Vortex pinning in MgB2 polycrystals

Rotational magnetic measurements were performed on two MgB2 polycrystalline samples prepared differently, only one of which, according to a recent study, shows hysteretically at 4.2 K a large abrupt decrease in magnetization (M) at constant field (H). The rotational results for this sample, at fields before the abrupt drop of M would occur, display at 4.2 K a rapid decrease of M at a value of the rotational angle which varies with the magnitude of H. Besides this evidence for vortices with two very different sizes of pinning forces, the sample also displays gradual rotational changes that testify to the existence of vortices with a continuous spread in the size of pinning force. The other sample, which does not show an abrupt drop of M hysteretically at 4.2 K, displays prominently only the gradual rotational changes at 4.2 K that again indicate a continuum in size of the vortex pinning forces. The primary difference between the two samples, in the abrupt drop (or no drop) of M, presumably originates from the results of the very diverse modes of their preparation.