On the Magnetic Anisotropy in Manganese-Iron Spinels

The magnetic anisotropy energy is measured as a function of temperature in eleven single crystal compositions corresponding to ${\mathrm{Mn}}_x{\mathrm{Fe}}_{\text{3−x}}{\mathrm{O}}_4$ , where $x$ is varied from 0.00 to 1.90. The interesting features observed are as follows. (1) Small manganese concentrations in magnetite $\text{(x=0)}$ are shown to decrease the temperature of the “isotropy point” known to exist in magnetite at low temperatures. (2) An unexpected variation in the temperature dependence and magnitude of the anisotropy energy in the $\text{x=0.4\hspace{0.17em}}\mathrm{to}\text{\hspace{0.17em}1.0}$ composition region is shown. (3) The anisotropy energy in the composition region between $\text{x=1.0\hspace{0.17em}}\mathrm{and}\text{\hspace{0.17em}1.8}$ is semiquantitatively shown to be related to cubic crystalline field splitting of the energy levels of ferric ions. (4) A large planar anisotropy is measured in a crystal composition given by $\text{x\hspace{0.17em}=\hspace{0.17em}1.90}$ and is discussed in terms of the expected tetragonal nature of trivalent manganese. (5) Finally, a minute magnetic annealing effect at a temperature of 370°K has been shown to exist in a crystal composition corresponding to $\text{x=0.40}$ and the magnitude of this effect is shown to be dependent on the direction relative to the crystal axes of the applied magnetic field during the annealing process.