Detection of ${\mathrm{Al}}_2$${\mathrm{O}}_3$: ${\mathrm{Fe}}^{2+}$ by EPR Relaxation-Time Measurements of ${\mathrm{Cr}}^{3+}$ Ions in Corundum

Angular-dependence experiments of relaxation times $T_1$ for the ${\mathrm{Cr}}^{3+}$ ion have already been reported in a ruby sample doped with silicon. They showed minima in $T_1$ which could not be interpreted in terms of harmonic cross relaxation. The presence of ${\mathrm{Fe}}^{2+}$ ions in the specimen may explain these minima. Ruby samples doped with both iron and silicon have been made to increase the ${\mathrm{Fe}}^{2+}$ concentration by charge compensation with ${\mathrm{Si}}^{4+}$ ions. The minima in relaxation time for ${\mathrm{Cr}}^{3+}$ are enhanced in these crystals and they occur at all the crossing points of the ${\mathrm{Cr}}^{3+}$ and ${\mathrm{Fe}}^{2+}$ lines. The variation of $T_1$ vs angle for ${\mathrm{Cr}}^{3+}$ is studied and the temperature dependence of $T_1$ is investigated from 4.2 to 1.2°K. Further work is in progress to see if our results are in agreement with the sign of the zero-field splitting parameter $D$ found by Lewiner et al. or with the one obtained by Ganapol'skii. Theoretical work by Stevens and Walsh and the acoustical-paramagnetic-resonance experiments by Lewiner et al. and Ganapol'skii on ${\mathrm{Fe}}^{2+}$ in corundum are inconsistent.