Neutron scattering studies of magnetic phase transitions in superconductors

Abstract

The rare earth (R) ternary superconductors belonging to the RMo 6X 8 (X ≡ S, Se) and RRh 4B 4 classes of materials have provided the first unambiguous examples of the coexistence of superconductivity and long-range magnetic order. For systems in which the interactions between rare earth moments are antiferromagnetic in nature, the magnetic order only weakly perturbs the superconductivity since there is no macroscopic magnetization associated with the magnetic state. There are now a rather large number of ternary materials which exhibit long-range antiferromagnetic order coexisting with super-conductivity over a wide range of temperature. The competitive nature of these two cooperative phenomena is illustrated for systems which display ferromagnetic interactions, such as HoMo 6S 8 and ErRh 4B 4, which first become superconducting at a temperature T c1 and then order magnetically at lower temperatures. At first a compromise long wavelength ( λ ≈ 10 2 A ̊ ) oscillatory magnetization is established at intermediate temperatures, but at a sufficiently low temperature T c2 the superconductivity is destroyed as ferromagnetism fully sets in. In the (Er 1− x Ho x )Rh 4B 4 alloy no modulated magnetic phase is found for the holmium-rich materials, and there appears to be a coexistence of ferromagnetic order and superconductivity over a substantial range of temperature and concentration. The microscopic nature of this coexistence is not yet fully understood, however. The behavior of these ternary re-entrant ferromagnets contrasts with the pseudobinary substitutional alloy systems such as (Ce 1− x R x )Ru 2, where rather sizable ferromagnetic correlations (ξ ≈ 10 2 Å) develop in the superconducting state, but the superconductivity persists. Longrange magnetic order only appears above the threshold in concentration where the superconducting state is fully suppressed.