NMR studies of EuB6 at low temperatures

We report results of Eu-153 and B-11 nuclear magnetic resonance (NMR) measu rements on EuB6 at temperatures between 0.1 and 150 K and in external magne tic fields between H = 0 and 7 T. The evolution of the Eu-153 NMR spectra a t low temperatures and low fields provides evidence for an unexpected and c omplex electronic ground state of EuB6, involving the coexistence of two ma gnetically very similar phases. The temperature and magnetic-field dependen ces of the B-11 spin-lattice relaxation rate T-1(-1)(T,H) are very well acc ounted for by magnon-driven relaxation. A spin-wave theoretical interpretat ion of the Eu-153-NMR spectra and the B-11 T-1(-1) measurements indicate th at a gap of the order of 1 K exists in the magnon excitation spectrum. [S01 63-1829(99)08329-0].