Structural, transport, and magnetic properties of pure and La-doped RuSr₂GdCu₂O₈

Abstract

The structural properties, the electrical resistivity, and the magnetic properties of the “ferromagnetic” superconductor Ru ( Sr_1- La_x)₂ GdCu₂O₈ are systematically investigated as a function of La doping, temperature, and external magnetic field. These compounds are characterized by superconductivity ( T_c = 45 K ) in the CuO₂ planes, coexisting with weak ferromagnetism in the RuO₂ planes. Pure Ru-1212 reveals properties similar to those observed in heavily underdoped high- T c materials. We present a detailed investigation of the dc and ac magnetic properties. Doping with La gives no significant structural changes but reduces the charge-carrier density and already at x = 0.03 superconductivity is completely suppressed while the magnetic ordering temperatures are slightly enhanced. On increasing x the charge carriers are localized at low temperatures and for x = 0.1 semiconducting transport properties dominate below room temperature.