Spin-lattice relaxation of trivalent praseodymium, terbium, and erbium in anhydrous lanthanum trichloride: Temperature and magnetic field dependence

We present the results of our measurements of the spinlattice relaxation rates of several trivalent rare earth ions, in the temperature range 0.25 - 4.2o K. We analyze these results within the framework of the Kronig-Van Vleck-Orbach theory, the main features of which we derive in a semi-rigorous manner. We used the pulse saturation method, operating at a microwave frequency of 15.5 GHz. Through additional measurements of our own and of others on the same samples at 9 GHz, we were able to check the frequency (or field) dependence of the relaxation rates. The non-Kramers Pr:LaC13 system was bottlenecked in the direct region and hence its relaxation rate was essentially frequency independent. The Kramers system Er:LaC13 showed a direct process frequency dependence which was very close to the fourth power law of Van Vleck; the non-Kramers system Tb:LaC13 had a direct relaxation process which varied with the square of the frequency, a variation close to that predicted. Actually, the Tb:LaC13. system is somewhat unusual in that a crystal field splitting of its ground doublet results in the composition of its two lowest states being a function of the magnitude of the applied field. This situation should result in a fairly strong field dependence for the Raman and Orbach coefficients, a dependence which is observed experimentally. In contrast, no frequency dependence was expected or observed for the Raman and Orbach coefficients for Er:LaC13 , The low temperatures used in these experiments were attained through the use of a helium-three cryostat. The construction of this cryostat and the associated ESR pulse spectrometer are described in some detail.U of I OnlyThesi