J. Phys. IV France 8 (1 998) Microwave behaviour in Z-type polycristalline hexaferrites

Abstract. The hexaferrites present today a great interest in the field of electromagnetic waves absorbents. We have studied Ba3CqFe24041 2-type hexaferrites.The origin of magnetic permeability in such polycrystalline materials is not so far perfectly known, especially in terms of relations between properties and microstructure. The purpose of this paper is to illustrate some of the issues involved in designing ferrite materials. On the one hand we have dealt with the relation between the elaboration conditions of the materials and their microstructure. On the other hand the influence of the microstructure on microwave behaviour has been studied. Investigated materials were in an intermediate state between the poly and single crystalline one. Co2Z-type hexaferrite (Ba3CozFe24041) with a planar anisotropy has been investigated. The values of the magnetic anisotropy fields B; and B Z of this strongly planar anisotropic ferrite are very different (respectively 1300 mT and 12 mT). This allows to exceed the Snoek law which applies to uniaxial anisotropic materials. T o take advantage of this property in the case of ceramic materials, grains must be orientated. This latter textured ceramic will be considered as single crystal like. Previous studies on hexaferrites [ I] have shown that magnetic permeability at microwave frequencies results from domain walls displacement and gyromagnetism. The purpose of this paper is to study the influence o f microstructure (grain size, density and orientation rate) on microwave behaviour