Abstract
Similar to the optimal-doped, weak-ferromagnetic (WFM induced by canted antiferro-magnetism, T Curie = 131 K) and superconducting (T c = 56 K) RuSr 2 GdCu 2 O 8 , underdoped RuSr 2 EuCu 2 O 8 (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 ave c1 (T) = B ave c1 (0)[1 - (T/T SVS ) 2 ], with B a c1 ve (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 Gd 3+ (J = S = 7/2) at 2.5 K to be close to N A k ln 8, as expected. © 2009 THE PHYSICAL SOCIETY OF THE REPUBLIC OF CHINA.