Abstract
We have performed 11B nuclear magnetic resonance (NMR) measurements to investigate vortex dynamics in the polycrystalline MgB 2 superconductor. 11B NMR spectrum, shift and transverse relaxation rate 1 / T 2 were measured down to 4.3 K at ∼ 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 , 1 / 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 T c 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.
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