Magnetic and superconducting properties of RuSr₂GdCu₂O₈

The high-temperature superconducor RuSr2GdCu2O8 (Ru1212) offers the rare opportunity to study how superconductivity develops in an already magnetically ordered state. In this compound the Ru moments order magnetically at TM 130 K and a G-type antiferromagnetic arrangement has been proposed from neutron-diffraction studies [1] with the spins aligned along the c axis. We have measured the magnetic-field dependence of the magnetization of Ru1212 at T < TM in fields up to B 50 T. At the lowest measuring temperature of 4.2 K no saturation of the magnetization was observed and at 50 T, assuming a Gd contribution of 7 μB/formula unit, the Ru contribution to the measured magnetic moment was estimated to be 2.1 μB/formula unit. This is consistent with a mixed-valence state of the Ru ions (Ru5+ and Ru4+) and in contrast to the neutron diffraction studies that estimate an ordered Ru moment of 1.18 μB. In the superconducting state of the sample (T < 50K) we show that the signature of a bulk Meissner state, diamagnetism, is absent. Nevertheless, we suggest that this could be the effect of the granular nature of the investigated samples. A discussion of the specific heat of Sr2GdRuO6, the precursor for the preparation of Ru1212, with respect to the reported specific heat of the latter compound strongly supports the claims that Ru1212 is a bulk superconductor.