TOWARDS A QUANTITATIVE DESCRIPTION OF FIELD-IMPRESSED ANISOTROPY OF SUSCEPTIBILITY

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