Micromagnetic simulations of thermally activated magnetization reversal ofnanoscale magnets

Numerical integration of a stochastic Landau-Lifshitz-Gilbert equation is u sed to study dynamic processes in single-domain nanoscale magnets at nonzer o temperatures. Special attention is given to including thermal fluctuation s as a Langevin term, and the fast multipole method is used to calculate di pole-dipole interactions. It is feasible to simulate these dynamics on the nanosecond time scale for spatial discretizations that involve on the order of 10(4) nodes using a desktop workstation. The nanoscale magnets consider ed here are single pillars with large aspect ratio. Hysteresis-loop simulat ions are employed to study the stable and metastable configurations of the magnetization. Each pillar has magnetic end caps. In a time-dependent field the magnetization of the pillars is observed to reverse via nucleation, pr opagation, and coalescence of the end caps. In particular, the end caps pro pagate into the magnet and meet near the middle. A relatively long-lived de fect is formed when end caps with opposite vorticity meet. Fluctuations are more important in the reversal of the magnetization for fields weaker than the zero-temperature coercive field, where the reversal is thermally activ ated. In this case, the process must be described by its statistical proper ties, such as the distribution of switching times, averaged over a large nu mber of independent thermal histories. (C) 2000 American Institute of Physi cs. [S0021-8979(00)78108-4].