Abstract
11B nuclear magnetic resonance (NMR) measurements have been performed on MgB2 powder sample at several magnetic fields ranging from 6.3 to 21.9 T. The isotropic Knight-shift, as well as quadrupole coupling frequency, was accurately determined from magic-angle spinning (MAS) spectra by analyzing its magnetic-field dependence. These NMR parameters were then used to interpret its static powder-pattern spectra observed at various magnetic fields, which reflect both the anisotropic Knight-shift tensor and the second-order quadrupole coupling tensor. We found that the Knight-shift tensor and the quadrupole coupling are both axially symmetric and their relative orientation to the molecular frame was also determined. This is the first experimental determination of the anisotropy of the Knight-shift of MgB2 in a powder form from an exact analysis of the powder lineshape of the 11B NMR spectra. The anisotropy was found to be very small (∼4 ppm), being in consistent with previous observations indicating that anisotropy of electronic conductivity in MgB2 is small.
