Low-temperature susceptibility and hysteresis of magnetite

Abstract

Measurement of susceptibility ( χ) 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 χ across the Verwey transition ( T v). A second frequency dependent anomaly in χ is found at 50 K. In both cases the frequency dependency of χ 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 T k. 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., χ at T v, is due to the partial destruction of the Vonsovskiǐ exchange interaction in the presence of high-fields, where according the magneto-electronic model of the Verwey transition, the Vonsovskiǐ exchange interaction is the mechanism by which electronic ordering occurs at T v. The acicular single domain sample displays no anomalous behavior in either χ or hysteresis parameters near T k or T v.