1. Y. Oka, Oyobutsuri, 57, 894 (1988) [in Japanese].
2. Y. Oka, Oyojiki-Gakkaishi, 17, 869 (1993) [in Japanese].
3. Y. Oka, Materials Science, 31, 16 (1994) [in Japanese].
4. Y. Oka, Proc. 4th Japan-Korea Joint Symposium, p26, Cheju Island, Korea (1994).
5. K. Yanata, K. Suzuki and Y. Oka, J. App[. Phys., 73, 4596 (1993).