B-11 NMR study of vortex motion in magnesium-diboride

Abstract

We have performed B-11 nuclear magnetic resonance (NMR) measurements to investigate vortex dynamics in the polycrystalline MgB) superconductor. B-11 NMR spectrum, shift and transverse relaxation rate 1/T-2 were measured down to 4.3 K at similar to 1.8 T. The spectrum below T-c exhibits a typical local field distribution for a vortex lattice under magnetic field. The peak point of the spectrum shifts toward low magnetic field due to incomplete field penetration. Below the irreversibility temperature T-irr, I/ T-2 data, probing the slow motion of vortices, shows a single peak with a small change of the rate, contrary to the results of nickel borocarbides. Also, the relaxation profile changes from Lorentzian decay below Tc to Gaussian decay at lower temperature. It strongly suggests that thermal fluctuation of vortices is reduced at low temperature and the vortex motion is much weaker compared with nickel borocarbides. (c) 2005 Elsevier B.V. All rights reserved.