Coexistence of 3d-Ferromagnetism and Superconductivity in [(Li1−xFex)OH](Fe1−yLiy)Se

Superconducting [(Li1−xFex)OH](Fe1−yLiy)Se (x≈0.2, y≈0.08) was synthesized by hydrothermal methods and characterized by single‐crystal and powder X‐ray diffraction. The structure contains alternating layers of anti‐PbO type (Fe1−yLiy)Se and (Li1−xFex)OH. Electrical resistivity and magnetic susceptibility measurements reveal superconductivity at 43 K. An anomaly in the diamagnetic shielding indicates ferromagnetic ordering near 10 K while superconductivity is retained. The ferromagnetism is from the iron atoms in the (Li1−xFex)OH layer. Isothermal magnetization measurements confirm the superposition of ferromagnetic and superconducting hysteresis. The internal ferromagnetic field is larger than the lower, but smaller than the upper critical field of the superconductor. The formation of a spontaneous vortex phase where both orders coexist is supported by 57Fe‐Mössbauer spectra, 7Li‐NMR spectra, and μSR experiments. Opposites united: Superconductivity and ferromagnetism are antagonistic phenomena, and their coexistence is extremely rare. In [(Li1−xFex)OH](Fe1−yLiy)Se superconductivity emerges in iron selenide layers at 43 K, while iron atoms in hydroxide layers exhibit ferromagnetic ordering at 10 K and generate a spontaneous vortex phase.