Micomagnetic calculations of the magnetic properties of thin metallic films

Abstract

A micromagnetic model has been developed to simulate the magnetisation in a metallic thin film. This is achieved by energy minimisation techniques. The energy of each grain in the medium includes Zeeman, anisotropy, magnetostatic and exchange. The magnetostatic energy is incorporated by considering each grain as a point dipole and its a magnitude was obtained in collaboration with J.J. Miles. The exchange coupling is incorporated by utilising the phenomenological expression introduced by Zhu and Bertram. The model consists of one thousand grains on a hexagonal lattice with periodic boundary conditions applied. The model was initially used to compute hysteresis loops and these agreed with loops produced by other micromagnetic models.
AC Erased states were obtained by a technique we have developed based on simulated annealing. This produced erased states which were of low energy and well correlated. The erased state was found to consist of vortices, which increased in size as the exchange coupling increased. The difficulty of obtaining zero magnetisation was also found to increase as the exchange coupling increased - a problem also found experimentally.
The Al plot was calculated from a comparison with the computed remanence curves. The Al plot undergoes a change from being negative in the purely magnetostatic case to positive in the presence of strong exchange coupling. This change in the form of Al agrees with experimental investigations.
Furthermore, the magnitude of the magnetostatic interaction was varied. It was shown that by keeping the exchange coupling constant and varying the magnetostatic interactions, it was possible to change the form of Al. Thus, it is the detailed balance of the exchange and magnetostatic interaction which is of importance.
The model was then extended to simulate magnetic force microscopy (MFM). The ac erased, dc demagnetised and recording medium consisting of two oppositely magnetised bits have been scanned to produce MFM images.
The image of the recording medium agrees with the experimental image. It is anticipated that the images which we have produced will enhance the understanding of experimental images.