Intrinsic and Anneal-Induced Anisotropy in Cobalt-Substituted W-Type Hexagonal Oxides

The intrinsic and anneal-induced anisotropy of cobalt-substituted hexagonal ferrous W compounds containing some vacancies in lattice sites normally occupied by metal ions were measured by the torque method. Six oriented polycrystalline samples, corresponding to $\text{y\hspace{0.17em}=\hspace{0.17em}0.03}$ , 0.06, 0.12, 0.24, 0.45, and 0.9 in the formula ${\mathrm{BaCo}}_y{\mathrm{Fe}}_{\text{17.9−y}}{\mathrm{O}}_{27}$ , and one single crystal of composition ${\mathrm{Ba}}_{\text{0.27}}{\mathrm{Sr}}_{\text{0.73}}{\mathrm{Co}}_{\text{1.845}}{\mathrm{Fe}}_{\text{16.155}}{\mathrm{O}}_{27}$ , were investigated. The results confirm the prediction that the cobalt ions and vacancies would be located in the spinel portion of theW lattice and therefore respond to magnetic annealing in much the same way as in the ferrite system ${\mathrm{Co}}_x{\mathrm{Fe}}_{\text{3−x}}{\mathrm{O}}_4$ . The contribution per cobalt ion to the anneal-induced anisotropy constant $K_u$ was found to be approximately the same in the two systems ($\mathrm{ca}{\text{\hspace{0.17em}3×10}}^{\text{−16}}\hspace{0.17em}\mathrm{erg}/\mathrm{cobalt}$ ion for annealing at 100°C). Other points of comparison include the dependence on cobalt concentration of $K_u$ and the relaxation times characterizing the annealing process, the activation energy and the variation of $K_u$ with temperature of anneal. The first-order intrinsic hexagonal anisotropy constant $K_1$ becomes more negative with increasing cobalt concentration reaching a value of ${\text{−4.35×10}}^6\hspace{0.17em}\mathrm{erg}/\mathrm{cc}$ at 27°C for the single crystal. The value of $K_3$ , the constant which determines the anisotropy in the basal plane, is much larger for this crystal than for any other hexagonal oxide reported so far ($K_3{\text{\hspace{0.17em}=\hspace{0.17em}−1.22×10}}^4\hspace{0.17em}\mathrm{erg}/\mathrm{cc}$ at 27°C and ${\text{+1.35×10}}^5\hspace{0.17em}\mathrm{erg}/\mathrm{cc}$ at −196°C). Two of the compositions ($\text{y\hspace{0.17em}=\hspace{0.17em}0.45}$ and 0.9) have cones of easy magnetization at −196°C.