Crystallographic and Magnetic Studies of the System (NiFe2O4)1−x + (NiMn2O4)x

The system (NiFe₂O₄)_1−x + (NiMn₂O₄)_x was selected to study the role of the Mn³⁺ ion in determining the crystallographic and magnetic properties of spinels. The ionic distribution was investigated by means of x‐ray diffraction. A cubic spinel structure is obtained throughout the system, the lattice parameter increasing monotonically with x from 8.34 to 8.39 A. It is found that in the range 0≤x≤0.5 that manganese progressively replaces iron on the six‐coordinated sites whereas, in the range 0.5≤x≤1, the manganese progressively replaces iron on the four‐coordinated sites. Therefore, manganese is not able to displace nickel from the six‐coordinated sites and NiMn₂O₄ is a cubic inverse spinel. These results are consistent with the thesis that the tetragonal distortions of ZnMn₂O₄, Mn₃O₄, MgMn₂O₄, and Li_0.5Mn_2.5O₄ are because of the six‐coordinated sites in these compounds essentially all being occupied by manganese ions. Magnetostriction measurements were made as a function of applied field to determine the sign and order of magnitude of the magnetic anisotropy of the polycrystalline samples. The magnetic anisotropy is found to pass through zero with increasing x, becoming highly positive ${\text{(K≃+10}}^5$ ergs/cc) at x=0.5. Heretofore only the Co²⁺ ion has been found to produce a positive anisotropy in the spinel structure. Magnetic moment measurements indicate that all of the manganese is trivalent in the range 0≤x≤0.5. However, magnetic moment measurements over the range 0.5≤x≤1 indicate that at x=1, NiMn₂O₄, the manganese is divalent and quadravalent on the four‐ and six‐coordinated sites respectively. NiMn₂O₄ appears to become ferrimagnetic at approximately 100°K.