1. W.J. Caspers, Theory of Spin Relaxation,Interscience, New York, 1964 (and references therein).
2. Spin-Lattice Relaxation in Ionic Solids,A.A. Manenkov and R. Orbach, eds., Harper & Row, New York, 1966 (and references therein).
3. R.M. Steffen and H. Frauenfelder, in Perturbed Angular Correlations, D. Karlsson, E. Matthias, and K. Siegbahn, eds., North-Holland, Amsterdam, 1965.
4. Frauenfelder and R.M. Steffen, in Alpha-, Beta-, and Gamma-Ray Spectroscopy, K. Siegbahn, ed., North-Holland, Amsterdam, 1966.
5. A. Abragam and R.V. Pound, Phys. Ref. 92, 943 (1953).