Anomalous spin-orbit torques in a ferromagnetic metal*

Current-induced spin-orbit torques have attracted a considerable amount of attention in recent years. The typical structure for studying the spin-orbit torque is a ferromagnetic metal/ nonmagnetic material bilayer. Most research has focused on the spin-orbit torque generated externally by the nonmagnetic material, overlooking the possibility that the ferromagnetic metal can generate spin-orbit torque on itself. We recently discovered that a ferromagnetic metal can in fact generate equal and opposite spin-orbit torques at its surfaces, which is termed as the anomalous spin-orbit torque, due to its analogy to the anomalous spin Hall effect. The strong anomalous spin-orbit torque may challenge our current understanding of spin-orbit torque in bilayer/multilayer systems. It also provides new opportunities that ferromagnet itself can be a source of spin-orbit torques.
[1] W. Wang, T. Wang, V.P. Amin, Y. Wang, A. Radhakrishnan, A. Davidson, S.R. Allen, T.J. Silva, H. Ohldag, D. Balzar, B.L. Zink, P.M. Haney, J.Q. Xiao, D.G. Cahill, V.O. Loren, X. Fan, “Anomalous spin-orbit torques in magnetic single-layer films”, Nature Nanotechnology 14, 819-824 (2019)