Spin lattice relaxation in rare earth ethylsulphates. I

Abstract

Abstract The results of Hartshorn bridge measurements at low temperatures on dysprosium and ytterbium ethylsulphate are presented and discussed. The g -values, Curie-Weiss temperatures, and magnetic specific heat constants are in good agreement with literature values. The perpendicular susceptibility of DyES is shown to be largely due to second order Zeeman effect; its theoretical value, calculated from the Powell and Orbach wave functions, agrees within 7% with the experiments. The spin-lattice relaxation times in DyES are in good agreement with those from other authors and, above 1.7°K, with Orbach's theory for the two-phonon resonance process. There are indications of the two-phonon process through the second excited Kramers doublet at k × 29.1° K above the ground state. Below 1.7°K deviations from the theory of the direct process occur, similar to the deviations found in many salts of the iron groep. In ytterbium ethylsulphate the temperature dependence of the inverse relaxation time may be described by a sum of T,T 9 and exp -59/ T or exp -46/ T terms. The values of τ estimated from Orbach's theory are, however, more than a thousand times larger than those experimentally found. In both salts the magnetic field dependence of the relaxation times agrees well with the Brons-Van Vleck formula in the two-phonon region. Significant deviations from the theory of Casimir and Du Pre are found only in the DyES single crystal ( c -axis direction), above the lambda point of liquid helium. The agreement with Eisenstein's theory is rather poor, but the estimated values of the lattice heat conductivity agree with values previously found in other crystals. With the same heat conductivity, it is possible to explain why no deviations are found in the ytterbium ethylsulphate single crystal.