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

Abstract

We used the proximity effect as a tool to achieve an ideal ("barrier-free") NS boundary for quantitative evaluation of transport phenomena that accompany converting dissipative current into supercurrent in NS systems with unconventional superconductors -single-crystal chalcogenide FeSe and granulated pnictide LaO(F)FeAs. Using features (limitations) of Andreev reflection in the NS systems with dispersion of the electron spin subbands, we revealed direct evidence for spin-polarized nature of transport and the absence of residual magnetization in iron-based superconductors in the normal state: In heterocontacts with single-crystal FeSe and granular LaO(F)FeAs, we detected a spin-dependent contribution to the efficiency of the Andreev reflection associated with the spin accumulation at the NS boundary, and a hysteresis of conductivity of FeSe in the ground 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.

Spin-dependent conductivity of iron-based superconductors in a magnetic field

“…In NS mode it includes four areas: highly conductive normal metal (Cu), oxide barrier, and two border areas of iron-based superconductor (in the normal and superconducting states). In this structure, only superconductor in the ground normal state may be responsible for negative magnetoresistance at low fields: in accordance with the characteristics of other parts of a contact, copper probes and oxide barriers [12], their magnetoresistance can reveal only slightly increasing behavior in weak magnetic fields. It is known that an increase in conductivity in a magnetic field (except, perhaps, for field values corresponding to large Zeeman energies, capable of, for example, inducing metamagnetism), leading to negative magnetoresistance, indicates either the presence of spin-dependent effects in transport of itinerant conduction electrons [14] or the degradation of weak-localization interference addition to the resistance [15].…”

“…[11,12], we have already obtained an evidence of the existence of such a contribution in the absence of magnetic field from the results of studying temperature properties of Andreev reflection in contacts. However, in a magnetic field, the relative magnitude of the above correction in our contacts obviously depends on the product of two probabilities, namely, the probability to preserve the dispersion of spin subbands along the length L in the N (F) area, ie, on the length of spin relaxation λ s , and the probability of conservation of Andreev reflection.…”

“…The normal layer thickness of the superconductor, L(F), formed due to shifting the NS boundary as a result of suppressing superconductivity by self magnetic field of the transport current I = 1 mA appeared to be of the order of λ T . Therefore, at helium temperatures, L(F) approximately amounts to (10 −5 ÷ 10 −6 ) cm (at I = 100 mA, it is ln100 times more [12]) while the elastic mean free path of electrons in contacted materials l el (5 K) ∼ 10 −6 ÷ 10 −8 cm (from the measurements on macroscopic samples).…”

“…This is a typical mesoscopic scale, non-ballistic due to even smaller elastic mean free paths (details ibid). In this paper, we present the results of studying the conductance of point-contact samples with ohmic characteristics similar to those used in [12].…”

“…We used this fact in [12] studying the Andreev reflection. In particular, our results showed that the Ginzburg-Landau parameter for considered superconductors κ = λ T /ξ T was ≥ 1, and hence, the discussed superconductors must have the properties of type II superconductors with the London penetration depth λ L ≃ 0.2 µ [ 13].…”