International Journal of Applied Electromagnetics and Mechanics 13 (2001/2002) 251–256 251 IOS Press Monte Carlo simulations of interaction effects on magnetic viscosity in ferromagnetic granular media C.G. Verdes a,∗ , B.R. Diaz d , R.W. Chantrell b , S. Thompson d and Al. Stancu c a Magnetic Materials Research Group, Center for Materials Science, University of Central Lancashire, Preston, PR1 2HE, UK b Department of Physics, University of Durham, Durham, DH1 3LE, UK c “Al. I. Cuza” University, Iasi, Romania d Department of Physics, University of York, York, YO10 5DD, UK Abstract. A Monte Carlo time and temperature dependent model was developed to simulate magnetization processes in fine ferromagnetic particle media. The model uses a combination between a time dependent Stoner-Wohlfarth model and a standard Metropolis algorithm to find the magnetization of single-domain particle systems. Both magnetostatic and exchange interactions are introduced explicitly. Variation of magnetic viscosity with interactions and temperature is studied. Results show that viscosity exhibits a maximum around coercive field. The value of this maximum is closely related to the strength and the origin of interactions as well as to temperature. Dipolar interactions decrease the viscosity and broaden the viscosity vs. field curve, while exchange interactions lead to an increase in viscosity around coercive field. An increase in temperature up to the critical temperature for the superparamagnetic behavior leads to an increase in viscosity. Further increase of temperature produces a decrease in viscosity. 1. Introduction Magnetic recording media are used to store information for long periods of time, typically years, therefore the need for stable storage media so that information can be accurately read when necessary. The need for high densities requires that written bits become smaller, susceptible to thermal fluctuations that can affect the quality of the record. In ferromagnetic materials, in general, the value of the magnetization is time dependent. This dependence generally increases with temperature and decreases with particle size. To characterize the time dependence of magnetization, one uses the “magnetic viscosity”, S = dM d(ln(t)) . Here, a computational model for single domain particle systems has been developed, a time and temperature dependent model capable to predict magnetic behavior of granular systems. The model uses a time-dependent Stoner-Wohlfarth approach technique to find the magnetization of a sample depending on the applied field, temperature, measuring time and previous state of the system. The model also * Corresponding author: C.G. Verdes, Fax: 0191 3743749; E-mail: claudiu-georgel.verdes@durham.ac.uk. 1383-5416/01/02/$8.00  2001/2002 – IOS Press. All rights reserved