Microwave electrodynamics of low Tc and high Tc systems with coexisting superconductivity and magnetism

We investigate the microwave electrodynamics of both artificial low Te supe rconducting / magnetic (S/M) layered structures and high TC cuprates. In pa rticular we focus on Nb/CuMn (superconducting / spin-glass) bilayers, and o n GdBa2Cu3O7-delta (GBCO) thin films, which show coexistence of superconduc tivity and long range ordered antiferromagnetism below T-N=2.2 K. In bath c ases we are interested in the influence of magnetism on superconductivity. Moreover, in the GBCO case, we want to shed light on the problem of the det ermination of the pairing symmetry in the cuprates. First we show surface i mpedance data at 10 GHz on Nb/CuMn bilayers. We extract information about t he induced order parameter in the magnetic layer and we compare it with the exotic behavior predicted for S/M proximity systems. Then we present micro wave surface impedance data at three different frequencies on GBCO c-axis o riented epitaxial thin films. Both the resistance and the reactance data on GBCO show an unusual low temperature behavior, mainly due to change in mag netic permeability. This result indicates that the paramagnetism of the rar e-earth ions has to be taken into account when extracting the superconducti ng penetration depth as a function of temperature, and thus determining the pairing state symmetry of the cuprates.