Possible Formation of Ultrafine Co_1-xCa_xFe₂O₄ Particles and Their Magnetic Properties

Abstract

The morphology and magnetic properties of ultrafine Co_1-xCa_xFe₂O₄ 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 σ_s vs. X appear convex with no linearity at all for 0≤X≤1. These results on a₀, T_c, and σ_s indicate that ultrafine Co_1-xCa_xFe₂O₄ 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, _IH_c, 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 _IH_c. 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 Co_1-xCa_xFe₂O₄ particles might become increasingly higher than the value for bulk CoFe₂O₄.