MODULATION AND CROSS-RELAXATION EFFECTS ON LINE SHAPE OF STRONGLY SATURATED NUCLEAR MAGNETIC RESONANCE IN SOLIDS

We have observed the nuclear-magnetic-resonance dispersion line shape in aluminum, under the common experimental condition of slow passage with magnetic field modulation, while applying an rf field H1 intense enough to produce saturation, and have found that the line shape does not agree with the very-weak-H1 theory of Bloembergen, Purcell, and Pound, or with Redfield's strong-H1 theory. We have qualitatively explained the line shape by assuming that the spin system in the rotating frame can be divided into two parts which have unequal temperatures, and which are able to cross-relax. For two limiting cases large H1 and H1 small, but still large enough to saturate we have calculated the line shape for all modulation frequencies, and have obtained semiquantitative agreement. The small-H1 calculation is based upon the theories of Provotorov and of Goldman. The large-H1 theory is a phenomenological extension of Redfield's theory. We have measured the dipole spin-lattice relaxation time and the cross-relaxation time by observing the signal as a function of modulation frequency. This method has the advantages of both the signal-to-noise ratio of steady-state resonance, and the measurement of time on a frequency scale. © 1968 The American Physical Society.