MgO Containing Fe++: Magnetic Properties and Clustering

Abstract

The magnetization curves to 7000 Oe at 2° and 4.2°K of magnesiowüstites with 1 to 5 cat.% Fe⁺⁺ have more curvature and a lower extrapolated saturation magnetization (using the Brillouin function) than expected for isolated Fe⁺⁺ ions. This behavior indicates Fe⁺⁺ ion clustering. The magnetization data were used to compute n̄, the average number of Fe⁺⁺ per cluster, and J̄, the average total angular momentum quantum number per cluster taking the isolated Fe⁺⁺ ion value for g. The observed J̄'s showed that the Fe⁺⁺–Fe⁺⁺ near‐neighbor interaction is ferromagnetic. The cluster size increased with Fe⁺⁺ content but J̄/n̄ decreased due to the predominance of the second‐near‐neighbor antiferromagnetic interaction. When magnesiowüstites containing 9 and more cat.% Fe were cooled from room temperature to 4.2° or 2°K in an applied magnetic field, the magnetization curves, which showed a small amount of hysteresis, were not symmetric about the origin. The shifted hysteresis was attributed to exchange coupling between neighboring ferromagnetic and antiferromagnetic regions. The antiferromagnetic order persists to rather low concentrations of Fe⁺⁺ (T_N for 9 cat.% Fe⁺⁺ is above 8°K), further evidence for Fe⁺⁺ clustering. Published values of the thermodynamic activities show a positive deviation from Raoult's law and thus a nonrandom clustered configuration is expected.