1. Central Research Laboratory, Hitachi, Ltd., Hitachi Laboratory Office, Information Storage Research Department, 1‐1, Omika‐cho, 7‐chome, Hitachi‐shi, Ibaraki‐ken 319‐12, Japan
2. Data Storage and Retrieval Systems Division, Hitachi, Ltd., Magnetic & Development Department, Advanced Technology Research & Development Division, 2880 Kozu, Odawara‐shi, Kanagawa‐ken 256, Japan