Direct observation of vortex images on lead micro-hole lattice by SQUID microscope

We have investigated the magnetic properties of a triangular micro-hole lat tice on Pb film (TriMHoLP) by means of a SQUID magnetometer and a SQUID mic roscope. A matching field Ho of TriMHoLP is 0.425 G. The zero-field-cooled magnetization curves are in accordance with those expected for typical supe rconductors. However, the curves taken in a field-cooling mode upon cooling are always negative irrelevant to the field polarity. Even in the zero fie ld, we have observed an appreciable amount of vortices and anti-vortices by the SQUID microscope. These strongly suggest that there must be a spontane ous magnetization at temperatures near T-c. A clear vortex density wave has been found for a half-filling state of TriMHoLP. (C) 2001 Elsevier Science B.V. All rights reserved.