Magnetic Anisotropy of Cobalt Ferrite (${\mathrm{Co}}_{1.01}$${\mathrm{Fe}}_{2.00}$${\mathrm{O}}_{3.62}$) and Nickel Cobalt Ferrite (${\mathrm{Ni}}_{0.72}$${\mathrm{Fe}}_{0.20}$${\mathrm{Co}}_{0.08}$${\mathrm{Fe}}_2$${\mathrm{O}}_4$)

A method of measuring the first magnetic anisotropy constant, $K_1$, of cubic crystals having a large anisotropy was developed which utilized measurements of the torque for directions of the applied field near the direction of easy magnetization. By this method $K_1$ of a cobalt ferrite crystal was found to be closely approximated by the empirical relationship $K_1=19.6\mathrm{}\times {10}^6 \exp (-1.90\mathrm{}\times {10}^{-5\mathrm{}}{T}^2)$ ergs/cc from 20° to 325°K. Above 325° the measured anisotropy depended upon the length of time the crystal was in the magnetic field. For a nickel cobalt ferrite crystal, $K_1$, as measured by the usual torque method, was given empirically by $K_1=\left[8.08\mathrm{} \exp \right(-3.57\mathrm{}\times {10}^{-5\mathrm{}}{T}^2)-9.78\mathrm{} \exp (-0.863\mathrm{}\times {10}^{-5\mathrm{}}{T}^2\left)\right]\times {10}^4$ ergs/cc from 20° to 600°K while $K_2$ increased from -4.7×${10}^4$ ergs/cc at 20.5°, passed through zero at 190°, rose to a maximum of 1×${10}^4$ at 280°K, and then above this temperature decreased rapidly to zero. The relationships for $K_1$ are of the same form as those found by Brükhatov and Kirensky for metallic ferromagnetic materials.