Abstract
Similar to the optimal-doped, weak-ferromagnetic (WFM induced by canted antiferromagnetism, T Curie =131 K) and superconducting (T c =56 K) RuSr 2 GdCu 2 O 8 , the underdoped RuSr 2 EuCu 2 O 8 (T Curie =133 K, T c =36 K) also exhibits a spontaneous vortex state (SVS) between 16 and 36 K. The low field superconducting hysteresis loop indicates a weak and narrow Meissner state region of average lower critical field B cl 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 diamagnetic signal indicates a vortex liquid region due to the coexistence and interplay between superconductivity and WFM order. No visible jump in specific heat was observed near T c . This is not surprising, since the electronic specific heat is easily overshadowed by the large phonon and weak-ferromagnetic contributions. Furthermore, a broad resistivity transition due to low vortex melting temperature would also lead to a correspondingly reduced height of any specific heat jump. © 2007 Elsevier B.V. All rights reserved.