Spontaneous magnetic moments in YBCO thin films

We have observed spontaneous magnetic moments with random signs in c-axis oriented thin films of the high-Tc cuprate superconductor YBCO, imaged with a scanning SQUID microscope. These moments arise when the samples become superconducting, and appear to be associated with non-ferromagnetic defects in the films. In contrast with granular high-Tc samples, which also show spontaneous magnetization with random signs, these samples shield diamagnetically.