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.