Nuclear Magnetic Relaxation ofP31in Diamagnetic ThP and the Paramagnetic State of UP

Abstract

The spin-lattice relaxation time $T_1$ and free-induction decay time constant ${T_2}^{*}$ for ${}_{}{}^{31}{}_{}{}^{}\mathrm{P}$ as functions of temperature and applied magnetic field have been studied in diamagnetic thorium monophosphide (ThP) and the paramagnetic state of uranium monophosphide (UP) (antiferromagnetic below $T_N\simeq 125$°K). Both compounds have the same NaCl-type structure and similar low electrical resistivities, so that the effect of the magnetic character of UP is manifested in the different behavior of the relaxation times in UP as compared with ThP. ${T_2}^{*}$ has been determined from the shape of the Bloch decay following a 90° pulse; $T_1$, from the amplitude of the Bloch decay following a 180°-90° pulse sequence. Pulse measurements have been carried out at 4, 8, and 12 MHz and 77-303°K; the values obtained are consistent with the previously reported continuous-wave (cw) data. In ThP, ${T_2}^{*}$ is 140±5 μsec and is independent of temperature and magnetic field, while $T_{1}T=16.2\mathrm{}\pm 0.8$ sec °K is constant. This supports the metallic behavior and the Korringa mechanism for the relaxation. The ratio of the experimental $T_{1}T{K_e}^2$ to the theoretical Korringa value is 1.95±0.25, which is comparable to that of other metals. In UP, ${T_2}^{*}$ values are 15-70 μsec, with the same temperature and field dependence as the cw linewidth. The $T_1$ values are three orders of magnitude shorter than in ThP, but increase with temperature, and $T_{1}T$ increases strongly with temperature. However, when the $T_{1}T$ values are combined with the Knight shift $K$ in UP, a Korringa-type relation $T_{1}T{K}^2=(38\mathrm{}\pm 6)\times {10}^{-6\mathrm{}}$ sec °K is obtained, with a value of 24±4 for the ratio of the experimental $T_{1}T{K}^2$ to the theoretical Korringa value. This ratio is higher than in most metallic materials. The Korringa-type relationship between $T_1$ and $K$ in UP suggests that the relaxation is founded in the same mechanism as the Knight shift, namely, an indirect coupling between the ${}_{}{}^{31}{}_{}{}^{}\mathrm{P}$ nuclei and the uranium localized moments via the conduction electrons. The results in UP are compared with those in $\beta -{\mathrm{UH}}_3$, which shows a different behavior in the paramagnetic state, and with other recent results.