Grain size dependence of the Wohlleben effect in Bi-2212 high temperature superconductors

We report on a study of the Wohlleben effect (WE) in powders of Bi-2212 high temperature superconductor (HTSC) consisting of isolated grains with dimensions ranging between 1 mu m<d<1 mm. Our main results are: (i) The WE is present in all powders studied; in particular, the effect is appreciable even for grains of dimension 1 mu m. (ii) The field cooled and zero field-cooled susceptibilities as well as the microwave absorption (MWA) are strongly suppressed for d<30-50 mu m. (iii) In aligned powders, the WE is strongest for magnetic fields parallel to the c-axes. Our data give evidence that the WE is an intragrain property, i.e., it is not predominantly determined by intergrain weak links, and that the spontaneous currents flow within the ab-planes. Furthermore, the presence of the WE in 1 mu m grains requires that the critical current density of the pi-contacts which are generally involved to explain the spontaneous currents in the WE, must be of order 10(5)-10(6) A/cm(2), even close to T-c. Such large critical current densities are hardly achievable in conventional tunneling junctions suggesting that the pi-contacts are highly transparent-eventually metallic-barriers. We argue that this result speaks strongly for an 'intrinsic' nature of the pi-contacts arising from an unconventional pairing state. (C) 1998 Elsevier Science B.V.