5f -electron magnetism in single crystal UN probed by N14 NMR

Abstract

Spin susceptibility and low-frequency dynamics of uranium $5f$ electrons have been investigated by nuclear magnetic resonance (NMR) on the $^{14}\mathrm{N}$ nuclei in paramagnetic and magnetically ordered phases for single crystalline and polycrystalline samples of uranium mononitride (UN). NMR spectra, shifts of the $^{14}\mathrm{N}$ NMR lines, and the spin-lattice relaxation times ${T}_{1}$ have been obtained in the temperature range $T=10--760\phantom{\rule{0.16em}{0ex}}\mathrm{K}$ in magnetic field $B=92.8\phantom{\rule{0.16em}{0ex}}\mathrm{kOe}$. It is shown that in the UN paramagnetic phase, temperature dependence of the $^{14}\mathrm{N}$ NMR line shift is proportional to the spin susceptibility of the uranium $5f$ electrons. Joint analysis of NMR and magnetic susceptibility data allows us to determine temperature dependence of spin fluctuation energy ${\mathrm{\ensuremath{\Gamma}}}_{\mathrm{nmr}}(T)$ of the uranium $5f$ electrons and to demonstrate that its temperature variation is close to $\mathrm{\ensuremath{\Gamma}}(T)\ensuremath{\propto}{T}^{0.5}$ dependence which is characteristic of the concentrated Kondo systems above the coherent state formation temperature. In the magnetically ordered UN phase the $^{14}\mathrm{N}$ NMR spectra consist of several lines that can be explained in terms of the model of type $I$ antiferromagnetic order corresponding to $1k$ structure in the presence of magnetic domains.