Magnetic behavior of quenched and aged CoFe2O4–Co3O4 alloys

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Abstract
The saturation magnetization (Ms) and intrinsic coercive force (Hc) of spinel‐structured CoFe2O4–Co3O4 samples were measured in the quenched condition and also after aging inside the miscibility gap which is present below 860°K in this system. Magnetic properties were interpreted through correlation with the sample's microstructures. The isothermal Ms of as‐quenched samples decreases approximately linearly with Co content. At low temperatures compensation compositions exist near 25 cation% Fe. From the composition dependence of Ms at low temperatures, and assuming Néel's model, it was concluded that the addition of Co to CoFe2O4 made the cation distribution more normal and that Co3+ entered B sites diamagnetically replacing Fe3+. The intrinsic coercive force of as‐quenched samples were small at room temperature but those at low temperatures were very large; Hc values at 77°K were 22, 15, and 4.8 kOe for 30, 35, and 50 cation% Fe samples, respectively. This was attributed to the presence of compensated M=0 regions in the samples due to composition fluctuations arising from spinodal decomposition which occurred during the quench. It was suggested that these act as large energy barriers to domain‐wall motion by a process similar to exchange anisotropy. Compensation does not occur at temperatures higher that 200°K; thus Hc at room temperature is small. The Hc values of samples fully aged at 500°C measured at room temperature were 7 and 4.5 kOe for 35 and 50 cation%Fe, respectively, while those measured at 77°K were 22 and 12.5 kOe. The maximum energy products (BH) at room temperature were 0.25 and 1.0×106 G Oe. Aging of 45 and 50 cation % Fe samples at 700°C did not cause any large increase in Hc because 700°C is outside the spinodal. The large Hc can be explained by single‐domain precipitates of composition near CoFe2O4 (which has a large crystalline anisotropy) produced by spinodal decomposition where isolated regions are formed due to the wavelike nature of the decomposition product. The small Hc obtained on aging outside the spinodal is interpreted by classical nucleation and growth decomposition which does not produce isolated ``CoFe2O4'' precipitates when it is the major component.

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