Anomalous physical properties of underdoped weak-ferromagnetic superconductor RuSr2EuCu2O8

Abstract

Similar to the optimal-doped, weak-ferromagnetic (WFM induced by canted antiferromagnetism, TCurie = 131 K) and superconducting (Tc = 56 K) RuSr2GdCu2O8, the underdoped RuSr2EuCu2O8 (TCurie = 133 K, Tc = 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/TSV S ) 2 ], with B ave c1 (0) = 7 G and TSV S = 16 K. The vortex melting transition (T melting = 21 K) below Tc 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 Tc for Eu-and Gd-compound. 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. Finally, with the baseline from the nonmagnetic Eu-compound, 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 NAk ln8 as expected.