NMR OF 24N IN 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 N14 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=K0+αχM, where K0=-(31.5±3.5)×10-4 is different from the Knight shift, + (10.7±1.5)×10-4, of N14 in isostructural ThN. The difference is probably due to some temperature-independent susceptibility of ∼10-3 cgs. The coefficient α is 4.20+0.25 (when χM is in cgs units), which is practically the same value as α for other uranium compounds; α is analyzed on the assumption of U+4 ions (5f2 configuration) and the recent value of 0.876 conduction electrons per uranium atom. In the model with uniform conduction-electron polarization, an exchange constant Jsf 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 Γ=-460 eV 3 is again negative, but is about eight times higher than |Γ| obtained from the paramagnetic Curie temperature (θ-320°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 α 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 N14 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 H0(T-θ) (similar to that in UP with the quadrupole contribution absent). These results are compared with those on N14 in lanthanide nitrides and with uranium compounds that have an ordered state. © 1969 The American Physical Society.