Spontaneous vortex state in the weak-ferromagnetic superconductor RuSr2EuCU2O8

Similar to the 56 K weak-ferromagnetic superconductor RuSr2GdCu2O8, a spontaneous vortex state (SVS) between 16 K and 36 K was observed for the weak-ferromagnetic superconductor RuSr2EuCu2O8 with the ferromagnetic Curie temperature T-Curie = 133 K and the superconducting transition temperature T-sc = 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-cl(ave)(T) = B-cl(ave)(0)[1 - (T/T-svs)(2)], with B-cl(ave)(0) =7 G and T-svs= 16 K. Full Meissner shielding signal for bulk sample in a very low applied field indicates an ab-plane lower critical field B-cl(ab)(0) similar to 2 G with an estimated anisotropic parameter gamma similar to 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 similar to 0.1 mu(B)/Ru, which generates a weak magnetic dipole field around 9 G in the CuO2 bilayers. The vortex melting transition temperature T-melting = 21 K < T-sc = 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. (C) 2007 Elsevier B.V. All rights reserved.