Effect of Fluoride-Compensated Co2+ on the Anisotropy of BaFe12O19

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Abstract
Samples in the series BaCoxFe12−xO19−xFx were prepared by firing appropriate mixtures of oxides and fluorides in a carefully dried oxygen atmosphere at 1350°C, with values of x ranging from 0 to 2.0. Single crystals of mixtures having x values of 0.3, 0.4, 0.5, 1.0, and 2.0 were grown from a flux composed of equimolar quantities of Na2CO3 and Fe2O3. The crystals had the BaFe12O19 structure for x values below 0.5 and the BaFe18O27 (W) structure for higher values of x. The mixture with x=0.5 yielded both kinds of crystals. Previous measurements of saturation magnetization in the easy direction had indicated that the Co2+ ions were substituting for Fe3+ in the tetrahedral sites. The positive anisotropy of the crystals decreased with increasing cobalt substitution, reaching a minimum at about x=0.47. A crystal having approximately this composition appeared to have very little anisotropy by qualitative tests. Torque measurements indicated that this crystal has a cone of easy magnetization. Comparisons of the magnetization of powders at a constant field (6000 Oe) with the calculated saturation value indicated that the powders approached saturation most closely at x=0.47, again suggesting a minimum in anisotropy at that concentration. The strong effect of tetrahedral Co2+ on the anisotropy is believed to be related to the presence of a trigonal component in the local crystalline field, owing to the presence of one fluoride ion and three oxygen ions in the immediate vicinity.

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