B-11 NMR study of ferromagnetic CaB6 single crystals

Abstract

B-11 nuclear-magnetic-resonance (NMR) measurements have been performed to microscopically investigate the electronic structure of the ferromagnetic state in three different compositions of calcium-hexaboride single crystals: Ca0.99B6, Ca0.995La0.005B6 and Ca0.99La0.01B6. Although three NMR. lines are expected for the nuclear spin 3/2 of B-11, since the crystal structure of CaB6 is cubic and so all boron sites are equivalent, a larger number of NMR, resonances have been observed. The frequency and intensity of these peaks distinctively change, depending on the angle between crystal axis and magnetic field. Analyzing this behavior, we find that the electric-field gradient (EFG) tensor at the boron site has its principal axis perpendicular to the six cubic faces, with a quadrupole resonance frequency nu(Q) approximate to 600 kHz. Even though the magnetization data highlight the ferromagnetic hysteresis, B-11 NMR. shift and linewidth data show no clear microscopic evidence of the ferromagnetic state in three different compositions of CaB6 single crystals.