Magnetic relaxation in a classical spin chain

With decreasing particle size, different mechanisms dominate the thermally activated magnetization reversal in ferromagnetic particles. We investigate some of these mechanisms for the case of a classical Heisenberg spin chain driven by an external magnetic field. For sufficiently small system size t he magnetic moments rotate coherently. With increasing size a crossover to a reversal due to soliton-antisoliton nucleation sets in. For even larger s ystems many of these soliton-antisoliton pairs nucleate at the same time. T hese effects give rise to a complex size dependence of the energy barriers and characteristic time scales of the relaxation. We study these quantities using Monte Carlo simulations as well as a direct integration of the Landa u-Lifshitz-Gilbert equation of motion with Langevin dynamics and we compare our results with asymptotic solutions for the escape rate following from t he Fokker-Planck equation. Also, we investigate the crossover from coherent rotation to soliton-antisoliton nucleation and multidroplet nucleation, es pecially its dependence on the system size, the external field, and the ani sotropy of the system.