Spin Relaxation ofCr3+in MgO: Evidence for Localized Modes

Measurements of the relaxation of ${\mathrm{Cr}}^{3+}$ spins in cubic sites in MgO have been extended by fast pulsed saturation up to 180 K and by line broadening from 130 to 430 K. Explanations based on harmonic lattice vibrations must be modified to account for an observed linear dependence of $\frac{1}{T_1}$ on $T$ above 350 K. The relaxation is accurately given over seven decades of $T_1$ by $\frac{1}{T_1}=AT+F\left(n\right)+Bcsch\left(\frac{\Delta }{T}\right)$, where $A=0.93\mathrm{}$ ${\sec }^{-1\mathrm{}}$ ${\mathrm{K}}^{-1\mathrm{}}$, $B=1.20\mathrm{}\times {10}^7$ ${\sec }^{-1\mathrm{}}$, $\Delta =537\mathrm{}\pm 5$ K. The expected Raman relaxation function $F\left(n\right)\mathrm{}$ accounts for more than 50% of the relaxation only between 30 and 40 K, and could have the usual form $T^n{J}_{n-1\mathrm{}}$ with $n=5\mathrm{} \mathrm{to} 7$. Apparently, motions localized around each ${\mathrm{Cr}}^{3+}$ ion dominate the ground-state relaxation. The csch function suggests relaxation via localized modes having temperature-independent amplitudes. The localized mode frequencies coincide with a high-density region found in the vibronic sideband of the $R$ line of ${\mathrm{Cr}}^{3+}$ in MgO.