Monte Carlo simulation of magnetization switching in a Heisenberg model for small ferromagnetic particles

Using Monte Carlo methods we investigate the thermally activated magnetization switching of small ferromagnetic particles driven by an external magnetic field. For low uniaxial anisotropy one expects that the spins rotate coherently while for sufficiently large anisotropy the reversal should be due to nucleation. The latter case has been investigated extensively by Monte Carlo simulation of corresponding Ising models. In order to study the crossover from coherent rotation to nucleation we use a specially adjusted update algorithm for the Monte Carlo simulation of a classical three-dimensional Heisenberg model with a finite uniaxial anisotropy. This special algorithm which uses a combined sampling can simulate different reversal mechanisms efficiently. It will be described in detail and its efficiency and physical validity will be discussed by a comparison with other common update-algorithms.  1999 Elsevier Science B.V. All rights reserved. Ferromagnetic particles are important for information storage in magnetic recording media (see, e.g., [1] for a review). Hence, much effort has been devoted to an understanding of the behavior of small magnetic particles experimentally [2–4], analytically [5], and in computer simulations. Thermally activated magnetization reversal by nucleation and subsequent domain wall motion has been studied mainly by Monte Carlo simulations of Ising models [6–10]. For other reversal mechanisms like coherent rotation it is necessary to study models with continuous degrees of freedom [8,10,11]. In these models an important issue for the efficiency of the Monte Carlo algorithm is the choice of the trial step of the single magnetic moments. In the following we will discuss different update-algorithms which can simulate thermally activated magnetization reversal in detail. 1 E-mail: [email protected]. 2 E-mail: [email protected]. We will consider a classical three-dimensional Heisenberg model which is defined by the Hamiltonian