Abstract
Measurements of the magnetization vector M of a polycrystalline (Ba,K)BiO3 sample turned in a fixed field H at 4.2 K show that the rotational behavior of MP, the penetrating vortex-flux component of M, depends sensitively on the magnetic state of the sample before rotation. For the field-cooled state at 240 Oe, the vortex lines represented by MP are seen to bifurcate into two groups, one of which rotates rigidly with the sample while the other is held by H and turns frictionally relative to the sample. This behavior, seen earlier in polycrystalline YBa2Cu3O7, is directly ascribable to a distribution in the strength of the vortex pinning forces. For the hysteretic state in which H is raised from a large negative value to +240 Oe, a vortex-flux bifurcation is again observed but with an important difference. The strongly pinned vortices (rotating rigidly) reflect the original negative polarity of H, whereas the weakly pinned vortices (turning frictionally) are those produced by the positive H. Hence, in this hysteretic state before rotation, there is a coexistence of vortices of opposing polarity.
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