Spontaneous vortex phase in a superconducting weak ferromagnet

In high-Tc superconducting compounds R1.5Ce0.5RuSr₂Cu₂O₁₀ the onset of superconductivity occurs at a temperature Tc much lower than the temperature of the phase transition to weak ferromagnetism, and the diamagnetic response arises at the temperature Td, which is much lower than Tc. The present paper contains experimental data and a theoretical analysis of this phenomenon. These results collectively show convincingly that between Tc and Td vortices are present in the sample in equilibrium without an external magnetic field (the spontaneous vortex phase).