Spontaneous magnetic moments in YBa₂Cu₃O_7-δ thin films

We have observed spontaneous magnetic moments with random signs in c-axis-oriented thin films of the high-Tc cuprate superconductor YBa2Cu3O7-δ, imaged with a scanning superconducting quantum interference device microscope. These moments arise when the samples become superconducting, and appear to be associated with nonferromagnefic defects in the films. In contrast with granular high-Tc samples, which also show spontaneous moments with random signs, the present samples shield diamagnetically.