Superconducting Properties of RuSr₂RECu₂O₈

The resistance R of the (weakly) ferromagnetic (T_M ≈ 133K) superconductor RuSr₂EuCu₂O₈ (Ru1212Eu) shows a maximum at T_max ≈ 30 K while T_c (R = 0) ≈ 10 K. Using dc-magnetization measurements we were able to identify T_max as the temperature where the intragrain superconducting transition takes place,whereas T_c fits well with the temperature where intergrain (Josephson)coupling between the grains is established. Our findings suggest that the wide(≈ 20 K) superconducting transition of Ru1212Eu in zero applied magnetic field is most probably due to resistive grain contacts. A similar situation is realised for RuSr₂GdCu₂O₈. These results contradict reports which attribute the wide superconducting transition of these compounds to the movement of spontaneously induced vortices [1]. Spontaneous vortex-state formation at T < T_c cannot be excluded, nevertheless we show that it is more likely that the vortices are pinned in the intergrain area (area between the grains). Single crystals would be required to unambiguously clarify the state of the individual grains. Furthermore, T_max for Ru1212Eu is shifted to lower temperatures when an external magnetic field is applied. This challenges reports [2], where an increase of the critical temperature with the application of an external magnetic field in specific heat measurements was interpreted as possible evidence for triplet pairing in the ruthenium cuprates.
[1] C. Y. Yang et al., Phys. Rev. B 72, 174508 (2005)
[2] J. L.Tallon et al., Phys. Rev. B 61, R6471 (2000)