Absorption and Dispersion in Porous and Anisotropic Polycrystalline Ferrites at Microwave Frequencies

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Abstract
In polycrystalline ferrites the relaxation of ferromagnetic resonance is often considerably increased by the random variations in the orientation of the anisotropy fields in different grains and by the presence of pores. The magnitude of this relaxation has been calculated most often in terms of scattering to spin‐waves, and the theories have been tested experimentally by measurements of the ferromagnetic resonance linewidth as a function of frequency or sample shape. In this paper it is discussed how an effective linewidth W and shift S of the resonance field can be derived from a measurement of the tensor susceptibility χ+, over a wide range of the applied magnetic field at a given frequency. The measuring apparatus is described and experimental results are presented for Ni0.36Zn0.64Fe2O4 with porosities from 0.2% up to 19%, and for a dense material of NiMn0.05Fe1.95O4, at a frequency of 9005 MHz, and applied magnetic fields from 1500–8000 Oe. These results are compared with the predictions of the spin‐wave model. Spin‐wave effects are clearly observed in anisotropy broadening as well as in porosity broadening. However, the perturbation of the spin‐wave manifold itself, respectively, by the nonuniformity of the anisotropy fields and by the inhomogeniety of the static demagnetizing fields around a pore, must be taken into account. The dependence of the relaxation parameters S and W on the applied magnetic field is modified significantly by this perturbation, particularly in the case of porosity broadening.