We report temperature and magnetic-field dependence of flux pinning force in bulk MgB2 with variable grain size. The samples are prepared by advanced methods, allowing minimizing effects of porosity, impurities, and inclusions of secondary phases. The effects of grain connectivity, flux-creep phenomena, and grain size on critical current density and flux pinning curves are analyzed. We have compared the field dependence of the pinning force for a range of samples with the predictions of theoretical models accounting for the effect of the grain size. There is qualitative agreement between grain-boundary pinning mechanism proposed by Hampshire and Jones J. Phys. C 21, 419 1987 and the experimentally observed grain-size dependence of pinning force in bulk MgB2.

This work was supported by the EU FP6 Project No. NMP3-CT2004-505724 and by the Spanish Ministry of Education and Science, MAT2005-06279-C03-01.

Peer reviewed

et al.