Magnetic domain wall visibility in bitter pattern imaging

A theoretical analysis of the imaging properties of the Bitter pattern method is made for both Bloch and Neel domain walls in ferromagnetic grains. We have formulated an analytic expression for the magnetic fields due to Néel and Bloch walls and determined the colloid density (and hence the optical contrast) that collects on the grain surface due to these fields. The results show that 180° Néel and Bloch walls are equally well imaged for walls of equal width, but since Néel walls are generally wider than Bloch walls, they should be significantly more visible. Furthermore, the optical contrast produced by accumulation of colloid decreases as the angle through which the magnetization rotates becomes smaller (an effect which is more pronounced for Néel walls than Bloch walls). We find also that both Néel and Bloch walls become less visible as depth of domain wall decreases, so that very shallow domain walls may be difficult to see compared with domain walls representative of the grain body domain structure.