque possibly given by the MoS2?
W. Zhang et al., APL Mater. 4, (2016)
Axel Hoffmann, Materials Science Division, Argonne National Laboratory

32. Significant Lineshape Changes
1. non-trivial Lorentzian-type signal with both symmetric and antisymmetric lineshape!
2. Angular rotation consistent with the spin torque theory.
~ cos2(f) sin(f)
W. Zhang et al., APL Mater. 4, (2016)
Axel Hoffmann, Materials Science Division, Argonne National Laboratory

33. Sizeable Torques from Interface with Monolayer!
Analytical fitting with the spin torque model using Py AMR rectification.
From Rashba-effect more field like torques expected
W. Zhang et al., APL Mater. 4, (2016)
Axel Hoffmann, Materials Science Division, Argonne National Laboratory

34. MoS2 Out-of-Plane Dependence
Virtually no non-reciprocity! Mostly Damping like-torque
Significant increase of torques for out-of-plane fields
W. Zhang et al., APL Mater. 4, (2016)
Axel Hoffmann, MSD, Argonne National Laboratory

35. Ferromagnetic Resonance
Spin Torque
Ferromagnetic Resonance
with Ag/Bi
Axel Hoffmann, MSD, Argonne National Laboratory

36. Spin Pumping with Ag/Bi
Permalloy/Ag
Permalloy/Bi
Permalloy/Ag/Bi
J. C. Rojas Sánchez et al., Nat. Comm. 4, 2944 (2013)
Axel Hoffmann, MSD, Argonne National Laboratory

37. Spin Pumping with Ag/Bi and Ag/Sb
W. Zhang et al., J. Appl. Phys. 117, 17C727 (2015)
Axel Hoffmann, MSD, Argonne National Laboratory

38. ST-FMR with Ag/Bi P = +10dBm, f = 4 GHz B. Jungfleisch et al.,
Phys. Rev. B 93, (2016)
Axel Hoffmann, MSD, Argonne National Laboratory

39. Gilbert Damping B. Jungfleisch et al., Phys. Rev. B 93, 224419 (2016)
Axel Hoffmann, MSD, Argonne National Laboratory

40. Interfacial Damping-like Torque
B. Jungfleisch et al.,
Phys. Rev. B 93, (2016)
Axel Hoffmann, MSD, Argonne National Laboratory

41. DC Current Modulated Linewidth
8% mA-1
no effect!
Axel Hoffmann, MSD, Argonne National Laboratory

42. Magnetic Films Group
Axel Hoffmann, Materials Science Division, Argonne National Laboratory

43. Argonne National Laboratory
Thanks to
Wei Zhang, Matthias B. Jungfleisch, Wanjun Jiang, Yaohua Liu, Hilal Saglam, Frank Y. Fradin, and John E. Pearson,
Argonne National Laboratory
Joseph Sklenar, Scott Grudichak, and John B. Ketterson
Northwestern University
Bo Hsu, Jiao Xiao, and Zhang Yang
University of Illinois at Chicago
$$$ Financial Support $$$ DOE-BES and NSF

44. Conclusions Permalloy/Pt
Complete angular dependence can unambiguously identify damping- and field-like torques
Presence of both results in dynamic non-reprocity
Permalloy/MoS2
Relatively large spin-orbit torques
Only interface / no bulk contribution!
Permalloy/Ag/Bi
Sufficient torque to drive magnetization dynamics
Axel Hoffmann, MSD, Argonne National Laboratory