High-temperature magnetization reversal in the inertial regime

Motivated by the remarkable experimental observation of all-optical femtosecond-scale magnetization reversal at relatively high temperatures, a stochastic inertial Landau-Lifshitz-Gilbert-Bloch (iLLGB) equation is written to describe nonequilibrium magnetization dynamics in ferromagnets at elevated temperatures and at short enough timescales that an inertial effect manifests. The effect of thermal agitations is described by a Fokker-Planck equation derived from the iLLGB equation including the longitudinal relaxation effect, which is solved with perturbation theory valid at elevated temperatures. Considering a uniaxially symmetric ferromagnet with uniaxial anisotropy, a thermal diffusion-driven exponential mode and alternating field-driven nutation mode of stable magnetization reversal are identified. Our theory proposes a magnetization reversal mechanism based entirely on transfer of angular momentum to the local magnetization, but which takes into account thermal fluctuations and inertial effect at the same time. The theory explains several key observations in all-optical magnetization reversal experiments; the absence of the need for a static field, the subpicosecond switching timescale, the relative roles of thermal and field effects, and the relevance of circularly polarized light. Our results have direct implications for magnetic recording devices operating close to room temperatures and in the ultrafast regime.