Abstract

In microwave circulators a magnetic material is biased by an applied field provided by external magnets. Due to the external magnets the weight and size of circulators are hard to reduce. In consequence, the use of internal magnetocrystalline anisotropy field to replace the external biasing field is suggested. Perpendicularly oriented polycrystalline barium ferrite films are potential candidates for this purpose. One reason is the high perpendicular magnetocrystalline anisotropy they have. Another reason is that in polycrystalline structure each grain is expected to be single domain so that saturation state can be reached without an external field. Ideally the hysteresis loops are a rectangle in the perpendicular direction and a line in the longitudinal direction.

However, the hysteresis loops of current films are not ideal. Hysteresis loops of the experimental film modeled in this dissertation have round corners as well as a relatively low intrinsic coercivity in the perpendicular direction and an "S" shape in the center as well as a relatively high saturation field in the longitudinal direction.

The experimental loops cannot be fitted with the traditional micromagnetic model. In the traditional model the material is composed of many magnetic cells having constant saturation magnetization. For barium ferrites all the cells are assumed to have uniaxial anisotropy with the c-axis following a certain distribution. Based on X-ray diffraction data, c-axes of the experimental film are well oriented in the perpendicular direction. With this assumption, the traditional model cannot get the round corner in the perpendicular direction; the lowest ratio of simulated intrinsic coercivity in the perpendicular direction to the simulated saturation field in the longitudinal direction is about 0.5 for the traditional model vs. about 0.06 for the experimental data and for the new model.

In order to fit experimental data, this dissertation originally assumes an additional cubic magnetic phase exists in the barium ferrite film and fits experimental loops in both directions with one parameter set. Traditionally, it is assumed that the hexagonal structure has uniaxial anisotropy only and the cubic structure has cubic anisotropy only. In fact, this is an approximation. Current anisotropy theory allows cubic anisotropy existing in the hexagonal structure, although the cubic anisotropy may be much smaller than the coexisting uniaxial anisotropy. (Abstract shortened by UMI.)