Superconductivity in the Bi-Sr-Ca(Y,Gd)-Cu-O system: DC magnetic susceptibility and microwave absorption investigations

Abstract

Microwave absorption and DC magnetic susceptibility measurements are carried out on Bi₂Sr₂Ca_2−xRe_xCu₂O_y (RE=Y, Gd; x=0, 0.25 and 0.5). These studies clearly establish the correlation between conventional DC magnetic susceptibility and changes in microwave reflectivity/absorption as the sample is cooled through the superconducting transition temperature (T_c). The results indicate that T_c, as determined by both the methods, initially increases with x, though for higher concentrations of x, superconductivity is suppressed. The Curie-Weiss temperature is found to increase with increasing concentration of Gd. No evidence for the local moment of Cu²⁺ is found in the EPR measurements. The EPR of Gd³⁺ is observable below 30 K for x=0.5 with a possible evidence for crystal-field interaction. Absence of Gd³⁺ EPR signal at higher temperatures, and also for x=0.25, might imply extra pathways for spin-lattice relaxation.