Field dependent spin - lattice relaxation

Abstract

The spin-lattice relaxation times,T(_1), of Cr(^3+)ions in a range of samples of artificial ruby have been measured at 4.2ºK, in the direct ( one phonon ) relaxation process regime, at frequencies of 9.3, 16, 22, 35.5, and 71 GHz, corresponding to magnetic fields, H, up to 25 kG. These results have been compared with the predictions of the Van Vleck theory of paramagnetic relaxation, that T(_1) should be proportional to H(^-2) for a non-Kramer's ion and H(^-4) for an isolated Kramer's doublet. The observed dependencies, between -0.3 and -1, at low magnetic fields are not in agreement with theory, but at high fields (between fields corresponding to 55.5 and 71 GHz) the average dependency for ΔM=1 transitions at polar angle 90 is T(_1) -H(^-2.55) indicating that the energy levels are tending to behave as isolated Kramer's doublets, due to their increasing separation in energy and the decreasing degree of mixing of pure spin states. At low fields the high degree of mixing of states and the masking effect of the zero field splitting on the field dependent energy separation of the energy levels appear to cause the low degree of dependence of T(_1) on H. In contrast to other published collations of T(_1) measurements performed en different samples at various frequencies, all of these results have been obtained under the same experimental conditions from the same samples, frequency of observation (magnetic field) being the only variable. In addition to the variation of T(_1) with H, a linear variation of T(_1) with mean lattice strain, ε , derived from observations of the line width broadening in the samples used, has been observed. The design and construction of the spectrometers used for these measurements is described in detail