Magnetic and electric properties of magnetite at low temperatures

TLDR: The low-temperature transition in magnetite is due to the ordering of the ferrous and ferric ions in the octahedral interstices of the spinel lattice as mentioned in this paper.

Abstract: The low-temperature transition in magnetite, according to Verwey, is due to the ordering of the ferrous and ferric ions in the octahedral interstices of the spinel lattice. This arrangement would require a symmetry change from cubic to orthorhombic. X-ray diffraction indicates and electric conductivity and magnetization measurements confirm that the transition leads to an orthorhombic structure. An external magnetic field applied while cooling through the transition establishes a preferred orientation for the $c$ axis throughout the whole crystal. Below the transition this $c$ axis can be switched to a new direction by a strong magnetic field, a process involving a co-operative rearrangement of the ferrous ions in new sites and relatively large changes in dimensions. In stoichiometric, synthetic, single crystals the transition occurs at 119.4\ifmmode^\circ\else\textdegree\fi{}K and is marked by an abrupt decrease in the conductivity by a factor of 90 in a temperature interval of 1\ifmmode^\circ\else\textdegree\fi{}. No thermal hysteresis is observed. The conductivity of a crystal cooled in a strong magnetic field is anisotropic below the transition as given by the relation $\ensuremath{\sigma}=A+B(1+{cos}^{2}\ensuremath{\theta})$, where $\ensuremath{\theta}$ is the angle between the $c$ axis and the direction of measurement. The ratio $\frac{B}{(A+B)}$ increases rapidly as the crystal is cooled to 90\ifmmode^\circ\else\textdegree\fi{}K, indicating a progressive increase in the long-range order. The $c$ axis is the direction of easy magnetization below the transition, and the anisotropy energy is very much larger below than above; the anisotropy constants have been determined at 85\ifmmode^\circ\else\textdegree\fi{}K.