Paramagnetic Relaxation Measurements on Ce, Nd, and Yb in CaWO4by an Electron Spin-Echo Method

Abstract

Lattice relaxation measurements have been made for ${\mathrm{Ce}}^{3+}$, ${\mathrm{Nd}}^{3+}$, and ${\mathrm{Yb}}^{3+}$ at axial sites in CaW${\mathrm{O}}_4$ by a novel spin-echo method. In this method the spin system is inverted by adiabatic rapid passage, and the magnetization is later sampled by a two-pulse echo sequence. Temperatures were in the range from 1.4 to 9°K, the transfer of cold helium gas being used to maintain temperatures of 4.7°K and above. The relaxation behavior of all the ions appeared to be best described by a direct process ($w\propto T$) going over into a Raman process ($w\propto T^n$) in the vicinity of 4°K. The results could not, however, be fitted to the $T^9$ law, which arises in simple theoretical treatments of the Raman process, but yielded powers of the absolute temperature between 10 and 11. Orbach processes did not seem to be relevant. Two causes for this behavior are considered: dispersion in the phonon spectrum, and the inadequacy of an approximation which is commonly made in treating the denominator of the Raman integral. The Yb relaxation rate showed a marked angular variation in the Raman region. This is discussed in terms of a possible admixture of the two processes which, in the simple theory, lead to $T^7$ and $T^9$ laws.