Current density and magnetic field distribution in hard thin film superconductors

Abstract

In thin film superconductors with magnetic fields applied perpendicular to the film plane, demagnetization effects lead to strong deviations of the flux density distribution B( r) from the predictions of the conventional critical state model. In particular if the magnetization M is taken as a probe for the critical current density j c of a thin film, the radial stray field component B r parallel to the film plane has to be taken into consideration. Especially in thin circular films with thicknesses lower than the London penetration depth, λ, the circular critical current density, j c =1/ μ 0( ∂B z / ∂B− ∂B z / ∂r), is determined by the gradient ∂ B r / ∂z rather than by ∂ B z /∂r. Using B z ( r)-data, measured by means of the magneto-optical Faraday effect on c-axis oriented YBa 2Cu 3O 7− δ -films, we perform a selfconsistent calculation of the radial component B r ( r), of the magnetic induction as well as of the current density distribution, j c ( r), and the volume pinning force, f p .