The segregated microstructures of electroless-deposited CoNiP perpendicular magnetic recording media with various Hc(⊥) values-500, 1000 and 1500 Oe–were investigated using a selective chemical etching technique, in which Co-rich ferromagnetic regions were preferentially dissolved with an etchant of sodium hydroxide-m-nitrobenzenesulfonic acid-ethylenediamine aqueous solution. TEM observations of the etched 1500 Oe film revealed an "asteroid" pattern with sixfold symmetry, which was inferred to be a substructure with Ni-rich composition. It is thought that the Co-rich regions are segregated between the "arms" of the asteroid substructure. The shape of the substructure varied with the Hc(⊥) of the films; no substructure was observed in the 500 Oe film, while a substructure with a needlelike shape as well as a quasi-asteroid shape were observed in different regions of the 1000 Oe film. The differences in the segregation conditions are strongly dependent on the Hc(⊥) of the films. Further investigation of the 1500 Oe film in the thickness direction clearly indicated that the segregation occurs even in the initial deposition region (30 nm), and that the size of the substructure becomes larger with increasing film thickness.
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