NMR Studies of Magnetism in Borides with Low Carrier Density.

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YbB_<12> is known as a typical Kondo insulator. Since Igaw et al. at the Hiroshima University succeeded to grow large single crystals, progress have been made in the measurements of thermal and magnetic properties, transport properties, spectroscopies, and neutron scattering. In this project, we have performed nuclear magnetic resonance (NMR) experiments at the Yb, B and Lu sites for single-crystal samples of pure and 1% Lu doped YbB12. In undoped YbB12, the nuclear spin-lattice relaxation rate at the Yb nuclei shows an activated temperature dependence, indicating an energy gap in the spin excitation spectrum due to either f-p hybridization or Kondo singlet formation. The nuclear spin-lattice relaxation rate at the B nuclei shows similar temperature dependence as the Yb nuclei at high temperatures. However, it increases below 20 K and shows a broad peak near 6 K. The cause for this anomalous temperature dependence was identified to be localized moments induced near dilute defects. It h … More as been theoretically suggested that in Kondo insulators, non magnetic impurities or defects of rare earth elements could lead to localized magnetic moments induced.near defects. The results of B NMR appear to be related to such defects-induced local moments. In order to further confirm this, we measured the relaxation rate at the B and. Lu nuclei in the 1%-Lu doped sample. Both nuclei show similar temperature dependence of the relaxation rate, but the Lu nuclei show 10 times larger relaxation rate than the B nuclei. This indicates that the localized moments are induced near Lu sites.We also found a sharp peak in the relaxation rate at the B nuclei near 30 K in undoped YbB_<12>, which is due to cross relaxation between the Yb and B nuclei near 30 K where the resonance frequency for Yb and B nuclei become equal. This enables us to determine the Yb resonance frequency through the measurements on B nuclei.Strong recent attention has been paid on CaB_6 with very dilute La doping, since the ferromagnetism was reported for this system in spite of the absence of any "magnetic elements" such as transition metal elements. However, extremely small magnetization made it difficult to clarify whether the ferromagnetism is intrinsic to this system or due to impurity phases. If it is intrinsic, the ferromagnetic moment should reside mainly on the 2p orbitals of B sites, which we should be able to detect by B NMR. We have, measured anisotropy of the local field at the B sites in 0.5 %-La doped CaB_6 single crystals with high accuracy. We found that three crystals among five examined show finite internal field at B nuclei, suggesting intrinsic ferromagnetic moments. However, the values of the internal field are not correlated well with the bulk magnetization of each crystal. Less