Current-induced noise and damping in nonuniform ferromagnets

In the presence of spatial variation in the magnetization direction, electric current noise causes a fluctuating spin-transfer torque that increases the fluctuations of the ferromagnetic order parameter. By the fluctuationdissipation theorem, the fluctuations at thermal equilibrium are related to the viscous magnetization damping, which in nonuniform ferromagnets acquires a nonlocal tensor structure. At the hand of spin spirals, we demonstrate that the current-induced noise and damping increase with the gradient of the magnetization texture and becomes significant for narrow domain walls.Kavli Institute of NanoscienceApplied Science