Angular Dependence of Paramagnetic Relaxation forT22States in Rhombic Symmetry: General Theory and Application toK3(Co, Fe)(CN)6

Abstract

A general expression for the orientational dependence of the direct spin-lattice relaxation rate for strong-field ${}_{}{}^2{}_{}{}^{}T_2^{}{}_{}{}^{}$ salts in a rhombic symmetry is derived, and the results are applied to potassium cobalticyanide lightly doped with iron. It is found that almost all of the anisotropy of the direct rate for this salt is due to the anisotropy of the $g$ tensor. Measurements of the direct rate are reported for iron concentrations of 0.1% and 0.2%, Zeeman frequencies in $S$ and $X$ bands, temperatures from 1.2 to 4.2°K, and for a variety of orientations of the Zeeman field. It is found that the measured orientational dependence can be accommodated within the framework of the theory but that quantitative comparison is not possible. Further, the dependence of the direct rate on the fourth power of the magnetic field is verified for fields in the $c$ crystal direction, and experimental evidence for the presence of a phonon bottleneck is reported for the $c$ orientation and for frequencies of ∼10 Gc/sec. The results of an orientational study of ${\mathrm{Mo}}^{5+}$in Ti${\mathrm{O}}_2$ are also presented.