Local Hall-probe-based susceptometry of Tl₂Ba₂CaCu₂O₈ epitaxial films: critical state and flux dynamics in collinear ac and dc magnetic fields

Abstract

The ac screening response of a superconducting epitaxial Tl₂Ba₂CaCu₂O₈ film in static and time-varying magnetic fields collinear with the film normal is reported. The complex susceptibility components measured in terms of the fundamental- (T_H and T_H) and third-harmonic (|T_H3|) transmittivities of a local hall-probe susceptometer are compared with the models of flux penetration in a disc-shaped thin superconductor. In the absence of a dc field, the low-amplitude (h₀<~13.2G) and low-frequency (flt;~1073Hz) response is well described by the bean critical-state model applied to thin discs in a transverse ac field. The J_c calculated from the model compares well with the J_c measured in a four-probe transport geometry. When a dc field is superimposed over the ac field, signatures of a creeplike motion of vortices are seen in a temperature regime where |T_H3| is nonzero. Vortex-glass/collective-pinning theories for a three-dimensional vortex system are considered to understand this behavior in a high-temperature (T gt;~60K) low-field (μ₀H<~0.5T) regime. The irreversibility field derived from the onset temperature of |T_H3| varies as H(T)~(1-t)ⁿ on the H-T plane, with n=1.2. At low fields, this line also represents the variation of the vortex-glass temperature T_g with field inferred from the tail of the resistive transition.