Physical Properties of the Weak-Ferromagnetic Superconductor RuSr2EuCu2O8

Similar to the optimal-doped, weak-ferromagnetic (WFM induced by canted antiferromagnetism, T-Curie = 131 K) and superconducting (T-c = 56 K) RuSr2GdCu2O8, under-doped RuSr2EuCu2O8 (T-Curie = 133 K, T-c = 36 K) also exhibited a spontaneous vortex state (SVS) between 16 K and 36 K. The low field (+/-20 G) superconducting hysteresis loop indicates a weak and narrow Meissner state region of average lower critical field B-c1(ave)(T) = B-c1(ave)(0)[1 - (T/T-SVS)(2)], with B-c1(ave)(0) = 7 G and T-SVS = 16 K. The vortex melting transition (T-melting 21 K) below T-c, obtained from the broad resistivity drop and the onset of the diamagnetic signal, indicates a vortex liquid region due to the coexistence and interplay between superconductivity and the WFM order. No visible jump in specific heat was observed near T-c for Eu- and Gd-compounds. Finally, with the baseline from the nonmagnetic Eu-compound, the specific heat data analysis confirms the magnetic entropy associated with antiferromagnetic ordering of Gd3+ (J = S = 7/2) at 2.5 K to be close to N(A)k ln 8, as expected.