Measurements have been made at 9 GHz of the circularly polarized components of the susceptibility tensor in dense samples of Ni1-xCo xMn0.05Fe1.95O4, where x=0, 0.015, 0.027, and 0.05. The materials were prepared from powders, obtained by spray drying a mixed solution of sulphate salts, and subsequent thermal decomposition of the dried mixture. After isostatic pressing and conventional sintering, porosities below 0.8% were obtained. The susceptibility data were interpreted in terms of the quantities1 S and W, defined by χ+ = Ms/(H-ω/γ+S+jW/2). The variation of S and W over the spinwave region is in quantitative agreement with Schlömann's theory, 2 although the predicted singularities were not found. Outside the spinwave region W has a constant value: at high field W is proportional to x (16 Oe per % Co); at low fields the values are higher and not linear in x. The absence of the predicted singularities cannot be explained by taking into account exchange or intrinsic damping. Another possible explanation is that the spinwave energies themselves are influenced by the variations in anisotropy field. For a simple variation of the internal field, as given by H i=H0+Aq cosq·r, the interaction between spinwaves parallel to q can be calculated (neglecting exchange interaction). The frequency of these spinwaves is broadened over an amount 2γAq. In a random sample Aq can be taken to be independent of the direction of q. In order to express Aq in terms of the anisotropy constants K1 and K2, the rms value of the anisotropy field in this model is put equal to that in a random sample. Therefore 1/2A q2 = (16/21) [K12+ (2/11) K 1K2+ (5/143) K22]/ M s2. Using this A value, and summing S and W over-all directions of q, quantitative agreement is found with experiment.3 © 1969 The American Institute of Physics.
