Studies of 3d4 ions in Al2O3 by cross-relaxation

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Abstract
Measurements are reported of the spin-lattice relaxation time T 1m of the Cr3+ ion in Al2O3 in the presence of either Cr2+ or Mn3+ ions. Both resonant (RCR) and non-resonant (NCR) cross-relaxation processes are identified, the latter in zero magnetic field. By carefully choosing the frequency, RCR to Cr2+ and Mn3+ are clearly visible in a number of samples. Temperature-dependence measurements on the Cr2+-containing crystals give results in agreement with the theoretical calculations detailed here. The theory of NCR is developed and used to extract the energies of the first excited states from the Cr2+ and Mn3+ NCR data. The values obtained are the same as those reported from phonon spectroscopy measurements. Finally, the relation between measurements of T1m and theoretical calculations in any system is discussed in general and discrepancies are accounted for in terms of NCR to either trace impurities or clusters or both

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