Paramagnetic Meissner effect in Pb nanowire arrays

The Meissner effect is one of the basic properties of superconductors. Recently, many experiments have shown that small-size superconducting samples may be paramagnetic in a weak magnetic field, the so-called paramagnetic Meissner effect (PME). In this paper, we report the observation of the PME in Pb nanowire arrays. We find that the signal of the PME increases with decreasing diameter of the nanowires. In a lead nanowire array of diameter about 40 nm, the oscillations of the PME are observed in field-cooling temperature-dependent magnetization $M(T)$ curves. Surprisingly, the PME was also observed in zero-field-cooling $M(T)$ curves. We conclude that the PME is in association with the metastable states in superconductors. The PME plays an important role only if the proportion of surface superconductors is sufficiently large.