Fig. 1: Illustration of various spin torque mechanisms.
From: Spin current and spin-orbit torque induced by ferromagnets
a Schematics of spin-transfer torque (STT). The current applied along the vertical direction generates a spin-polarized current injected into the free layer and generates STT. b Schematics of the conventional spin-orbit torque (SOT). The current applied along the horizontal direction through a heavy metal layer with strong spin-orbit coupling generates a perpendicular spin current injected into the free layer and generates SOT. c, d Schematics of examples of ferromagnet-induced spin-orbit torque (FM-SOT). The perpendicular spin current generated by spin-orbit coupling in a FM exerts a torque to another FM c or the FM itself d. The detailed mechanism of d is non-trivial so we focus here on phenomenological illustration and defer the illustration of its detailed mechanism to Fig. 4 and related discussions. Throughout this article, blue (gray) layers denote magnetic (non-magnetic) layers, and yellow, red, and magenta arrows denote the directions of magnetization, spin flow, and spin torque, respectively. As the directions of spin torque depends on the details of the system, the directions of the magenta arrows do not have physical meanings but are for schematic purpose only. Orange spheres denote conduction electrons whose spin directions are depicted by black arrows.
