1. M.D. Daybell in Magnetism, Vol. 5, edited by H. Suhl (Academic Press, New York, 1973) p. 138.
2. J.J. De Jong, thesis, University of Leiden, 1974 unpublished.
3. C. Rizzuto, Rep. Prog. Phys. 37, 147 (1974).
4. K.H. Fischer, J. Low Temp. Phys. 17, 87 (1974).
5. H. Keiter, E. Müller-Hartmann and J. Zittartz, Z. Phys. 223, 48 (1969).