Doping Studies of the Ferromagnetic Superconductor RuSr2GdCu2O8

Solid solutions of the ferromagnetic superconductor Ru1-xMxSr2GdCu2O8 have been prepared for M = Sn or Nb. Up to 7.5% Sn and 20% Nb have been incorporated into the structure, and powder diffraction and physical measurements show that substitution occurs at the Ru site. A suppression of the ferromagnetic moment in the ruthenate layers is observed upon Sn and Nb substitution and the Curie temperature drops from 136 K in the undoped material to 103 K in Ru0.8Nb0.2Sr2GdCu2O8. The onset of superconductivity increases from 19 K in the 15% Nb doped sample to 38 K in the parent compound to 50 K in the 7.5% Sn doped sample, evidencing a tuning of the superconducting transition by charge transfer. Structural changes upon M doping have been determined from powder synchrotron X-ray diffraction and, for one sample, neutron diffraction using the high flux diffractometer GEM to compensate for the high absorption of natural Gd.