NMR ofN14in the Paramagnetic State of Uranium Mononitride

Uranium mononitride (UN) has the NaCl-type structure, is a good conductor of electricity, is antiferromagnetic (type I) below ∼53°K, and obeys the Curie-Weiss law in the paramagnetic state in the temperature range 77-300°K. A continuous-wave NMR study was carried out on ${}_{}{}^{14}{}_{}{}^{}\mathrm{N}$ in this temperature range and in applied magnetic fields of 6340-13 000 Oe by the use of a Varian V-4210A spectrometer. The NMR signals were detected only in the dispersion mode and only by using high modulations and rf fields. The positive temperature-dependent Knight shift can be represented by $K=K_0+\alpha {\chi }_M$, where $K_0=-(31.5\mathrm{}\pm 3.5)\times {10}^{-4\mathrm{}}$ is different from the Knight shift, + (10.7±1.5)×${10}^{-4\mathrm{}}$, of ${}_{}{}^{14}{}_{}{}^{}\mathrm{N}$ in isostructural ThN. The difference is probably due to some temperature-independent susceptibility of ∼${10}^{-3\mathrm{}}$ cgs. The coefficient $\alpha$ is 4.20+0.25 (when ${\chi }_M$ is in cgs units), which is practically the same value as $\alpha$ for other uranium compounds; $\alpha$ is analyzed on the assumption of ${\mathrm{U}}^{+4\mathrm{}}$ ions ($5f^2$ configuration) and the recent value of 0.876 conduction electrons per uranium atom. In the model with uniform conduction-electron polarization, an exchange constant $J_{\mathrm{sf}}$ of - (1.0±0.2) eV is obtained that has the same sign as in the lanthanides, but is about one order of magnitude larger; the variation is probably due to the larger extent of the $5f$ electronic functions. In the Ruderman-Kittel-Kasuya-Yosida (RKKY) model, the $s-f$ coupling constant $\Gamma =-460\mathrm{}$ eV ${\mathrm{\AA }}^3$ is again negative, but is about eight times higher than $\left|\Gamma \right|$ obtained from the paramagnetic Curie temperature ($\theta \simeq -320\mathrm{}°$K) and the magnetic contribution to the resistivity. The variation of T is mainly due to the sensitivity of the RKKY sums, but also may be due to the simple RKKY model used. The constant value of $\alpha$ in uranium compounds, which have an ordered magnetic state, is an indication of the nonapplicability of the RKKY model to the analysis of the Knight shifts. The linewidth of ${}_{}{}^{14}{}_{}{}^{}\mathrm{N}$ in UN depends on the temperature and magnetic field. The linewidth is analyzed in terms of a constant quadrupole interaction (similar to that which contributes to the total linewidth in ThN) and a magnetic contribution that is proportional to $\frac{H_0}{(T-\theta )}$ (similar to that in UP with the quadrupole contribution absent). These results are compared with those on