Relaxation ofMn2+andFe3+Ions in Magnesium Oxide

The relaxation of ${\mathrm{Mn}}^{2+}$ and ${\mathrm{Fe}}^{3+}$ ions in MgO has been studied down to helium temperatures by saturation recovery and by cross-saturation techniques using two frequencies. Data on samples containing between 10 and 2000 ppm paramagnetic impurity are interpreted within the framework of current theories of cross relaxation and spin-lattice relaxation. The first part of the paper is devoted to cross-relaxation processes. At helium temperatures we observed that cross saturation occurs in almost all samples in a seemingly random fashion between resonances separated by more than 50 linewidths. This behavior is due to cross relaxation by a forbidden three-spin process. Other cross-relaxation processes were also observed and are discussed. The second part of the paper is devoted to spin-lattice relaxation. At helium temperatures the spin-lattice relaxation time was observed in most samples to be concentration-dependent and shorter than what is expected from the conventional process of single ions relaxing directly to the lattice. It was observed that the spin-lattice relaxation time could be changed substantially by heating the sample in hydrogen or oxygen to change the relative concentration of divalent and trivalent iron and by varying the rate at which the samples were cooled to room temperature. It was concluded that while there is no evidence to indicate the exact nature of the relaxation mechanism, our data are consistent with the assumption that the spins which are weakly coupled to the lattice relax by cross relaxation to sites which are strongly coupled to the lattice. It appears, however, that the ${\mathrm{Fe}}^{2+}$ spins, which are strongly coupled to the lattice, do not play an important role in the relaxation process. Finally, from observations of the relative relaxation rate as a function of the relative resonance intensities, we have concluded that the average rate for cross relaxation to the strongly coupled spins is a function of the concentration of weakly coupled spins, and that in performing the average over the weakly coupled spins it is necessary to include all spins that are in levels that are connected by transitions that are fast compared to the cross-relaxation rate.