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The paper describes measurements made at 34.6 Gc/s on synthetic sapphire (Fe3+ in Al2O3) and chromium-doped rutile (Cr3+ in TiO2) in the temperature range 1.4°K to 56°K. Spin-lattice relaxation times of up to 5 milliseconds at helium temperatures are reported. The main features of the temperature dependence of the relaxation times show a broad similarity with the results previously reported by the authors for ruby, but there are insufficient data to show whether the relaxation times are substantially independent of frequency as was found for ruby.

Measurements of spin-lattice relaxation time in ruby have been made at 34 6 Gc/s by a pulse saturation method at temperatures from 1.4° to 90°K With weak concentrations the values for the first-order transitions (e.g. 22 msec at 4.2°K) are of the same order of magnitude as those reported at lower frequencies, and the variation of relaxation time with temperature is in fair agreement with theory. A successive increase in relaxation time with the order of the transition, pronounced at 1.4°K, decreases with increasing temperature, and at about 77°K a common value of about 44 μsec is obtained for all transitions. The main effects of increasing concentration are to reduce the relaxation time and alter its temperature dependence.

The potentialities of microwave spectrometer techniques in assessing the purity and crystalline quality of synthetic maser crystals are illustrated by a comparison of the paramagnetic properties of ruby samples grown from the vapour phase and from powder by flame fusion. Typical spectra presented are discussed in relation to complementary chemical and spectrographic analysis, and it is shown the information obtained is of direct relevance to the accurate control of methods of crystal growth.