Absence of Meissner state and robust ferromagnetism in the superconducting state of UCoGe. Possible evidence of spontaneous vortex state

We report ac magnetic susceptibility and dc magnetization measurements on the superconducting ferromagnet UCoGe (with superconducting and Curie temperatures of TSC-0.5 K and TCurie-2.5 K, respectively). In the normal, ferromagnetic state (TSC < T < TCurie), the magnetization curve exhibits a hysteresis loop similar to that of a regular itinerant ferromagnet. Upon lowering the temperature below TSC, the spontaneous magnetization is unchanged, but the hysteresis is markedly enhanced. Even deeply inside the superconducting state, ferromagnetism is not completely shielded, and there is no Meissner region, a magnetic field region of H < Hc1 (a lower critical field). From these results, we suggest that UCoGe is the first material in which ferromagnetism robustly survives in the superconducting state and a spontaneous vortex state without the Meissner state is realized. (author)