Kinetic Ising systems as models of magnetization switching in submicron ferromagnets

Abstract

Experimental techniques, such as magnetic force microscopy (MFM), have recently enabled the magnetic state of individual submicron particles to be resolved. Motivated by these experimental developments, we use Monte Carlo simulations of two-dimensional kinetic Ising ferromagnets to study the magnetic relaxation in a negative applied field of a grain with an initial magnetization m0=+1. The magnetostatic dipole–dipole interactions are treated to lowest order by adding to the Hamiltonian a term proportional to the square of the magnetization. We use droplet theory to predict the functional forms for some quantities, which can be observed by MFM. One such quantity is the probability that the magnetization is positive, which is a function of time, field, grain size, and grain dimensionality. The relaxation is characterized by the number of droplets larger than a field-dependent critical size, which form during the switching process. Our simulations of the kinetic Ising model are in excellent agreement with droplet-theoretical predictions. The qualitative agreement between experiments and our simulations of switching in individual single-domain ferromagnets indicates that the switching mechanism in such particles may involve local nucleation and subsequent growth of droplets of the stable phase.