A Study of Domain Walls in Uniaxial Magnetic Materials

Abstract

<p> An investigation of domain walls in some uniaxial magnetic materials is reported in this thesis. Firstly, a method for measuring the wall energy anisotropy in orthoferrites, which causes cylindrical magnetic (bubble) domains to be elliptical is described. In Sm0.55Tb0.45FeO3 a measured anisotropy energy of 1.7% of the wall-energy density at room temperature is responsible for eccentricities as large as 0.4 at average bubble radii equal to 85% of the bubble strip-domain transition radius. The relationship between material parameters and wall-energy anisotropy is discussed. The hypothesis that in orthoferrites walls parallel to the a axis are Bloch walls while walls parallel to the b axis are Néel walls is investigated by measuring the wall anisotropy as a function of the quality factor of the material by varying the temperature of the sample. The measurements seem to verify the predicted dependence of wall anisotropy on the quality factor and thus the hypothesis.</p> <p> A method for measuring the temperature dependence of the wall-energy density in orthoferrites and the saturation magnetization in garnets is described. The advantage of the method is that it uses a single isolated bubble domain without the need to destroy the bubble in order to obtain the measurements. This method led to the derivation of the temperature sensitivities of bubble domains in orthoferrites and garnets in terms of the material parameters. Optimum plate thicknesses to minimize the variation of bubble diameter with temperature are considered. Also, the condition for zero temperature sensitivity of bubbles in some uniaxial materials is derived in terms of the material parameters.</p> <p> Finally, a study of the current requirement to cut a bubble domain from a strip domain or another bubble in uniaxial plates is reported in this thesis.</p>