Domain observations of titanomagnetites during hysteresis at elevated temperatures and thermal cycling

Abstract

Domain observations using Bitter patterns have been made on x = 0.6 titanomagnetites up to the Curie point. The principal results are: (1) there is no major increase in the number of domain walls before they are lost near the Curie point; (2) with increasing temperature, the fields at which saturation is reached and departed from during hysteresis decrease; (3) with increasing field, the temperatures at which saturation is reached during thermal cycling decrease; and (4) particles of the order of tens of microns were found to be in a saturated or near saturated state after cooling from the Curie point to room temperature in the ambient geomagnetic field. These results were observed on several different grains showing clear body domains. They attest to the predicted importance of the magnetostatic energy in the external field at elevated temperature. They also suggest that models of multidomain thermoremanent magnetization need not involve major increases in the number of walls at elevated temperature. Clearly the metastable single domain particles will be important in TRM due to the large moment they carry. Also discussed are the exotic patterns commonly found in titanomagnetites, and their possible role in determining magnetization state.