Microwave Electrodynamics of low TC and high TC Systems with Coexisting Superconductivity and Magnetism

Abstract

We investigate the microwave electrodynamics of both artificial low T C superconducting/magnetic (S/M) layered structures and high T C cuprates. In particular we focus on Nb/CuMn (superconducting/spin-glass) bilayers, and on GbBa 2 Cu 3 O 7-δ (GBCO) thin films, which show coexistence of superconductivity and long range ordered antiferromagnetism below T N = 2.2 K . In both cases we are interested in the influence of magnetism on superconducivity. Moreover, in the GBCO case, we want to shed light on the problem of the determination of the pairing symmetry in the cuprates. First we show surface impedance data at 10 GHz on Nb/CuMn bilayers. We extract information about the induced order parameter in the magnetic layer and we compare it with the exotic behavior predicted for S/M proximity systems. Then we present microwave surface impedance data at three different frequencies on GBCO c-axis oriented epitaxial thin films. Both the resistance and the reactance data on GBCO show an unusual low temperature behavior, mainly due to change in magnetic permeability. This result indicates that the paramagnetism of the rare-earth ions has to be taken into account when extracting the superconducting penetration depth as a function of temperature, and thus determining the pairing state symmetry of the cuprates.