Magnetic and Electric Properties of Magnetite at Low Temperatures

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°K and is marked by an abrupt decrease in the conductivity by a factor of 90 in a temperature interval of 1°. 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 $\sigma =A+B(1+{cos}^2\theta )$, where $\theta$ is the angle between the $c$ axis and the direction of measurement. The ratio $\frac{B}{\mathrm{}(A+B)\mathrm{}}$ increases rapidly as the crystal is cooled to 90°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°K.