Spin current and spin-orbit torque induced by ferromagnets

Spin torque is typically classified based on how the spin current is generated and injected into a magnet for manipulation. Spin-orbit torque arises from the spin-orbit interaction in a nearby normal metal, while spin-transfer torque results from exchange interactions in another ferromagnet. Recent studies have suggested that a ferromagnet itself can also generate a spin current through spin-orbit coupling, leading to the emergence of ferromagnet-induced spin-orbit torque as another class of spin torque. This novel torque mechanism not only inherits the advantages of spin-orbit torque architectures, such as separate reading and writing paths in memory applications but also offers the flexibility to control the generated spin direction by manipulating the orientation of the ferromagnet responsible for generating the spin current. In this article, we review the phenomena related to spin currents generated by ferromagnets, explore their physical descriptions in heterostructures, and discuss several spin torque architectures based on this effect. Ferromagnet-induced spin-orbit torque not only introduces new physical consequences by combining spin-orbit and exchange interactions but also offers a promising building block in spintronics with significant potential for diverse applications.