Cooperative response of the magnetic and superconducting subsystems in magnetic superconductor RbEuFe4As4*

We present magneto-optics and magnetization based studies of the magnetic flux evolution in single crystals of the magnetic superconductor RbEuFe4As4 during field cooling and warming, and magnetic field cycling at temperatures above and below the magnetic transition point, Tm. The vortex patterns emerging at T below and ~ Tm on different crystal facets in the fields of different orientations, reveal a peculiar magnetization process, where the magnetic subsystem serves as an internal magnetic flux pump while the superconducting subsystem controls the entry of magnetic flux quanta into the bulk. The interplay of magnetic susceptibility amplifying the magnetic induction and vortex pinning attenuating the magnetic flux entry, results in a field and temperature dependent critical state that mimics a paramagnetic Meissner effect. The observed anisotropic vortex dynamics is associated with a peculiar fine scale Eu-spin helical structure modified by the vortex field and resulting in a nontrivial spatial current distribution, which yields a self-consistent inhomogeneous enhancement of the sample magnetization.