Low-temperature susceptibility and hysteresis of magnetite

Measurement of susceptibility (chi) and hysteresis loops below room temperature for large multidomain hydrothermal magnetite and an acicular single domain magnetite are reported. Hydrothermal samples all display frequency dependent anomalies in chi across the Verwey transition (T-v), A second frequency dependent anomaly in chi is found at 50 K. In both cases the frequency dependency of chi is attributed to discontinuities in the hopping frequency of electrons at T-v and 50 K respectively, The coercive force (H-c) of the hydrothermal samples displays sharp discontinuous behavior at the magnetocrystalline anisotropy isotropic point (T-k) On plotting the hysteresis parameters on 'Day' plots, the hydrothermal grains move towards more pseudo-single domain-like regions on cooling through Tk It is suggested that the difference in low-temperature anomalous behavior between 'high-field' parameters, e.g., H-c at T-k, and 'low-field' parameters, e.g., chi at T-v, is due to the partial destruction of the Vonsovskii exchange interaction in the presence of high-fields, where according the magneto-electronic model of the Verwey transition, the Vonsovskii exchange interaction is the mechanism by which electronic ordering occurs at T-v. The acicular single domain sample displays no anomalous behavior in either chi or hysteresis parameters near T-k or T-y. (C) 1999 Elsevier Science B.V. All rights reserved.