Physical principles of spin torque

Abstract

Spin torque refers to the exchange of spin angular momentum between a transport spin current carried by electrons and a ferromagnet. The macroscopic manifestation of this angular momentum exchange is a torque exerted on the ferromagnet by the presence of the spin current. The spin current is often accompanied by a net charge-current transport, although this is not always necessary. There are two types of torque commonly associated with such interactions, one is exchange-like and the other energy nonconserving. These two types of torques have different vectorial relationship with the electron spin polarization and the magnet's moment. The exchange-like torque is in the direction perpendicular to the plane formed by the magnet moment and the spin polarization and is therefore often called the "perpendicular torque." The energy-nonconserving torque lies in the plane, hence the name the "in-plane torque." The perpendicular torque has been known for many decades, as it gives rise to exchange-like coupling between ferromagnetic thin films across a spacer layer of either a nonmagnetic metal or a tunnel barrier. A detailed understanding of the in-plane spin torque has emerged more recently. The in-plane spin torque is associated mainly with nonequilibrium and noncollinear transport of spin current across interfaces between nonmagnetic and ferromagnetic materials. It originates from the dephasing of an electron's spin precession as it enters or leaves a ferromagnet-nonmagnetic interface. The in-plane spin torque gives rise to new dynamic behaviors of the ferromagnet that is the subject of many interesting investigations and with potential for applications. Its physical origin and implications are the main subjects of this review.