DISK SUBSTRATE FOR A PERPENDICULAR MAGNETIC RECORDING MEDIUM AND A PERPENDICULAR MAGNETIC RECORDING MEDIUM USING THE SUBSTRATE

摘要

AN OBJECT OF THE INVENTION IS TO PROVIDE A DISK SUBSTRATE FOR A PERPENDICULAR MAGNETIC RECORDING MEDIUM, THE SUBSTRATE EXHIBI TING SUFFICIENT PRODUCTI VI TY, SERVING A FUNCTION AS A SOFT MAGNETIC BACKING LAYER OF THE PERPENDICULAR MAGNETIC RECORDING MEDIUM, AND SCARCELY GENERATING NOISES. ANOTHER OBJECT OF THE INVENTION IS TO PROVIDE A PERPENDICULAR MAGNETIC RECORDING MEDIUM USING SUCH A SUBSTRATE.THE DISK SUBSTRATE 10 COMPRISES AT LEAST A SOFT MAGNETIC UNDERLAYER 2 FORMED ON A NONMAGNETIC BASE PLATE 1 BY MEANS OF AN ELECTROLESS PLATING METHOD. THE THERMAL EXPANSION COEFFICIENT OF THE SOFT MAGNETIC UNDERLAYER 2 IS LARGER THAN A THERMAL EXPANSION COEFFICIENT OF THE NONMAGNETIC DISK-SHAPED BASE PLATE 1. A SATURATION MAGNETOSTRICTION CONSTANT AS SATISFIES A RELATION AS -1 x 10-5