Ferromagnetism and superconductivity in pure and doped RuSr₂GdCu₂O₈

Abstract

We report on systematical investigations of the electrical resistivity and the magnetic properties of the “ferromagnetic” superconductors Ru(Sr_1−xLa_x)₂GdCu₂O₈ as a function of La doping. Pure Ru-1212 is characterized by superconductivity (T_c≈40K) carried by the CuO₂ planes coexisting with weak ferromagnetism related to the RuO₂ planes. The superconducting properties are similar to those observed in heavily under-doped high-Tc materials. For T<Tc one finds an incomplete Meissner-phase together with a vanishing lower critical field Hc1. A spontaneous vortex phase is established due to the presence of strong internal fields. Doping the system with La³⁺ on the Sr²⁺ site leads to a suppression of superconductivity and an increase of the resistivity in the normal conducting-phase, respectively. At the same time no significant changes in the Cu–O–Ru bond angle can be detected. This denotes the sensitive dependence of the electronic properties on the hole concentration in the Cu–O planes.