Spin-torque switching: escape from a potential well with tunable damping

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Abstract
The problem of escape from a potential well is the central problem of chemical rate theory, as well as of theories of other thermally activated processes such as magnetic switching in hard disks and tapes. A key parameter in this problem is the damping constant; in 1940, Kramers[1] worked out the rate for various damping constants by solving a Fokker-Planck equation, later adapted to magnetic switching by Brown[2]. However, in neither case does one have independent control over the damping rate. Recently, experiments have been done on "spin-torque switching", in which a spin-polarized current is injected into a ferromagnetic element[3], of interest for information storage applications. In this letter we develop a Fokker-Planck formulation of spin torque, finding that the current changes the effective damping (Eq. 9). Thus we now have a barrier-escape problem in which we can control the rate by tuning the damping. The current-dependence of our calculated switching rate is in qualitative agreement with measurements[4].

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