NMR OF $sup 14$N IN 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 \ensuremath{\sim}53\ifmmode^\circ\else\textdegree\fi{}K, and obeys the Curie-Weiss law in the paramagnetic state in the temperature range 77-300\ifmmode^\circ\else\textdegree\fi{}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 $K={K}_{0}+\ensuremath{\alpha}{\ensuremath{\chi}}_{M}$, where ${K}_{0}=\ensuremath{-}(31.5\ifmmode\pm\else\textpm\fi{}3.5)\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}4}$ is different from the Knight shift, + (10.7\ifmmode\pm\else\textpm\fi{}1.5)\ifmmode\times\else\texttimes\fi{}${10}^{\ensuremath{-}4}$, of $^{14}\mathrm{N}$ in isostructural ThN. The difference is probably due to some temperature-independent susceptibility of \ensuremath{\sim}${10}^{\ensuremath{-}3}$ cgs. The coefficient $\ensuremath{\alpha}$ is 4.20+0.25 (when ${\ensuremath{\chi}}_{M}$ is in cgs units), which is practically the same value as $\ensuremath{\alpha}$ for other uranium compounds; $\ensuremath{\alpha}$ is analyzed on the assumption of ${\mathrm{U}}^{+4}$ ions ($5{f}^{2}$ 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\ifmmode\pm\else\textpm\fi{}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 $s\ensuremath{-}f$ coupling constant $\ensuremath{\Gamma}=\ensuremath{-}460$ eV ${\mathrm{\AA{}}}^{3}$ is again negative, but is about eight times higher than $|\ensuremath{\Gamma}|$ obtained from the paramagnetic Curie temperature ($\ensuremath{\theta}\ensuremath{\simeq}\ensuremath{-}320\ifmmode^\circ\else\textdegree\fi{}$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 $\ensuremath{\alpha}$ 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 $\frac{{H}_{0}}{(T\ensuremath{-}\ensuremath{\theta})}$ (similar to that in UP with the quadrupole contribution absent). These results are compared with those on $^{14}\mathrm{N}$ in lanthanide nitrides and with uranium compounds that have an ordered state.