Possible Formation of Ultrafine Co1-xCaxFe2O4 Particles and Their Magnetic Properties

The morphology and magnetic properties of ultrafine Co1-xCaxFe2O4 particles prepared by chemical coprecipitation have been investigated. For particles heat treated at 800°C, an incremental linear relationship was found to exist between the lattice constant, ao, and the nominal composition of CaFe2O4, X, for 0≤X≤0.7. The curie temperature, Tc, of these ferrites also increases from 518°C to 550°C for 0≤X≤0.5. The saturation magnetization, σs, of ultrafine CoFe2O4 particles is 75 emu/g at 300 K. Curves of σs vs. X appear convex with no linearity at all for 0≤X≤1. These results on a0, Tc, and σs indicate that ultrafine Co1-xCaxFe2O4 particles form as a solid solution, at least when X is in the range 0≤X≤0.7. In samples heat treated at 600°C or more, the grain size drops markedly as X increases. For example, for an 800°C treatment the size is 63 nm when X=0, but only 33 nm when X=0.2. In addition, the amount of a Fe2O3 formed by heat treatment increases as X increases. The largest value of the coercivity, IHc, obtained is 2750 Oe (0.2≤X≤0.3) for a sample heat treated at 800°C. This value is approximately equal to the intensity of the anisotropic magnetic field, Ha, calculated from the crystalline magnetic anisotropy constant, K, , of bulk CoFe2O4. There are two possible causes of the high IHc. One is that the single-domain CoFe2O4 particles might be separated by α-Fe2O3 particles, the other is that, as X increases, the K, of Co1-xCaxFe2O4 particles might become increasingly higher than the value for bulk CoFe2O4.