“Ferromagnet-Induced Spin-Orbit Torques”

Dr. Kyung-Jin Lee
Department of Physics, KAIST

Aug. 11 (Wed.), 09:30 AM
Online seminar
https://kaist.zoom.us/j/85087535578
회의 ID: 850 8753 5578
암호: 177566

Abstract:
Spin-orbit torque (SOT), which is considered as a write scheme for next-generation MRAMs, arises from the charge-to-spin conversion via spin-orbit coupling (SOC). For commercialization of SOT-MRAMs, it is of critical importance to enhance the charge-to-spin conversion efficiency, which requires a detailed understanding of various SOC effects. In addition to the widely studied spin Hall effect of non-magnet (NM), recent studies found that SOC of ferromagnet (FM) also contributes to the SOT [1], including (i) interface torque [2-4], (ii) anomalous torque [5-7], and (iii) orbital torque [8-10]. In this talk, we will discuss about these FM-induced SOTs, which provide additional knobs to enhance the net SOT efficiency. The talk will include
(i) The NM/FM interfaces generate spin currents polarized normal to the film plane [2,3], which allows for field-free SOT switching of perpendicular magnetization at a low current density [4].
(ii) Transverse spin currents are allowed within bulk FM [5,6], which serve as a source of SOT. Various features of this effect can be described by a recently generalized spin drift-diffusion formalism [7].
(iii) The spin Hall effect is concomitant with more fundamental orbital Hall effect [8]. As the orbital Hall effect is usually much stronger than the spin Hall effect, a direct injection of orbital currents to FMs can enhance the SOT via SOC of FM [9]. We will show an SOT experiment, necessitating the orbital Hall effect to explain the result.

Reference:
[1] D. Go et al. Theory of current-induced angular momentum transfer dynamics in spin-orbit coupled systems, Phys. Rev. Research 2, 033401 (2020).
[2] S.-h. C. Baek et al., Spin currents and spin-orbit torques in ferromagnetic trilayers, Nat. Mater. 17, 509 (2018).
[3] V. P. Amin, J. Zemen, and M. D. Stiles, Interface-generated spin currents. Phys. Rev. Lett. 121, 136805 (2018).
[4] D.-K. Lee and K.-J. Lee, Spin-orbit torque switching of perpendicular magnetization in ferromagnetic trilayers, Sci. Rep. 10, 1772 (2020).
[5] W. Wang et al., Anomalous spin–orbit torques in magnetic single-layer films, Nat. Nanotechnol. 14, 819 (2019).
[6] V. P. Amin, Junwen Li, M. D. Stiles, and P. M. Haney, Intrinsic spin currents in ferromagnets, Phys. Rev. B 99, 220405(R) (2019).
[7] K.-W. Kim and K.-J. Lee, Generalized spin drift-diffusion formalism in the presence of spin-orbit interaction of ferromagnets, Phys. Rev. Lett. 125, 207205 (2020).
[8] H. Kontani et al., Giant orbital Hall effect in transition metals: Origin of large spin and anomalous Hall effects, Phys. Rev. Lett. 102, 016601 (2009).
[9] D. Go and H.-W. Lee, Orbital torque: Torque generation by orbital current injection, Phys. Rev. Research 2, 013177 (2020).
[10] D. Lee et al., Orbital Hall effect in magnetic bilayers, submitted.