Zeeman Spin-Spin Relaxation in Platinum Metal

Abstract

Pulsed dispersion measurements with high radio-frequency fields are reported here for the ${\mathrm{Pt}}^{195}$ nuclear-spin system in platinum metal. Under the conditions used, these data provide a measure of the Zeeman spin-spin relaxation rate (i.e., the saturation rate) in the rotating reference frame. The results have been analyzed using a thermodynamic approach similar to that given by Provotorov and based on Redfield's hypothesis of a spin temperature in the rotating frame. Data have been obtained with pulses both long and short compared with the spin-lattice relaxation time $T_1$. Good agreement between experiment and theory is found using Zeeman spin-spin relaxation times calculated by means of time-dependent perturbation theory. These calculations are essentially an extension of Provotorov's saturation theory to high radio-frequency fields. For this purpose, the saturation process is considered to be generated by nonsecular dipolar coupling terms in the rotating frame rather than the traditional approach of using the radio-frequency field as the relevant perturbation. Such an approach is closely analogous to the Zeeman spin-spin relaxation theory for spin systems in static fields given by Hartmann and Anderson. These results serve to corroborate other measurements of the pseudodipolar interaction and predominant Ruderman-Kittel exchange coupling in platinum. A reinterpretation of earlier measurements of $T_1$ versus $H_{\theta }$, which deviate from the well-tested theories of Redfield and of Hebel and Slichter, is suggested. This reinterpretation would require an anomalously long Zeeman spin-spin relaxation time for platinum in small dc fields, i.e., nonsecular pseudodipolar coupling terms differing considerably in magnitude from the secular part. Experiments are suggested to test this hypothesis.