Critical fields and the spontaneous vortex state in the weakly ferromagnetic superconductor RuSr2GdCu2O8 -: art. no. 174508

A spontaneous vortex state (SVS) between 30 and 56 K was observed for the weak-ferromagnetic superconductor RuSr2GdCu2O8 with the ferromagnetic Curie temperature T-C=131 K and the superconducting transition temperature T-c=56 K. The low-field (+/- 20 G) super-conducting hysteresis loop indicates a narrow Meissner state region within the average lower critical field B-c1(T)=B-c1(0)[1-(T/T-0)(2)], with average B-c1(ave)(0)=12 G and T-0=30 K. Full Meissner shielding signal in very low applied field indicates an ab plane B-c1(ab)(0)similar to 4 G with an estimated anisotropic parameter gamma similar to 7 for this layered system. The existence of a spontaneous vortex state between 30 and 56 K is the result of weak-ferromagnetic order with a net spontaneous magnetic moment of similar to 0.1 mu(B)/Ru, which generates a weak magnetic dipole field around 10 G in the CuO2 bilayers. The upper critical field B-c2 varies linearly as (1-T/T-c) up to 7-T field. The vortex melting line B-m varies as (1-T/T-m)(3.5) with melting transition temperature T-m=39 K and a very broad vortex liquid region due to the coexistence and the interplay between superconductivity and weak-ferromagnetic order.