POLYCRYSTALLINE FERRITE FOR 150-MHZ RECORDING APPLICATIONS

As recording frequencies are increased above 50 MHz, the permeability of conventional Mn-Zn ferrites becomes prohibitively small with high losses. A material with low losses in this frequency range is needed. Ferrites with the general formula Ba2Me2Fe12O22, referred to as ferroxplana, have a large negative crystalline anisotropy resulting in a flat in-plane permeability spectra beyond 200 MHz. Unfortunately , single-crystal ferroxplana is difficult to use, owing to its poor machinability. Processes to fabricate highly oriented polycrystalline ferroxplana have been developed. Coprecipitation and molten salt firing were used to synthesize the starting materials. Powders were oriented, sintered, and hot isostatically pressed to theoretical density. A highly oriented polycrystalline sample was successfully fabricated, which had flat permeability spectra up to 200 MHz and a mu' = 54 at 150 MHz. This is the highest reported value for polycrystalline ferroxplana.