1. A large number of references are to be found for instance in: L.M. Levinson, M. Luban, and S. Shtrikman, Phys. Rev., 187, 715 (1969)
2. G. Gorodetsky, B. Lüthi, and T. J. Moran, Intern. J. Magnetism, 1, 295 (1971).
3. U. Atzmony, K. Hardy, and J.C. Walker, J. Phys. (Paris), 32 Cl-920 (1971).
4. U. Atzmony, M.P. Dariel, E.R. Bauminger, D. Lebenbaum, I. Nowik, and S. Ofer, to be published.
5. G.J. Bowden, D. St. P. Bunbury, A.P. Guimaraes, and R.E. Snyder, J. Phys. C. Proc. Phys. Soc., London, 2, 1367 (1968).