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Measurements of spin-lattice relaxation times of single crystals of ruby have been carried out using the pulse saturation technique at a frequency of 9.27 kMc/sec and, generally, in the helium temperature range. Results for Verneuil-grown specimens agree with those already reported: concentration-dependent relaxation times, almost independent of crystal orientation. For specimens grown by the so-called vapor-phase modification, the relaxation time is longer and independent of concentration over a range 0.005 to 0.2 at.% Cr 3+ . It exhibits strong angular dependence of the form predicted by Donoho, and calculation and experiment are in satisfactory quantitative agreement. It is concluded that vapor-phase rubies represent a "clean" magnetic system, and the suggestion is made that fast-relaxing impurity ions may account for the concentration-dependent parameter usually observed.

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