Microwaves properties of M and Z type hexaferrites

Bayum hexaferrites are anisotropic iron oxides which can present high values of permeability. We have studied Ba(Co,Ti)(x)Fe12-2xO19 (M type) and Ba3Co2Fe24O41 (Z type) compositions. Powders are synthetized using a classical ceramic method (M type) or a chemical one (Z type) consisting in the coprecipitation of carbonates. In both cases, the resultant powder is ground in aqueous suspension and casted under magnetic field in a plaster matrix. Materials are sintered under different conditions of atmosphere, time and temperature. They are characterized by radioelectric measurements in a coaxial line with or without a static magnetic field. A parametric study concerning the various stages of the elaboration has been carried cut in order to optimize density, orientation rate and phase purity. In both cases (Z & M type hexaferrite), permeability vs frequency curves have been fired by means of a least squares method, in Lorentzian contributions which describe as well gyromagnetism as walls contributions. In all cases, magnetic losses are the sum of the both contributions : gyromagnetism and walls. Nevertheless this last one presents a more or less important character of relaxation which could be linked to microstructure. Measurements of permeability under a static magnetic field confirm the previous result. In the case of Z type hexaferrites, the evolution of the relaxation character of the walls would be the result of the conjunction of two parameters : the ferrous ions, relaxing impurities leading to a local variation of anisotropy, and granular cohesion which ensures the continuity of magnetic flux and promotes cooperative phenomena like wail relaxation.