Preisach diagrams and anhysteresis: do they measure interactions?

Abstract

Traditionally, it is assumed that the off-diagonal distribution on the Preisach diagram and the inverse anhysteretic susceptibility as a function of DC field strength measure the spectrum of magnetostatic particle interactions in single-domain (SD) samples. To test this assumption, we have measured Preisach diagrams and anhysteretic remanence acquisition curves at a series of temperatures up to the Curie point for samples containing SD and small pseudo-single-domain (PSD) magnetites and maghemites. Preisach diagrams were also determined at room temperature only for a suite of PSD to multi-domain (MD) magnetites with mean sizes ranging from 2 to 100 μm. In the latter Preisach diagrams, the diagonal distributions, which are thought to represent microcoercivity spectra, changed from peaked, SD-like distributions for the smallest grains to typical quasi-exponential MD distributions for the largest grains. The transverse or off-diagonal distributions broadened enormously over the same grain-size range, probably indicating that self-demagnetizing fields become more influential in the larger grains. In the SD and small PSD samples, the diagonal and transverse Preisach distributions contracted about equal amounts at each high-temperature step. Both distributions changed approximately in proportion to bulk coercive force, HC(T), and not in proportion to saturation magnetization, Ms(T). Such a temperature dependence was expected for the diagonal distribution, which depends on crystalline and shape anisotropies. It was not anticipated for the transverse distribution, as magnetostatic interactions should vary with temperature as MS(T). Inverse anhysteretic susceptibilities at first changed like MS(T), but at the higher temperatures they also decreased in proportion to HC(T). Therefore the off-diagonal Preisach distribution seems to be affected more by microcoercivity than by magnetostatic phenomena such as particle interaction or self-demagnetization, at least in this suite of samples, and inverse anhysteretic susceptibility is also a dubious measure of interaction strength. A preliminary theoretical model is presented in which an interaction field acting perpendicular to the easy axis of anisotropy considerably reduces the particle coercivity. Thus the diagonal Preisach distribution is not independent of interactions. However, it remains unclear why the off-diagonal distribution is not independent of coercivity.