Detecting magnetic flux distributions in superconductors with polarized x rays

The magnetic flux distribution arising from a high-${T}_{c}$ superconductor is detected and visualized using polarized x rays. Therefore, we introduce a sensor layer, namely, an amorphous, soft-magnetic ${\mathrm{Co}}_{40}{\mathrm{Fe}}_{40}{\mathrm{B}}_{20}$ cover layer, providing a large x-ray magnetic circular dichroism (XMCD). Temperature-dependent XMCD spectroscopy on the magnetic layer has been performed. Exploiting the temperature dependence of the critical current density of the superconductor we find a quantitative correlation between the XMCD signal and the in-plane stray field of the superconductor. Magneto-optical Kerr effect experiments on the sensor layer can simulate the stray field of the superconductor and hence verify the correlation. We show that the XMCD contrast in the sensor layer corresponds to the in-plane magnetic flux distribution of the superconductor and can hence be used to image magnetic structures in superconductors.