Direct Evidence for the Coexistence of Superconductivity and Ferromagnetism

Abstract

In 1958 Matthias et al.1 proposed the simultaneous occurrence of superconductivity and ferromagnetism in certain substituted, cubic Laves-phase, intermetallic compounds. These compounds are generally of the type Ce1−xRxRu2, where R represents a magnetic rare earth. Their conclusion was based on the slow depression of the superconducting transition temperature T c (x) of CeRu2 upon addition of RRu2, the depression of the ferromagnetic ordering temperature θc(x) of RRu2 upon addition of CeRu2, and the intersection ofT c (x) and θ c (x) at some finite temperature and rare earth concentration x*. The phase diagram for the system Ce1−x GdxRu2 is shown in Fig. 1. Note that in the neighborhood of x = 0.13 the two critical curves intersect at about 4°K. The solid lines in Fig. 1 were obtained by low-field magnetic susceptibility measurements.2,3 The temperature at the midpoint of the transition to diamagnetism is taken as T c ; below T c diamagnetic shielding masks any magnetic behavior of the sample. Values of θC for the Gd-rich sample are determined from their paramagnetic behavior at higher T. The dashed lines are extrapolations assuming that the concentration dependences of T c (x) and θ c (x) persist above and below x*, respectively. One therefore asks whether samples in the concentration region about x* are either superconducting or ferromagnetic, or both, at temperatures below the dashed lines of Fig. 1. T c transitions have been observed1,4 for certain systems with magnetic rare earth concentrations slightly above their x*. Recently θC values below T c for x < x* were inferred3 from susceptibility data taken above T c .