Abstract
An anomalous magnetic field-effect has been observed in the microwave surface impedance of epitaxial YBa2Cu3O7−x films on LaAlo3 substrates. The application of dc and microwave magnetic fields up to 20 mT caused a significant decrease of microwave surface reactanceXs (≤3%) and resistanceRs (≤20%) at 8.5 GHz, 77 K and at 87 GHz, 4.2 K. The field effects onXs andRss were found to correlate within the framework of the two-fluid model. The observed effect is attributed to the presence of magnetic impurities in the superconducting films, giving rise to pair breaking. Alignment of the impurity spins suppresses the spin-flip scattering rate and, as a result, leads to a recovery of microwave superconductivity.
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