Anomalous NMR spin-lattice relaxation inSrB6andCa1−xLaxB6

We report the results of ${}^{11}\mathrm{B}$ nuclear magnetic resonance (NMR) measurements of ${\mathrm{SrB}}_{6}$ and ${\mathrm{Ca}}_{0.995}{\mathrm{La}}_{0.005}{\mathrm{B}}_{6}$ below room temperature. Although the electrical resistivities of these two materials differ substantially, their ${}^{11}\mathrm{B}\ensuremath{-}\mathrm{NMR}$ responses exhibit some strikingly common features. Both materials exhibit ferromagnetic order, but their ${}^{11}\mathrm{B}\ensuremath{-}\mathrm{NMR}$ spectra reveal very small hyperfine fields at the boron sites. The spin-lattice relaxation ${T}_{1}^{\ensuremath{-}1}$ varies considerably with external field but changes with temperature only below a few K. We discuss these unusual results by considering various different scenarios for the electronic structure of these materials.