Anomalous NMR spin-lattice relaxation in ${\mathrm{SrB}}_6$ and ${\mathrm{Ca}}_{1-x}{\mathrm{La}}_x{\mathrm{B}}_6$

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 ${}^{11}\mathrm{B}-\mathrm{NMR}$ responses exhibit some strikingly common features. Both materials exhibit ferromagnetic order, but their ${}^{11}\mathrm{B}-\mathrm{NMR}$ spectra reveal very small hyperfine fields at the boron sites. The spin-lattice relaxation $T_1^{-1}$ 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.