Relaxation processes in a multilevel spin system investigated by linewidth analysis of the multifrequency high-frequency EPR spectra

The line shape measurements of the high-field high-frequency electron paramagnetic resonance transitions for paramagnetic Pb3+ in calcite performed at four frequencies between 95 and 285 GHz as function of temperature are reported. The linewidth analysis is based on a recent theory of relaxation for multilevel spin systems at high temperatures and is used to identify the spin lattice relaxation (SLR) mechanisms at such high microwave frequencies. In comparison with previous results obtained at lower microwave frequencies, this analysis emphasizes for a multilevel spin system two relevant new features: (a) the direct SLR process by modulation of the hyperfine interaction significantly contributes to the linewidths even at room temperature and (b) the SLR time of some simple transitions measured from the linewidth under conditions of negligible saturation could be very different from that measured by the continuous saturation method. The difference between the perpendicular-detected transitions and the parallel-detected transition concerning the contributions of the different SLR processes to the SLR times measured by the two methods mentioned above comes out to be very significant.