Measurements of the spin-lattice relaxations times of Cr 3+ and Fe 3+ ions in Al 2 O 3 are reported for crystals in which Fe 2+ ions are also known to be present. Marked cross-relaxation to Fe 2+ is observed when the Fe 2+ level splitting equals that of Cr 3+ or Fe 3+ and a quadrupole-electric-field interaction Hamiltonian is used to explain the results. Other cross-relaxation mechanisms are discussed. It is also shown that an analysis of the temperature dependence of the measured relaxation time and its minimum gives a value for the zero-field splitting of Fe 2+ , even though the Fe 2+ concentration is very small.
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Bates et al. (1975) studied this question.
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