George W. Crabtree

Neutron-scattering studies of magnetic superconductors

After nearly twenty years of theoretical and experimental attempts to understand the mutual effects of superconductivity and magnetism on each other, the existence of long-range order of both kinds in the same material was finally established recently with the advent of the families of ternary compounds which have come to be known as “magnetic superconductors” [1,2]. The general use of this term does not necessarily imply a simultaneous coexistence (e.g., at a given temperature) of long-range magnetic order and superconductivity, although the very existence of superconductivity in crystals which contain ordered lattices of magnetic ions is itself initially very surprising and is believed to be due to the relative weakness of the exchange spin-flip (and therefore pair-breaking) interaction between the closed shell (necessarily f-like) of electrons on the magnetic ions and the conduction-band electrons. Nevertheless, in many of these compounds, magnetic long-range order (LRO) can simultaneously coexist with superconductivity over certain temperature ranges. This is particularly true of the case in which the magnetic LRO is of the antiferromagnetic type. The case of ferromagnetic LRO leads to the phenomenon of “re-entrance” and will be discussed in detail later. The best studied families of these compounds are the ternary borides with the chemical formula ReRh4B4 (where RE is a rare earth ion), and the Chevrel phase compounds with the formula REMo6S8 or REMo6Se8. There exist by now several review articles on these extremely interesting classes of materials, which the reader may consult for more detailed experimental information [3,4]. In this article, we shall concern ourselves with neutron scattering studies of these compounds. Reviews of the recent theoretical developments of the subject may be found in many other papers of this symposium.