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The effect of heat treatment on the magnetic properties and microstructure of electroless plated CoNiP films for use as perpendicular magnetic recording media was investigated, in order to clarify the film microstructure in the as deposited state, as well as to evaluate their thermal stability. The as-deposited film consisted of a mixture of ≪002≫ perpendicularly oriented hep structure, and fcc structure without any obvious crystal orientation. Heat treatment at temperatures above 400Â°C changed the dominant crystal structure from hep to fcc. The crystallites grow larger with heat treatment, causing a sharp change in magnetic properties. These changes were due to the disappearance of the magnetocrystalline anisotropy induced by the cobalt hep structure, and the decrease in the shape magnetic anisotropy due to the change in film topology. On the basis of these results, it is suggested that a compositional inhomogeneity due to the segregation of Co-rich regions exists in the as-deposited films; this segregation disappears with heat treatment at higher temperature, so that the crystal structure of the film changes to an fcc structure.