Characterisation of advanced metal particle recording media pigments

Abstract Advanced metal particle (MP) pigments used in high density digital storage applications on flexible substrates have been characterised using a wide range of techniques. We have examined the chemical composition of the particles and find that the core consists of an iron–cobalt alloy in a structure BCC, analogous to that of α-Fe. Surface layers on the particle used to provide oxidation resistance, have been found to contain some form of oxides of Co, Al and Y as well as iron oxide which we find to be in a spinel form. From Mossbauer spectroscopy we have determined the proportion of the iron atoms in the oxide layer and find that by controlling the percentage of Y in the surface, a much thinner oxide layer results with a corresponding increase in the overall saturation magnetisation. From high resolution TEM images we have determined the particle size of each sample and compared those values with those obtained from X-ray line broadening. We find that in each case the X-ray `size' is about a quarter of the physical size excluding the oxide layer. This indicates the polycrystalline character of the particles and the crystallite size is in broad agreement with the activation volume of magnetisation reversal determined from an analysis of magnetic viscosity in the materials. These results are indicative of incoherent magnetisation reversal in the particles which may account for the lower than expected coercivity values in such pigments.