A COHERENT MODEL FOR THE COMPLEX PERMEABILITY IN POLYCRYSTALLINE FERRITES

We demonstrate that a grain size dependence exists for the rotational permeability of a series of MnZn polycrystalline ferrites, analagous to that predicted by the Globus model for wall permeability. To account for this behaviour, we have developed a model which considers crystalline ferrite grains with intrinsic complex permeability µi, surrounded by thin, nonmagnetic grain boundaries. The effectively measured permeability of the polycrystal (μe) is related in our model to the intrinsic permeability, the grain size (D) and the grain boundary thickness (δ) according to the equation For small grains, the polycrystal effective permeability obtains the observed linear dependence on grain size, for large grains µe approaches µ i, which is in fact the rotational permeability of a single crystal. In the situation where µi is complex, and displays the relaxational type of frequency dependence where Snoeks relationship is obeyed, our model predicts that this relationship will remain intact independent of the grain size. Finally, as the origin of µi (rotational or domain wall) is not explicitly specified in our model, we believe that this model represents a good alternative to the Globus model in describing the initial domain wall permeability in polycrystalline ferrites. © 1990, IEEE. All rights reserved. © 1990 IEEE