Paramagnetic species and magnetic surface structures in high-TC superconductors as revealed by electron spin resonance and low-field nonresonant microwave absorption.

Abstract

Paramagnetic species in the normal conducting state of various high Tc superconducting oxides such as the La-Ba (or Sr) -Cu-O, RE-Ba-Cu-O (RE=Y, Gd, Dy, Ho, Er and Tm) and Bi-Ca-Ba-Cu-O systems were characterized by ESR spectroscopy. Low-field nonresonant microwave absorption spectra were observed for the superconducting state of the above high Tc superconductors. The nonresonant absorption is caused to the reduction of the shielding effect by the supercurrent and intergrain Josephson current on the surface of above samples. Magnetic hysteresis and transient response observed in the nonresonant absorption arise from frozen-in magnetic fluxes, which are in a metastable state. The critical field obtained from the flux trapping allows one to estimate the average size of the superconducting clusters with a uniform phase in the spin-glass-like high Tc superconductors. It is suggested that the low-field nonresonant microwave absorption method is applicable to the characterization of thin-layer high Tc superconductors as well as novel composite types of superconductors.