Rotational vortex-flux characteristics of a type-II superconductor

The rotation of a superconducting sample in a fixed field H, starting in a hysteretic state, is known to produce a bifurcation of the vortex-flux density into a trapped rotational component BR, which turns rigidly with the sample, and a frictional component BF, which stays fixed relative to H, reflecting the effects of vortex pinning and unpinning. Our rotational magnetic measurements on polycrystalline (Ba,K)BiO₃ at 4.2 K now show that both BR and BF and their changes with H are the same for starting states on different branches of a major hysteresis loop. Moreover, for starting states after zero-field cooling, we find that BR is zero for all H but that BF rises at Hc1 and rapidly joins the BF values for the hysteretic starting states at higher H. Hence, regardless of the magnetic history before sample rotation, the frictional component BF depends essentially only on H (at fixed temperature) and thus equilibrates with the magnetic environment.