Observations of viscous magnetization in multidomain magnetite

Abstract

The viscous behavior of multidomain magnetite has been directly observed in both natural and synthetic samples using Bitter pattern imaging. A computer‐controlled fully automated microscope fitted with a heating stage, field coils, and digital camera was used to record viscous acquisition and decay sequences as a function of time, temperature, and field. Domain walls (DW) were observed to move continuously through a series of quasi‐static states over many hours rather than instantaneously. Viscosity was observed only on grains oriented near to the {111} surface. Generally, DWs moved perpendicular to their surface in the direction of the applied field; however, because observed domains respond primarily to the movement of main domains underneath the surface, occasionally, DWs moved in the opposite direction to the applied field. Small variations in temperature were found to strongly influence the viscosity, supporting the idea that viscosity is thermally activated. Viscous and nonviscous domain structures were examined using magnetic force microscopy. These images revealed that the domains displaying viscous behavior tended to be narrow (∼2 μm in width). Larger domains on grains oriented near the {111} surface did not display viscosity, reflecting the greater energy required to move larger domain structures. This may explain why no viscosity was observed on the {110} surfaces, as the domains were wider, that is, ∼6–10 μm. A complex spiraling vortex‐like magnetic domain structure was imaged. Etch pit analysis found a corresponding dislocation pit at the same location. It is suggested that this corresponds to the microstructure around a screw dislocation line.