Paramagnetic Meissner effect in Nb

The paramagnetic Meissner effect (PME), or Wohlleben effect, in which the field-cooled magnetization of superconducting samples is paramagnetic below T-c, has been reported to occur in some samples of a variety of high-T-c cuprate superconductors. It has been proposed that the effect arose in granular hole-doped cuprates from current loops with pi phase shifts of the superconducting order parameter at some grain-boundary junctions. It is argued that such behavior would be expected to occur in a d-wave superconductor, but not in a conventional s-wave superconductor. To test this hypothesis, we have searched for the occurrence of the effect in Nb, and have confirmed a recent report by Minhaj et al. of its occurrence in some Nb samples. For these studies, the effects of stray fields and field gradients in the measurement volume of the superconducting quantum interference device magnetometer have been carefully considered to rule out the possibility that measurement artifacts might be responsible for the apparent paramagnetic behavior in Nb. The M(T) and M(H) curves obtained in Nb samples that show the PME also show remarkably strong resemblance to those curves reported for the cuprate materials exhibiting the PME. Evidence is presented that the effect arises from inhomogeneously trapped flux, and is strongly influenced by sample geometry and surface effects. These results suggest that, for the effect to be observable, T-c on the sample surface must be different from the bulk T-c. The occurrence of the PME in Nb strongly suggests that the observation of this effect is unrelated to d-wave superconductivity.