C. Heinzel et al., PARAMAGNETIC MEISSNER EFFECT ANALYZED BY 2ND HARMONICS OF THE MAGNETIC-SUSCEPTIBILITY - CONSISTENCY WITH A GROUND-STATE CARRYING SPONTANEOUS CURRENTS, Physical review. B, Condensed matter, 48(5), 1993, pp. 3445-3454
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.