Magnetic Resonance Line Shapes at the Onset of Saturation

Magnetic resonance absorption line shapes and widths in Li metal and CaF₂ crystals have been measured as a function of radio-frequency power level. The measurements have been made on the Li⁷ resonance in Li at 77^∘{}K and 215^∘{}K and on the F¹⁹ resonance in CaF₂ at room temperature. The measurements have been made from low rf power into the region of appreciable saturation, but not into the very high saturation region considered by Redfield. A decrease in δH, the peak-to-peak absorption derivative line width, was observed in both systems at sufficiently large values of H₁, the rf magnetic field. The changes of line width in Li at various values of T₁ and of characteristic motional frequency of the nuclei show good qualitative agreement with the criteria proposed in the recent theory of Tomita. Line-width data obtained by Redfield in aluminum are included for comparison. In the case of CaF₂, where the spin-lattice relaxation proceeds via paramagnetic impurities, the decrease in δH occurs at appreciably higher saturation levels than is the case in Li and Al.