Spin-Relaxation ofHo3+in Yttrium Ethyl Sulfate between 1.2 and 25 K

$T_1$ was measured directly for samples containing 1 and 0.1 at.% ${\mathrm{Ho}}^{3+}$ using the method of pulse-saturation recovery between 1.1 and 3.5 K at $\mathrm{Ku}-$ and $X$-band frequencies. These data are interpreted as due to a sum of a phonon-limited direct relaxation process, a phonon-limited Orbach process which proceeds through an excited state with $\frac{\delta E}{k}=8\mathrm{}$ K, and a $T^7$ Raman process. Cross-relaxation effects among the hyperfine lines were noted in these $T_1$ data for the 1-at.% samples. Linewidths were also measured between 1.2 and 25 K and the more concentrated samples exhibited a minimum near 10 K. Above 15 K the relaxation-induced linewidth increased as $T^7$. The minimum in the linewidth is explained on the basis of decreased dipolar broadening, as the nonmagnetic state at 8 K is populated, followed by line broadening due to a dominant Raman relaxation mechanism. $\frac{T_2}{T_1}$ is estimated to lie between 0.9 and 1.8 for this $T^7$ Raman process.