  Spin-orbit torques in bilayers of ferromagnetic and nonmagnetic materials
hold promise for energy efficient switching of magnetization in nonvolatile
magnetic memories. Previously studied spin Hall and Rashba torques originate
from spin-orbit interactions within the nonmagnetic material and at the bilayer
interface, respectively. Here we report a spin-orbit torque that arises from
planar Hall current in the ferromagnetic material of the bilayer and acts as
either positive or negative magnetic damping. This planar Hall torque exhibits
unusual biaxial symmetry in the plane defined by the applied electric field and
the bilayer normal. The magnitude of the planar Hall torque is similar to that
of the giant spin Hall torque and is large enough to excite auto-oscillations
of the ferromagnetic layer magnetization.
