2026-07-072026-07-07http://salesiana.dossiersoluciones.com/handle/123456789/18124The magnetic field dependent critical current density $j_c(B)$ of a MgB$_2$ bulk sample has been obtained by means of magnetization hysteresis measurements. The $j_c(B)$ 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 $B_{sb}$. The temperature dependence of the crossover field $B_{sb}(T)$ is in agreement with a model of randomly distributed weak pinning centers via the spatial fluctuations of the transition temperature ($δT_c$-pinning), while pinning due to the mean free path fluctuations ($δl$-pinning) is not observed.11 pages, 6 figuresSuperconductivitytext