Characterisation of advanced metal particle recording media pigments

Advanced metal particle (MP) pigments used in high density digital storage applications on flexible substrates have been characterised using a wide ra nge of techniques. We have examined the chemical composition of the particl es and find that the core consists of an iron-cobalt alloy in a structure B CC, analogous to that of alpha-Fe. Surface layers on the particle used to p rovide 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 inc rease in the overall saturation magnetisation. From high resolution TEM ima ges 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 th e oxide layer. This indicates the polycrystalline character of the particle s and the crystallite size is in broad agreement with the activation volume of magnetisation reversal determined from an analysis of magnetic viscosit y in the materials. These results are indicative of incoherent magnetisatio n reversal in the particles which may account for the lower than expected c oercivity values in such pigments. (C) 1998 Elsevier Science B.V. All right s reserved.