Anomalous ac susceptibility behaviour in ion-irradiated YBa₂Cu₃O_7-δ epitaxial thin films

Abstract

Epitaxial thin films of YBa₂Cu₃O_7-δ, prepared by the low-pressure rf-magnetron sputtering technique, have been irradiated with 100 MeV ⁵⁸Ni ions at a dose exceeding 10¹² ions cm⁻² at 77 K. Low-field magnetic ac-susceptibility measurements (X _ac ^−T ) are performed before and after the irradiation to study the irradiation-induced effects in the films. A superconducting transition accompanied by an anomalous change in the X _ac ^−T response. that is a positive susceptibility peak just below the transition temperature, is seen for the first time in the ion-irradiated films. This ion-irradiation-induced positive-susceptibility trait has been attributed to the paramagnetic Meissner effect. It is proposed that the ion irradiation at higher fluences leads to the formation of a π junction through the partial removal of oxygen ions in the chain sites and results in the observed extrinsic paramagnetic Meissner effect.