The features of ac magnetic susceptibility in thin superconducting films with a high critical current

Abstract

The features of ac magnetic susceptibility appearing in thin superconducting films with a high critical current were investigated for the first time. These features were found in some films of YBa2Cu3O7–δ in magnetic-field and temperature dependencies of the ac susceptibility in the form of Z-like step for the real component, χm′(H) or χm′(T), and V-like dip for the imaginary component, χm″(H) or χm″(T), where the index m indicates measurements at a maximum of the polar diagram χ′′(χ′) (“maximum of losses”). Out of the maximum of losses the effect is also observed and its value depends on the depth of entrance and exit of vortices. It is suggested that the effect is caused by dependencies of the critical current and flux creep on a magnetic field. The magnetic susceptibility for a thin disk was calculated approximately using modified Clem-Sanchez formulas, in which a dependence of the critical current density on the applied field, jc(H), was introduced. The calculations based on the real dependence jc(H) in the films studied and with a simplest account for the flux creep allowed to obtain the dependencies χm′(H) and χm″(H), qualitatively similar to experimental, including the Z- and V-features at a large critical current. It is shown that for these samples the Clem-Sanchez relations between the critical current density, on the one hand, and components of the ac magnetic susceptibility and also the amplitude of the ac field at the maximum of losses, on the other hand, are violated.