Spontaneous vortex phase and the Meissner-state in the magneto-superconductor Eu1.5Ce0.5RuSr2Cu2O10-δ (Eu-2122)

The superconducting ceramic material R1.5Ce0.5RuSr2Cu2O10-delta (Eu-2122) is also weak-ferro-magnetically ordered at T-M much greater thanT(C) (T-C=32 and T-M similar to 122 K), thus, superconductivity (SC) coexists with ferromagnetism. SC is confined to the CuO2 planes, whereas the weak-ferromagnetic (WFM) state is due to the Ru sublattice. The observed saturation moment of the Ru sublattice is about 0.7 mu (B) per unit cell, which is translated into an internal field B-int= 4 piM (higher than H-c1) of a few hundreds of G. This B-int induces vortices in the superconducting planes which have been observed in a ceramic material for the first time by means of magneto-optical imaging. Below T-C, at zero applied field, magnetic flux (the mixed state) is present in the sample in equilibrium and disappear above T-C. On the other hand, under an external field of 50 Oe, the flux density persists up to T-M. These measurements confirm our predicted spontaneous vortex phase (SVP) model, which assumes that in WFM-SC materials vortices are present in the sample in equilibrium without an external field. We also show the existence of the Meissner state in the WFM-SC Eu-2122 system in which Ru is substituted by Nb. The reduced Ru moment, and as a result the weak SVP, do not mask the expulsion of the flux lines at TC.