Vortex dynamics in magnesium-diboride superconductor by using B-11 NMR

Abstract

B-11 nuclear-magnetic-resonance (NMR) measurements were performed to investigate vortex dynamics in the polycrystalline superconductor MgB2. B-11 NMR, spectrum, shift and transverse relaxation rate 1/T-2 were measured down to 4.3 K at similar to1.8 T. The spectrum below T exhibits a typical local field distribution for a vortex lattice under a magnetic field. The peak point of the spectrum shifts toward low magnetic field, due to imperfect field penetration. This decrease in the shift starts to appear below the detuning temperature of a NMR, resonant circuit, which is measured to match the irreversibility temperature T-irr of magnetization. 1/T-2 data, probing the slow motion of vortices, show a single peak with a small change in the rate, in contrast to the results for nickel borocarbides. This strongly suggests that thermal fluctuation of vortices is reduced and the vortex motion is much smaller, compared with nickel borocarbides.