Anomalous magnetoresistance of ultrathin films of ${\mathrm{DyBa}}_2$${\mathrm{Cu}}_3$${\mathrm{O}}_{7\mathrm{-}\mathit{x}}$ near the superconductor-insulator transition

An anomalous peak in the magnetoresistance was observed in granular ultrathin films of ${\mathrm{DyBa}}_2$ ${\mathrm{Cu}}_3$ ${\mathrm{O}}_{7\mathrm{-}\mathit{x}}$ whose low-temperature behaviors span the zero-field superconductor-insulator transition. The magnitude of this peak increased with the weakening of the superconductivity of the films. Frustration effects in a Josephson coupled superconducting array are not sufficient to account for the peak. Instead, this unusual behavior may result from an interaction of the carriers with antiferromagnetically ordered ${\mathrm{Cu}}^{2+}$ ions on the copper-oxygen sheets, providing a crucial link between superconductivity and magnetism.