1. O. Hartmann, Phys. Rev. Lett. 39, 832 (1977)
2. R. S. Hayano, Y. J. Uemura, J. Imazato, N. Nishida, K.Nagamine, T. Yamazaki, H. Yasuoka and Y. Ishikawa, contribution to this conference.
3. R. Kubo and T. Toyabe, in Magnetic Resonance and Relaxation (ed. R. Blinc, North-Holland Pub. Co., Amsterdam) p. 810.