Coexistence of 3d-Ferromagnetism and Superconductivity in [(Li_1-Fe_x)OH](Fe_1-yLi_y)Se

Superconducting [(Li_1-xFe_x)OH](Fe_1-yLi_y)Se (x≈0.2, y≈0.08) was synthesized by hydrothermal methods and characterized by single-crystal and powder X-ray diffrac- tion. The structure contains alternating layers of anti-PbO type (Fe_1-yLi_y)Se and (Li_1-xFe_x)OH. Electrical resistivity and magnetic susceptibility measurements reveal superconductivity at 43K. An anomaly in the diamagnetic shielding indicates ferromagnetic ordering near 10K while superconductivity is retained. The ferromagnetism is from the iron atoms in the (Li_1-xFe_x)OH layer. Isothermal magnetization measurements confirm the superposition of ferromagnetic and superconduct- ing 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.