In-plane field-effect correction in quantitative magnetooptical analysis

Abstract

A bstractWe present a detailed analysis of the calibration procedure to be performed in order to extract from magneto-optical images the local values of magnetic induction and electric current density on a plane over and inside the superconducting sample respectively. The interaction between the spontaneous magnetization of the indicator ®lm and the full magnetic ®eld distribution in the specimen region has been considered. In particular, the generally disregarded interaction of the indicator ®lm with in-plane magnetic ®elds, that is with the component parallel to the indicator surface, is retained in the model. The current density distribution is obtained by the numerical inversion of the Biot±Savart law. In order to incorporate the in-plane ®eld e ect into the inversion scheme, we have constructed an iterative procedure which successfully takes into account the full magnetic induction ®eld. A set of experimental data on a high-temperature superconducting YBa2Cu3O7¡x ®lm is presented to show the actual impact of the new analysis method, for what concerns both the local values of the magnetic induction and the electrical current density evaluation.

Evidence of vortex curvature and anisotropic pinning in superconducting films by quantitative magneto-optics

“…The QMO measurements consisted of several zero field cooling of the superconductor, down to different temperatures, followed by applying a set of increasing magnetic field directed along the z axis. Each applied field was kept constant for 3 s, until a magneto-optical image was acquired by our standard set-up [10]. After the calibration process to convert the measured light intensity to B z values, the numerical method returns, from each image, the distributions of the full induction field at the superconductor surface (B x (x,y,d/2), B y (x,y,d/2) and B z (x,y,d/2), where the reference z=0 plane is at half thickness of the sample), and the local electrical current density averaged over the sample thickness, J x (x,y) and J y (x,y).…”

“…The iterative algorithm evaluates the in-plane magnetic field components from the current density obtained by Biot-Savart inversion of the B z data, afterward it corrects the measured B z data itself until the convergence is reached; for details see [10]. This correction is needed for the quantitative evaluation of the magnetic induction distribution, but it is even weightier for current density reconstruction.…”

“…Each applied field was kept constant for 3 s, until a magneto-optical image was acquired by our standard set-up [10]. After the calibration process to convert the measured light intensity to B z values, the numerical method returns, from each image, the distributions of the full induction field plane is at half thickness of the sample), and the local electrical current density averaged over the sample thickness, J x (x,y) and J y (x,y).…”

“…We extended the correction procedure already elaborated for strip geometry [9] to a modelindependent QMO analysis accounting for the in-plane magnetic fields at the superconductor surface [10]. Yielding the local measurement of the actual magnetic induction in the z direction, B z (x,y), and the evaluation of the in-plane magnetic field components and of current density distribution, our method enables to study the 3D critical state of thin superconductors.…”

Twofold role of columnar defects in iron based superconductors

“…The irradiated samples were preliminary characterized by means of the Magneto-Optical imaging with an Indicator Film (MOIF) technique [27,28] that makes use of the magnetization of the indicator film combined with the Faraday effect to directly visualize magnetic field distributions. A nonlinear calibration combined with an iterative algorithm [29] allows obtaining the quantitative measurement of the magnetic field and the reconstruction of the current density distribution (J) [30]. MOIF measurements were performed on Ba 0.58 K 0.42 Fe 2 As 2 and BaFe 2 (As 0.67 P 0.33 ) 2 single crystals partially irradiated and partially kept pristine by protecting them with a suitable screen during the irradiation process.…”