Paramagnetic Relaxation of Chromium Ion in Ruby and Potassium Alum

Measurements have been made of spin-lattice relaxation times T 1 of single Cr ion impurities in ruby and alum, using the decay method at liquid helium temperature. In ruby, T 1 is dependent on the Cr ion concentration C , for specimens having more than 0.01% Cr, whereas in alum T 1 is nearly independent of C . The difference between them is explained in terms of spin-lattice relaxation via exchange pairs. The relaxation time T 1 in ruby varies as C -1 between 0.044% and 0.28%, which agrees with the theoretical prediction. At higher concentrations, T 1 decreases more rapidly with C and its temperature dependence becomes faster than T -1 . In very dilute crystals, T 1 at 4.2°K is 200 msec and 3 msec in ruby and alum respectively. The comparison of the values of T 1 for both crystals is given on the basis of the Van Vleck mechanism, and it is shown that the orbit-lattice interaction between Cr and ligand ions is larger in ruby than in alum.