Coexistence of Singlet Superconductivity and Magnetic Order in Bulk Magnetic Superconductors and SF Heterostructures

Abstract

The basic physics of bulk magnetic superconductors (MS) related to the problem of the coexistence of singlet superconductivity (SC) and magnetic order is reviewed. The interplay of the exchange (EX) and electromagnetic (EM) interaction between conduction electrons and localized magnetic moments in the formation of the coexistence phase is discussed. It is shown that the singlet SC and uniform ferromagnetic (F) order practically never coexist in bulk materials. In the case of their mutual coexistence the F order is modified into a domain-like or spiral structure depending on magnetic anisotropy. It turns out that the domain-like structure is realized in several superconductors such as ErRh4B4, HoMo6S8, HoMo6Se8 with electronic order and (probably) the spiral structure in AuIn2 with nuclear magnetic order. The latter problem is also discussed. The coexistence of SC with antiferromagnetism (AF) is more favorable. A rich phase diagram is realized in AF systems with SC and weak-ferromagnetism. A number of properties of magnetic superconductors in the magnetic field are very peculiar, especially near the (ferro)magnetic transition temperature, where the upper critical field becomes smaller than the thermodynamical critical field. The Josephson junctions based on MS with spiral magnetic order is discussed, in which the induced triplet pairing amplitude gives rise to π-contact (with the π-phase difference in the ground state). The interplay of the superconducting and the magnetic phase, ϕ, θ, respectively, in such a contact renders possibilities for an interplay of the charge and spin current, and for a new type of coupled Josephson-qubits in a single Josephson junction. The coexistence of superconductivity and ferromagnetism may be easily achieved in artificially fabricated superconductor/ferromagnet heterostructures. Due to the proximity effect, the Cooper pairs penetrate into the F layer and one has the unique possibility to study properties of superconducting electrons under the influence of a large exchange field. Moreover, varying in a controllable manner the thickness of the ferromagnetic and superconducting layers it is possible to change the relative strength of two competing orderings. The Josephson junctions with ferromagnetic layers reveal many unusual properties that are quite interesting for applications, in particular the recently fabricated so-called π-Josephson junction.