Langevin simulation of thermally activated magnetization reversal in nanoscale pillars

Numerical solutions of the Landau-Lifshitz-Gilbert micromagnetic model incorporating thermal fluctuations and dipole-dipole interactions (calculated by the fast multipole method) are presented for systems composed of nanoscale iron pillars of dimension 9 nm X 9 nm X 150 nm. Hysteresis loops generated under sinusoidally varying fields are obtained, while the coercive field is estimated to be 1979 +/- 14 Oe using linear field sweeps at T = 0 K. Thermal effects are essential to the relaxation of magnetization trapped in a metastable orientation, such as happens after a rapid reversal of an external magnetic field less than the coercive value. The distribution of switching times is compared to a simple analytic theory that describes reversal with nucleation at the ends of the nanomagnets. Results are also presented for arrays of nanomagnets oriented perpendicular to a flat substrate. Even at a separation of 300 nm, where the field from neighboring pillars is only approximate-to 1 Oe, the interactions have a significant effect on the switching of the magnets.