THE VARIATION OF PERMEABILITY WITH FERRITE TILE DENSITY

The effectiveness of ferrites as specular radar absorbers may be related to the volumetric concentration of ferrite in the absorber. This relationship is indicated when the performances of ferrite composites are compared, and when the composites are compared with ferrite tiles. The density of a ferrite tile is dependent on both composition and the conditions of preparation. A series of samples with the composition Ni0.5Zn0.5Fe2O4 was prepared by seven different methods to yield ferrites with densities varying from 3.82 to 5.33 g/cm3. Micrographs are presented to illustrate the structural dissimilarities. The highest density was produced by hot isostatic pressing, a procedure that produced two regions of differing grain size. There was relatively little grain growth in the exterior region, but grains in the interior nearly doubled in size. The greatest effects of density on permeability were noted in the 100-200-MHz region. There, the real part of permeability (mu') increased with density up to 5.1 g/cm3. The imaginary part (mu") also increased with ferrite density. Since the microstructure greatly influences the magnetic loss, and the many processes used resulted in greatly varying microstructural differences, the increase of both mu' and mu" with density was conclusive. The presence of linewidth broadening as a result of energy absorption by various spin-wave modes was confirmed.