Spin-Lattice Relaxation of Cr³⁺ in Coexistence with Ti³⁺ in Al₂O₃

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The shortening of spin-lattice relaxation time caused by a second spin having a fast spin-lattice relaxation is studied. In ruby (Cr; Al₂O₃) containing about 0.02% of Ti³⁺ as an additional impurity, a change of spin-lattice relaxation times of Cr³⁺ has been observed with a variation of paramagnetic concentrations and the frequency difference between Cr³⁺ and Ti³⁺ resonance transitions, at liquid helium temperatures and using the decay method. Experimental results are compared with the predictions of rate equations. At a crossing of the resonance lines of Cr³⁺ and Ti³⁺, the relaxation time of Cr³⁺ for specimens having about 0.1% of Cr can be reasonably described by the rate equations. For 0.02% Cr, simultaneous flip-flop between Cr³⁺ and Ti³⁺ becomes a slow process. In absence of the crossing, the relaxation is also found to be accelerated by the presence of Ti³⁺.