Proton Polarization and Relaxation in Ytterbium-Doped Yttrium Hydroxide

Measured proton-spin-lattice relaxation times and proton-polarization enhancements are reported for four samples of Y${\mathrm{}\left(\mathrm{HO}\right)\mathrm{}}_3$:Yb with Yb content ranging from 0.01 to 0.27 at.%. The protons were dynamically polarized above their thermal equilibrium values by mechanically rotating the sample in a spin refrigerator. Relevant theories are discussed. The crystal-field splitting for the ${\mathrm{Yb}}^{+3\mathrm{}}$ ion in Y${\mathrm{}\left(\mathrm{HO}\right)\mathrm{}}_3$ is estimated from data for several other rare-earth hydroxides. Experimental data are presented for the rate of the enhanced-proton-polarization buildup and are in agreement with the theory. The temperature and field dependences of the proton-spin-lattice relaxation rate are shown to agree with theory. A dependence of this rate on the ${\mathrm{Yb}}^{+3\mathrm{}}$-ion concentration which differs from the theory is reported. In the low-temperature region where the ion-spin-lattice relaxation is dominated by the direct process the proton rate has a first-power concentration dependence as expected, but at higher temperatures where the Raman process is dominant an exponential dependence of the proton rate on the ${\mathrm{Yb}}^{3+}$ concentration is observed, but not explained. Attempts are made to qualitatively explain observed changes of the proton line shape and the relaxation-recovery curves with changing concentration.