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

Abstract

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 y = 0.03, 0.06, 0.12, 0.24, 0.45, and 0.9 in the formula BaCoyFe17.9-yO27, and one single crystal of composition Ba0.27Sr0.73Co 1.845Fe16.155O27, were investigated. The results confirm the prediction that the cobalt ions and vacancies would be located in the spinel portion of the W lattice and therefore respond to magnetic annealing in much the same way as in the ferrite system CoxFe 3-xO4. The contribution per cobalt ion to the anneal-induced anisotropy constant Ku was found to be approximately the same in the two systems (ca 3×10-16 erg/cobalt ion for annealing at 100°C). Other points of comparison include the dependence on cobalt concentration of Ku and the relaxation times characterizing the annealing process, the activation energy and the variation of Ku with temperature of anneal. The first-order intrinsic hexagonal anisotropy constant K1 becomes more negative with increasing cobalt concentration reaching a value of -4.35×106 erg/cc at 27°C for the single crystal. The value of K3, 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 (K3 = -1.22×104 erg/cc at 27°C and +1.35×105 erg/cc at -196°C). Two of the compositions (y = 0.45 and 0.9) have cones of easy magnetization at -196°C. © 1960 The American Institute of Physics.