Verwey-like Transition in <i>β</i>-Fe_4+δSe₅ Nanowires of Fe-vacancy Ordering

The magnetotransport and the source-drain frequency dependence of β-Fe_4+δSe₅ nanowires are investigated. The nanowires feature Fe-vacancy ordering and mixed valence of Fe. The resistance shows a first-order metal-insulator transition of transition temperature at ~30K in the absence of magnetic fields. Colossal positive magnetoresistance is observed at temperatures around 30K. The metal-insulator transition presents uniaxial anisotropy with the c-axis in field-orientation dependence. The frequency dependence in resistance below the metal-insulator transition temperature is similar to features in magnetite nanowires at temperatures lower than the Verwey temperature. Our results reveal that the Fe-vacancy-ordered β-Fe_4+δSe₅ nanowires undergo a magnetic first-order phase transition strongly involving spin-orbital coupling with d_z2 orbitals .