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 $T_{\mathrm{S}\mathrm{C}}\sim 0.5$~K and $T_{\mathrm{C}\mathrm{u}\mathrm{r}\mathrm{i}\mathrm{e}}\sim 2.5$~K, respectively). In the normal, ferromagnetic state ($T_{\mathrm{S}\mathrm{C}}<T<T_{\mathrm{C}\mathrm{u}\mathrm{r}\mathrm{i}\mathrm{e}}$), the magnetization curve exhibits a hysteresis loop similar to that of a regular itinerant ferromagnet. Upon lowering the temperature below $T_{\mathrm{S}\mathrm{C}}$, 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 $H<H_{\mathrm{c}1}$ (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.