NMR studies ofEuB6at low temperatures

We report results of ${}^{153}\mathrm{Eu}$ and ${}^{11}\mathrm{B}$ nuclear magnetic resonance (NMR) measurements on ${\mathrm{EuB}}_{6}$ at temperatures between 0.1 and 150 K and in external magnetic fields between $H=0$ and 7 T. The evolution of the ${}^{153}\mathrm{Eu}$ NMR spectra at low temperatures and low fields provides evidence for an unexpected and complex electronic ground state of ${\mathrm{EuB}}_{6}$, involving the coexistence of two magnetically very similar phases. The temperature and magnetic-field dependences of the ${}^{11}\mathrm{B}$ spin-lattice relaxation rate ${T}_{1}^{\ensuremath{-}1}(T,H)$ are very well accounted for by magnon-driven relaxation. A spin-wave theoretical interpretation of the ${}^{153}\mathrm{Eu}\ensuremath{-}\mathrm{NMR}$ spectra and the ${}^{11}\mathrm{B}$ ${T}_{1}^{\ensuremath{-}1}$measurements indicate that a gap of the order of 1 K exists in the magnon excitation spectrum.