Magnetocrystalline anisotropy of out-of-phase magnetic susceptibility in hematite

 Out-of-phase component of AC magnetic susceptibility (opMS) in four single crystals of hematite from Minas Gerais, Brazil was investigated. The phase angle ranges from 1° to 6° along the c-axis and slightly exceeds 25° along the basal plane (in the standard field 400 A/m at the operating frequency 1220 Hz). The opMS in the basal plane shows strong field dependence and virtually no frequency dependence. Along the c-axis, opMS is at least three orders lower and increases with field only slowly; it is similar at the frequencies 976 Hz and 3,904 Hz, while it is clearly higher at 15,616 Hz. The opMS vs. T curve measured along basal plane drops acutely between 680 °C and 710 °C, evidently indicating the Curie (Néel) temperature of hematite. The same curve along c-axis passes more or less parallel to the abscissa in the entire temperature interval investigated.

Directional opMS was measured in 320 independent directions from which the opAMS tensor was calculated using standard linear technique. The minimum opAMS directions are parallel to the c-axis, while the maximum and intermediate opAMS directions lie within basal plane. The differences between measured values and those calculated from the opAMS tensor are relatively small in the vicinity of the maximum opAMS direction and very high in the vicinity of the minimum opAMS direction. All this indicates that the second rank tensor is not the best representative of the spatial variation of the directional opMS of hematite single crystals. In multi-crystal assemblages, however, summation of many oriented grains filters out the non-tensorial parts of the grain opAMS and the resultant opAMS is well represented by a tensor. This result is similar to that of in-phase magnetic susceptibility in hematite investigated in Hrouda et al. (2020).