Nuclear-Magnetic-Resonance Study of ${\mathrm{Co}}^{2+}$ in CdS

Nuclear spin-lattice relaxation times, $T_1$, have been measured over a temperature range $T=(1.1-300)\mathrm{} °$K and frequency range $\nu =2-15\mathrm{}$ MHz in a single crystal of CdS doped with 13-ppm cobalt. Minima in $T_1$ vs $T$ are observed, and absolute values of the effective electron relaxation time, ${\tau }_e$, may be calculated at the temperatures of the minima, e.g., at 14 °K, ${\tau }_e=8.0\mathrm{}\times {10}^{-8\mathrm{}}$ sec. At low temperatures, $T\lesssim 5 °$K, ${\tau }_e$ is dominated by a resonant Orbach process involving the two ground-state Kramers doublets ($S=\pm \frac{3}{2}$ and $S=\pm \frac{1}{2}$) which, according to our measurements, are split by $\Delta =(4\mathrm{}\pm 1) °$K at zero magnetic field. At higher temperatures, $T\gtrsim 20 °$K, ${\tau }_e$ is dominated by a nonresonant process, and these data are well fitted from 40 to 300 °K by ${\tau }_e^{-1\mathrm{}}\propto T^5{J}_4\left(\frac{210}{T}\right)$, where $J_4$ is a transport integral and 210 °K is the CdS Debye temperature.