Coexistence of 3d-ferromagnetism and superconductivity in [(Li(1-x)Fex)OH](Fe(1-y)Liy)Se

TL;DR

This study reports the synthesis and characterization of a novel layered material where superconductivity at 43 K coexists with ferromagnetism near 10 K, supported by multiple spectroscopic techniques.

Contribution

It demonstrates the coexistence of 3d ferromagnetism and superconductivity in a bulk iron-based compound, revealing a spontaneous vortex phase.

Findings

Superconductivity at 43 K confirmed by resistivity and susceptibility.

Ferromagnetic order appears near 10 K from iron atoms in specific layers.

Evidence of a spontaneous vortex phase from magnetic measurements.

Abstract

Superconducting [(Li(1-x)Fex)OH](Fe(1-y)Liy)Se (x ~ 0.2, y ~ 0.08) was synthesized by hydrothermal methods and structurally characterized by single crystal X-ray diffraction. The crystal structure contains anti-PbO type (Fe(1-y)Liy)Se layers separated by layers of (Li(1-x)Fex)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 emerges from the iron atoms in the (Li(1-x)Fex)OH layer. Isothermal magnetization measurements confirm the superposition of ferromagnetic with superconducting hysteresis. The internal ferromagnetic field is larger than the lower, but smaller than the upper critical field of the superconductor, which gives evidence for a spontaneous vortex phase where both orders coexist. 57Fe-M\"ossbauer spectra, 7Li-NMR spectra, and muSR experiments consistently support this rare situation, especially in a bulk material where magnetism emerges from a 3d-element.