Abstract
Similar to the 56K weak-ferromagnetic superconductor RuSr2GdCu2O8, a spontaneous vortex state (SVS) between 16K and 36K was observed for the weak-ferromagnetic superconductor RuSr2EuCu2O8 with the ferromagnetic Curie temperature TCurie=133K and the superconducting transition temperature Tsc=36K. The low-field (±20G) superconducting hysteresis loop indicates a weak and narrow Meissner state region of the average lower critical field Bc1ave(T)=Bc1ave(0)[1-(T/TSVS)2], with Bc1ave(0)=7G and TSVS=16K. Full Meissner shielding signal for bulk sample in a very low applied field indicates an ab-plane lower critical field Bc1ab(0)∼2G with an estimated anisotropic parameter γ∼8.5 for this layered system. The existence of a spontaneous vortex state between 16K and 36K is the result of weak-ferromagnetic (canted antiferromagnetic) order with a very small net spontaneous magnetic moment of ∼0.1μB/Ru, which generates a weak magnetic dipole field around 9G in the CuO2 bilayers. The vortex melting transition temperature Tmelting=21K<Tsc=36K 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.
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