1. Newman, D.J.: Adv. Phys.20, 197 (1971)
2. Cone, R.L., Meltzer, R.S.: Spectroscopy of rare earth ions in crystals. Macfarlene, R.M., Kaplianskii, A.A. (eds.). Amsterdam: North Holland 1987
3. Ahrens, K., Schaack, G.: Phys. Rev. Lett.42, 1488 (1979)
4. D�rfler, W., Schaack, G.: Z. Phys. B?Condensed Matter59, 283 (1985)
5. Thalmeier, P., Fulde, P.: Z. Phys. B?Condensed Matter26, 323 (1977)