Determination of critical current densities of YBa 2Cu 3O 7−δ thin films from AC susceptibility measurements

Abstract

The temperature dependent AC susceptibilities χ( T)=χ'( T)-iχ″ ( T) of laser ablated YBa 2Cu 3O 7−δ thin films were measur ed for different AC fields aligned perpendicular to the film surface. According to the critical-state model calculations of Sun et al. [Phys. Rev. B 44 (1991) 5275], the maximum of the imaginary part of the AC susceptibility χ″ ( T) occurs when the amplitude of the applied field H AC equals approximately the full penetration field H ∗. With the relation H ∗∞ J c l, where l is the thickness of the film, an estimate of the values of the critical current density J c at the χ″ peak temperatures has been made. The J c values obtained in this way are in good agreement with those determined by AC screening measurements also performed in this study. For a temperature range about 10 K below T c, the critical current density J c( T) exhibited a temperature dependence J c ( T) ∞ (1- T/ T c) n , with n≈2.5. By taking J c( T) as an input value, we could calculate AC susceptibilities of the film s studied. The calculated χ' ( T) and χ″ ( T) closely resemble the measured curves. The present work has therefore demonstrated that the theoretical calculations of Sun et al. can be used to explain the observed AC susceptibilities of high- T c thin films.