Study of the itinerant electron magnetism of Fe-based superconductors by the proximity effect

• We study electron transport through NS boundary in the interior of FeSe and LaO(F)FeAs.
• Spin polarization of electron transport is observed.
• It is proved that in this case Andreev conductance is sensitive to spin rotation symmetry.
• Hysteresis of magnetoresistance also confirms the spin polarization of charge carriers.
• We suggest that superconductivity in the systems is connected with itinerant magnetism.

We offer a unconventional method for the study of superconductors using own magnetic field of the transport current as a tool to achieve an ideal (”barrier-free”) NS boundary inside the superconductor due to the proximity effect. Since the probability of Andreev reflection at such a boundary is of order of 1, it is possible with the same probability to judge the nature of phenomena accompanying the conversion of the dissipative current in the supercurrent at it. Thus, in systems with NS boundaries inside unconventional superconductors, monocrystalline chalcogenide FeSe and granulated pnictides LaO(F)FeAs, we have direct evidence of spin-polarized nature of transport and the absence of residual magnetization in their ground normal state: In heterocontacts with these superconductors we detected the spin-dependent contribution to the efficiency of the Andreev reflection associated with the spin accumulation at the NS boundary, and a hysteresis of FeSe conductivity in the ground normal state in low external magnetic fields. Based on our findings, we conclude that in iron-based superconductors, the itinerant electron magnetism is predominant, magnetism of iron atoms being localized.