PARAMAGNETIC MEISSNER EFFECT ANALYZED BY 2ND HARMONICS OF THE MAGNETIC-SUSCEPTIBILITY - CONSISTENCY WITH A GROUND-STATE CARRYING SPONTANEOUS CURRENTS

It has been reported recently by Braunisch et al. [Phys. Rev. Lett. 68 , 1908 (1992)] that some ceramic high-T(c) superconductors (HISC's) ex hibit a paramagnetic signal if cooled through the superconducting tran sition temperature in small external magnetic dc fields (H(dc) < 1 Oe) . In the present work, this paramagnetic Meissner effect (PME) is expe rimentally confirmed and further experimental details are reported on this phenomenon. This is accomplished by applying a recently developed compensation technique based on the measurement of the second harmoni c of the magnetic ac susceptibility in external dc fields. This techni que allows one to detect sensitively the dc magnetization of a HTSC sa mple and makes it possible to observe the PME under conditions where i ts dc superconducting-quantum-interference-device signal is totally ma sked by the dominating diamagnetic behavior of most of the sample. The experiments were performed on melt processed Bi2Sr2CaCu2O8-x samples, some of which did not exhibit the PME. Thus, a comparison was possibl e between samples with and without the PME, subjected to different fie ld-cooling procedures. The experimental results are consistent with th e occurrence of a reversible transition into a PME state carrying spon taneous orbital currents resulting in corresponding paramagnetic momen ts, which can be reversibly aligned by small external dc fields. This PME state is attributed to loops containing relatively strong weak lin ks, which are able to remain superconducting close to the bulk T(c) of the grains (e.g., at T/T(c) = 0.9) and in external dc fields of at le ast 30 Oe.