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-221 2 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 effec t is appreciable even for grains of dimension 1 mu m. (ii) The field c ooled and zero field-cooled susceptibilities as well as the microwave absorption (MWA) are strongly suppressed for d<30-50 mu m. (iii) In al igned 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, an d that the spontaneous currents flow within the ab-planes. Furthermore , the presence of the WE in 1 mu m grains requires that the critical c urrent density of the pi-contacts which are generally involved to expl ain 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 h ardly achievable in conventional tunneling junctions suggesting that t he pi-contacts are highly transparent-eventually metallic-barriers. We argue that this result speaks strongly for an 'intrinsic' nature of t he pi-contacts arising from an unconventional pairing state. (C) 1998 Elsevier Science B.V.