The work I want to describe was started in Bristol in 1959. The purpose was to test theories of spin-lattice relaxation on a system where one could make reasonably quantitative calculations. For this it seemed desirable to study as many aspects of the magnetic properties of one particular system as possible, and to do this both experimentally and theoretically at the same time. The system we chose was cobalt-doped magnesium oxide. This requires some explanation, as at first sight it does not appear to be a particularly simple system. Figure 1 shows how the energy levels of the free cobaltous ion are modified by the crystal field in MgO. The Co2+ substitutes for Mg2+ and is surrounded by a regular octahedraon of O2– ions. The 4F level splits into three, and these are further split by spin-orbit interaction. The important point is that the lowest level is a Kramers doublet.
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M. H. L. Pryce (1965) studied this question.