Micromagnetic modelling of thin film media

Abstract

The magnetic recording industry is continuously trying to increase the density of recording media. There is a continuous need, therefore, to fully understand the magnetic processes that occur in such media. To enable this understanding, theoretical studies are conducted in the form of micromagnetic modelling. This thesis is concerned with the micromagnetic modelling of thin film media. Experiments have shown that thin film media consists of closely packed, irregularly shaped grains with non-magnetic boundaries.

To simulate a model of the physical structure, the Voronoi Construction Technique was implemented to give the required irregular structure. The grains were assumed to have uniaxial anisotropy and the magnetisation within the grain was assumed to vary throughout. To achieve non-uniformity, the grains were divided into triangles and the magnetisation within each triangle was assumed to vary linearly. The effect on the magnetisation within the grains due to the influences from an externally applied field, an anisotropy field, a magnetostatic field and an exchange field was observed. The motion of the magnetic moments under these influences was modelled by the Landau-Lifshitz equation.

The most time consuming calculation in the modelling process is the magnetostatic field calculation. Therefore, continuous research into more efficient methods of calculating this field is carried out. The model initially uses a dipole approximation to calculate the total contribution from the magnetostatic field. A more accurate magnetostatic calculation, based on volume and surface charges of the triangles, was implemented to calculate the close range magnetostatic effects. The integrals were found to have singularities when the point of evaluation lay on one of the vertices of a triangle to be integrated. The Shift Method was introduced to overcome the problem which translated the problem vertex slightly away from the point of evaluation.

Vast differences in the hysteresis loops when using the two methods of calculating the magnetostatic field were seen. The dipole approximation appeared to be too inaccurate in calculating the magnetostatic field. With the introduction of the more accurate method, the model compared well against previous findings.