Mechanism of Transition in Magnetite at Low Temperatures

Abstract

When magnetite is cooled through -160\ifmmode^\circ\else\textdegree\fi{}C it is known to undergo a transition (cubic to orthorhombic) that is influenced by the presence of a magnetic field. Our experiments are in agreement with the following mechanism of the transition: The orthorhombic $c$ axis is parallel to one of the original cubic axes and is the axis of easiest magnetization. Generally, different regions of the original crystal will transform with their $c$ axes lying along different cubic axes, and when no field is applied there are 6 different orientations which different regions assume. When a field is applied during cooling a $c$ axis tends to lie along the original cubic axis that is nearest to the applied field, the $a$ and $b$ axes having less but different tendencies to lie parallel to the field.Six magnetic crystal anisotropy constants are derived from torque curves measured in the (100) and (110) planes. From them magnetization curves are calculated for the [100] and [110] directions, and these are in agreement with experiment.