Spin-Lattice Relaxation ofTi3+Ions in RbAl(SO4)2·12H2O

Abstract

Spin-lattice relaxation measurements with small concentrations of ${\mathrm{Ti}}^{3+}$ ions in single crystals of RbAl${(\mathrm{S}{\mathrm{O}}_{4})}_{2}$\ifmmode\cdot\else\textperiodcentered\fi{}12${\mathrm{H}}_{2}$O have been carried out between temperatures of 4.2 and 1.6\ifmmode^\circ\else\textdegree\fi{}K. The experimental method involved the continuous-wave saturation technique in which comparison was made with a ${\mathrm{Cr}}^{3+}$ impurity of known relaxation time. Over the above temperature range, the values of ${T}_{1}$ varied between 3.3 and 63 msec, indicating Raman behavior at 4.2\ifmmode^\circ\else\textdegree\fi{}K. In general, the magnitudes of the relaxation times are much greater than had been expected, and this fact has been explained by applying the Van Vleck theory to the crystal-field model previously reported for this salt. Agreement between theory and experiment is remarkably good at 4.2\ifmmode^\circ\else\textdegree\fi{}K, but breaks down at the lower temperatures, where the direct process begins to dominate. Cross relaxation with the ${\mathrm{Cr}}^{3+}$ impurity is discussed as a possible explanation for this discrepancy, and a qualitative comparison is made between the relaxation behavior of ${\mathrm{Ti}}^{3+}$ and ${\mathrm{Cr}}^{3+}$ ions in the light of current theories of spin-lattice interaction.