The magnetic field dependent critical current density jc(B) of a MgB2 bulk sample has been obtained by means of magnetization hysteresis measurements. The jc(B) curves at different temperatures demonstrate a crossover from single vortex pinning to small-bundle vortex pinning, when the field is larger than the crossover field Bsb. The temperature dependence of the crossover field Bsb(T) is in agreement with a model of randomly distributed weak pinning centers via the spatial fluctuations of the transition temperature (δTc-pinning), while pinning due to the mean free path fluctuations (δl-pinning) is not observed.
@article{arxiv.cond-mat/0108172,
title = {Evidence for Transition Temperature Fluctuation Induced Pinning im MgB$_2$ Superconductor},
author = {M. J. Qin and X. L. Wang and H. K. Liu and S. X. Shi},
journal= {arXiv preprint arXiv:cond-mat/0108172},
year = {2007}
}