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 B(R), which turns rigidly with the sample, and a frictional component B(F), 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 B(R) and B(F) 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 B(R) is, zero for all H but that B(F) rises at H(c1) and rapidly joins the B(F) values for the hysteretic starting states at higher H. Hence, regardless of the magnetic history before sample rotation, the frictional component B(F) depends essentially only on H (at fixed temperature) and thus equilibrates with the magnetic environment.