Reentrant reversible remanent magnetization in superconducting submicron YBaCuO particles

Reversible properties of the remanent magnetic moment, M-rem, in supercondu cting submicron- and micron-size Y-Ba-Cu-O particles and thin films are inv estigated with a SQUID magnetometer. In micron-size particles and thin film s a monotonic increase of M-rem was observed when decreasing the temperatur e. In submicron particles, the same monotonic behaviour is observed at firs t but at low temperatures an unusual reentrant behaviour was found: M-rem d ecreased when cooling the sample below 6 K. This phenomenon is independent of the direction of the thermal cycle. Possible mechanisms of this effect, such as midgap states in the quasiparticle excitation spectrum and defect-i nduced localization of Cooper pairs, are discussed. (C) 1999 Elsevier Scien ce B.V. All rights reserved.