Optical spin-orbit torque and its application for spin-wave excitation*

Spin current generation through the spin-orbit interaction in non-magnetic materials lies at the heart of spintronics. In particular, the electrical generation of a spin-orbit torque through the spin Hall effect in heavy metals and Rashba effect in interfaces is intensively studied. For the fast magnetization dynamics, however, the optical generation of a spin-orbit torque is more promising than its electrical counterpart.
In this work, we report optical spin-orbit torques in heavy metal/ferromagnet heterostructures [1]. The strong spin-orbit coupling of heavy metals induces photo-excited carriers to be spin-polarized, and their transport from heavy metals to ferromagnets induces a torque on magnetization. These results resemble, in many ways, the electrically generated spin-orbit torque that also utilizes the strong spin-orbit coupling of heavy metals. We thus call the optically generated damping-like torque in heavy metal/ferromagnet heterostructures. as optical spin-orbit torque (OSOT).
Owing to its fast timescale, OSOT can be used to excite phase-coherent spin waves in the frequency range from GHz to THz [2,3]. Employing OSOT, we investigate the frequency, amplitude, and damping of spin waves. From the frequency analysis, we determine the spin wave stiffness. From the amplitude analysis, we show that FM acts as a cavity for spin waves. From the damping analysis, we observe a clear dependence on the FM thickness, which is attributed to the spin pumping effect of spin waves.

References
G.-M. Choi, J. H. Oh, D.-K. Lee, S.-W. Lee, K. W. Kim, M. Lim, B.-C. Min, K.-J. Lee, and H.-W. Lee, Nature Commun. Vol. 11, p. 1482 (2020).
G.-M. Choi, D.-K. Lee, K.-J. Lee, and H.-W. Lee, Phys. Rev. B Vol. 102, p. 014437 (2020).
G.-M. Choi, J. Magn. Magn. Mater. Vol. 516, p. 167335 (2020).