Anisotropic Spin-Lattice Relaxation Rates of Some Rare-Earth-Doped Lanthanum Trichlorides

Abstract

We have measured the temperature dependence and the angular anisotropy of the spin-lattice relaxation rate $\frac{1}{T_1}$ of cerium, praseodymium, neodymium, samarium, terbium, and erbium in single crystals of anhydrous lanthanum trichloride. The experiments were performed at $X$-band frequencies using pulse-saturation techniques on samples containing ≤2 at.% rare-earth doping for temperatures between 1.65 and 4.2°K. Angular measurements at constant temperature were made for magnetic-field orientations between 0° and 90° with respect to the crystal $c$ axis. The results are interpreted as involving an angular-dependent direct process and isotropic Orbach and Raman mechanisms. These results are compared with theoretically predicted rates obtained from our computer calculations, which are based upon Orbach's approach and require a knowledge of the static crystal-field parameters. Since rare-earth trichlorides have been the object of much optical investigation, these parameters are available for all ions we treat except cerium. Except for this ion, most of the predicted relaxation rates agree with experiments to within a factor of 5 or less, and for all ions the form of the angular anistropy is followed almost exactly. Theoretical treatments which involve the mixing of excited-state wave functions into the ground magnetic doublet through the anisotropic Zeeman interaction are required to explain the observed angular dependence. One of the ions, praseodymium, exhibits a phonon-limited direct-process relaxation rate which is interesting because the apparent hot-phonon equilibration time is about 80 nsec and corresponds to an equilibration path length of only 160μ. Afterneutron bombardment the bottlenecked relaxation rate changed and the equilibration time was apparently reduced to 53 nsec. Prior irradiation of this same sample with 100-keV x rays produced no observable change in the relaxation time. No dependence upon crystal size was observed for the phonon bottleneck in ${\mathrm{Pr}}^{3+}$-doped La${\mathrm{Cl}}_3$.