Ferromagnetism and superconductivity in pure and doped RuSr2GdCu2O8

We report on systematical investigations of the electrical resistivity and the magnetic properties of the ferromagnetic superconductors Ru(Sr 1-x La x ) 2 GdCu 2 O 8 as a function of La doping. Pure Ru-1212 is characterized by superconductivity (T C 40 K) carried by the CuO 2 planes coexisting with weak ferromagnetism related to the RuO 2 planes. The superconducting properties are similar to those observed in heavily under-doped high-T c materials. For T< T c one finds an incomplete Meissner-phase together with a vanishing lower critical field H c1 . A spontaneous vortex phase is established due to the presence of strong internal fields. Doping the system with La 3+ on the Sr 2+ 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.