The coercive force spectrum of magnetite at high temperatures: Evidence for thermal activation below the blocking temperature

Abstract

Thermal activation of magnetization is the mechanism proposed by Neel to explain TRM (thermoremanent magnetization) and VRM (viscous remanent magnetization). Neel's theory is difficult to test because in a small constant field, magnetization changes occur only at or near particle blocking temperatures. By increasing the field, the blocking temperature can be lowered to any desired temperature, including room temperature. Isothermal hysteresis or demagnetization data, particularly coercive forces, therefore provide information about thermal fluctuations at temperatures well below the usual blocking temperatures of low-field TRM or VRM. We apply thermal fluctuation analysis (Dunlop) to high-temperature measurements of average coercive force and of the coercive force spectrum (from alternating-field demagnetization). The samples incorporate representative fractions (average grain sizes from 0.04 to 0.22 μm) of the lower pseudo-single-domain (PSD) range of magnetite and one fraction (1–5μm size spread) from the upper PSD range. The ordinary coercive force HC has the same temperature dependence as the softest 20–30 per cent of the coercive force spectrum, whereas the remanent coercive force HR has the same temperature dependence as the median demagnetizing field. HR is, therefore, a more meaningful average over the coercive force spectrum than HC. The predicted dependence of Neel's ‘fluctuation field’Hq on temperature, on grain size and on microscopic coercive force HK are followed quite closely by the smaller PSD grains, but not by the 1–5 μm grains. By extrapolation, the critical size for (room temperature) superparamagnetic behaviour in magnetite is found to be (250+25–10) A, in good agreement with previous experimental and theoretical estimates. On the basis of fluctuation analysis, domain walls with a minimum width ˜0.1μm carry the permanent magnetic moment of small multidomain magnetite grains. This wall moment, thermally activated as a unit, rotates in singledomain-like fashion. Wall displacement plays a secondary role in two-domain particles, at least in those<0.22 pm in size: fluctuation fields at the TRM blocking temperature, as inferred from the Neel wall displacement model of TRM, are inconsistent with observed (and predicted) magnitudes and size variation of Hq. However, in 1–5μm grains, although PSD moments are generally accepted as the explanation of their enhanced low-field TRM intensity, hysteresis and alternating-field demagnetization results do not give unequivocal evidence of the existence of PSD moments. High-field isothermal magnetic processes in these larger ‘PSD’ grains are equally well accounted for by wall displacements alone.