Voltage-Driven Versus Current-Driven Spin Torque in Anisotropic Tunneling Junctions

Nonequilibrium spin transport in a magnetic tunnel junction comprising a single magnetic layer in the presence of interfacial spin-orbit interaction (SOI) is studied theoretically. The interfacial SOI generates a spin torque of the form
T
=
T
_||
M
×(
z
×;
M
)+
T
_⊥
z
×
M
, even in the absence of an external spin polarizer. For thick and large tunnel barriers, the torque reduces to the perpendicular component
T
_⊥
, which can be electrically tuned by applying a voltage across the insulator. In the limit of thin and low tunnel barriers, the in-plane torque
T
_||
emerges, proportional to the tunneling current density. Experimental implications on magnetic devices are discussed.