Towards a quantitative description of field-impressed anisotropy of susceptibility

Abstract

A simplified model of domain rearrangement in multidomain particles following the application of an alternating (AF) or direct (DF) field is used to quantify the dependence of anisotropy of magnetic susceptibility on magnetic history. To be able to account for changes in the average susceptibility (i.e. the average of three mutually perpendicular susceptibility measurements) of the sample following field treatment, the model has to be refined by introducing interaction between domains. By fitting theoretical curves of anisotropy versus peak AF to experimental results for samples containing different size fractions of magnetite particles, of average grain size ranging from 0.7 to 58 μm, four relevant parameters can be computed (for each sample), which enables a good fit to be obtained. By allowing two of these parameters to change systematically when a DF is applied (and an IRM is acquired), the anisotropy dependence on direct field strength can also be modelled. This includes the sign change observed as the field-impressed ellipsoid changes from prolate to oblate, together with increases in the average susceptibility as the field increases. However, some details, such as discrepancies between predicted and actual increases in the average susceptibility at high fields, suggest that further refinement of the model is required.