MAGNETIC HYSTERESIS AND ROTATIONAL HYSTERESIS PROPERTIES OF HYDROTHERMALLY GROWN MULTIDOMAIN MAGNETITE

Abstract

A series of hysteresis and rotational hysteresis measurements have been made on a suite of sized hydrothermally grown multidomain magnetite samples. These measurements consisted of hysteresis measurements made between room temperature and the Curie temperature, remanent hysteresis measurements at room temperature and rotational hysteresis measurements also made at room temperature. It was found that several of the measured and calculated parameters, e.g. the coercive force and rotational hysteresis parameters, display slight grain-size dependences across the entire range of samples up to the largest sample, which has a mean grain size of 108 μm, whereas other results, e.g. Henkel plots, were grain-size independent. These results suggest that there is no clear pseudo-single domain to ‘true’ multidomain behaviour transition. On comparison of high-temperature hysteresis with micromagnetic calculations there appears to be a change in the dominant domain-wall pinning mechanism with temperature. It is suggested that this effect could provide a possible mechanism for domain wall reorganization models that have been developed to explain partial thermoremanence cooling behaviour. The room-temperature rotational hysteresis results indicate that in addition to anisotropy, which controls most of the magnetic behaviour, there is a much smaller very high intrinsic anisotropy. It is tentatively suggested that this very high intrinsic anisotropy could be related to metastable remanences in multidomain magnetite. On comparison with published ‘crossover’ template plots it is seen that the low dislocation density hydrothermally produced samples display behaviour that does not entirely correspond with the standard templates, implying that the template plots need to be reassessed.