Abstract
Similar to the 56 K weak-ferromagnetic superconductor RuSr <sub>2</sub> GdCu <sub>2</sub> O <sub>8</sub> , a spontaneous vortex state (SVS) between 16 K and 36 K was observed for the weak-ferromagnetic superconductor RuSr <sub>2</sub> EuCu <sub>2</sub> O <sub>8</sub> with the ferromagnetic Curie temperature T <sub>Curie</sub> = 133 K and the superconducting transition temperature T <sub>sc</sub> = 36 K. The low-field (±20 G) superconducting hysteresis loop indicates a weak and narrow Meissner state region of the average lower critical field B <sub>c 1</sub> <sup>ave</sup> (T) = B <sub>c 1</sub> <sup>ave</sup> (0) [1 - (T / T <sub>SVS</sub> ) <sup>2</sup> ], with B <sub>c 1</sub> <sup>ave</sup> (0) = 7 G and T <sub>SVS</sub> = 16 K. Full Meissner shielding signal for bulk sample in a very low applied field indicates an ab-plane lower critical field B <sub>c 1</sub> <sup>ab</sup> (0) ∼ 2 G with an estimated anisotropic parameter γ ∼ 8.5 for this layered system. The existence of a spontaneous vortex state between 16 K and 36 K is the result of weak-ferromagnetic (canted antiferromagnetic) order with a very small net spontaneous magnetic moment of ∼0.1 μ <sub>B</sub> /Ru, which generates a weak magnetic dipole field around 9 G in the CuO <sub>2</sub> bilayers. The vortex melting transition temperature T <sub>melting</sub> = 21 K < T <sub>sc</sub> = 36 K from the broad resistivity transition and the onset of diamagnetic signal indicate a very broad vortex liquid region due to the coexistence and the interplay between superconductivity and weak-ferromagnetic order. © 2007 Elsevier B.V. All rights reserved.