Optical contrast of inclined boundaries in birefringent magnetic materials

Abstract

The theory of electromagnetic propagation in birefringent magnetic materials is generalized to include propagation in an arbitrary direction with respect to the crystal axes. Simple transmission matrices are derived by using the electric displacement vector rather than the electric field vector. These are used to predict the behaviour of interference fringes observed across inclined magnetic domain walls, inclined twin lamellae and inclined grain boundaries in a thin plate of rare earth orthoferrite viewed between crossed polarizers. Some experimental observations of the variations of the fringe patterns with the settings of the polarizers relative to the crystal axes are presented. The predicted behaviour is in very good agreement with experiment. It is possible to determine whether an inclined domain wall turns over on interaction with a defect, and to detect the presence of a domain wall at a twin lamella.