Local Hall-probe-based susceptometry ofTl2Ba2CaCu2O8epitaxial films: Critical state and flux dynamics in collinear ac and dc magnetic fields

Abstract

The ac screening response of a superconducting epitaxial ${\mathrm{Tl}}_{2}{\mathrm{Ba}}_{2}{\mathrm{CaCu}}_{2}{\mathrm{O}}_{8}$ 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- ${(T}_{{H}^{\ensuremath{'}}}$ and ${T}_{{H}^{\ensuremath{''}}})$ 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 ${(h}_{0}l~13.2\mathrm{G})$ and low-frequency $(fl~1073\mathrm{Hz})$ 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 $(Tg~60\mathrm{K})$ low-field $({\ensuremath{\mu}}_{0}Hl~0.5\mathrm{T})$ regime. The irreversibility field derived from the onset temperature of $|{T}_{H3}|$ varies as $H(T)\ensuremath{\sim}(1\ensuremath{-}{t)}^{n}$ 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.