Direct measurements are reported of the dependence of the anisotropy torque on the orientation of an applied magnetic field in the $\mathrm{ac}$ and $\mathrm{bc}$ crystallographic planes of ferrimagnetic ${\mathrm{Fe}}_{1-x}{\mathrm{Ga}}_{2-x}{\mathrm{O}}_3$ for $x=1.11\mathrm{}$. Temperatures between 4.2 and 294°K and applied fields up to 110 kOe were used. Anisotropic spontaneous magnetizations, as well as isotropic susceptibilities and field-dependent anisotropy energies, were determined by comparing the experimental data with a new thermodynamic analysis of the magnetic properties of spontaneously magnetized orthorhombic crystals. The larger of the values obtained at 4.2°K for the zerofield anisotropy energy (3.22×${10}^6$ erg/${\mathrm{cm}}^3$ and 5.65×${10}^6$ erg/${\mathrm{cm}}^3$ in the $\mathrm{ac}$ and $\mathrm{bc}$ planes, respectively) is the largest value of this quantity measured so far in any insulator. The contributions of classical dipolar interactions to the zero-field anisotropy energy at 0°K are calculated to be 1.22×${10}^6$ and 4.19×${10}^6$ erg/${\mathrm{cm}}^3$ in the $\mathrm{ac}$ and $\mathrm{bc}$ planes, respectively. The remainder of the anisotropy energy is shown to be probably due to one-ion anisotropy or antisymmetric exchange or both. The largest values obtained for the fractional anisotropy of the spontaneous magnetization (0.24 and 0.55 in the $\mathrm{ac}$ and $\mathrm{bc}$ planes, respectively, at 294°K) are, to our knowledge, the largest values of this quantity measured so far in any material. It is found, moreover, that the anisotropic part of the spontaneous magnetization does not tend to zero as the temperature approaches 0°K. Direct measurements of the anisotropic magnetization are reported for 276 and 294°K, and the results are found to agree satisfactorily with those deduced from the torque measurements.

• Received 28 October 1968

DOI:https://doi.org/10.1103/PhysRev.179.541