Self-induced magnetic flux structure in magnetic superconductor*

We image the temperature evolution of the distribution of the magnetic induction in single crystals of the magnetic superconductor EuRbFe4As4. In contrast to the traditional Meissner flux expulsion, the samples cooled in constant magnetic fields show unusual vortex patterns revealing a strong enhancement of the internal magnetic flux density upon approaching the magnetic transition temperature. The observed patterns demonstrate a cooperative response of the magnetic subsystem, which acts as an internal pump of the magnetic flux, and the superconducting system, which controls the delivery of vortices into the bulk of the sample. We suggest possible current distributions responsible for the observed flux patterns.