Oxygen Pressure-Dependent Phases and Properties of Iron Oxide Thin Films on α-Al₂O₃(0001)

Iron oxides, such as FeO, α-Fe₂O₃, γ-Fe₂O₃, and Fe₃O₄, exhibit a range of fascinating properties. Hematite (α-Fe₂O₃, R3-c) is an antiferromagnetic material with a Néel temperature of 948 K and an indirect band gap of 2.0 eV. It undergoes a Morin transition, where its magnetic ordering begins to cant above 260 K, resulting in a weak ferromagnetic state. This distinctive property positions hematite as a candidate for altermagnetic materials, a recently recognized third fundamental type of magnetism. In contrast, Fe₃O₄, or magnetite (Fd3-m), is ferrimagnetic with a Curie temperature of 858 K and a smaller indirect band gap of 0.1 eV. At room temperature, magnetite features a metallic cubic inverse spinel structure, which transitions to a monoclinic structure at 120 K, leading to a metal-insulator transition known as the Verwey transition. In this study, we have grown α-Fe₂O₃(0001) and Fe₃O₄(111) thin films on α-Al₂O₃(0001) using MBE. Our objective is to control the phases of these iron oxide thin films by varying the oxygen concentration. The surface and crystal structures of the thin films will be analyzed using in-situ RHEED and XRD. At a high oxygen pressure of 6 × 10^-5 Torr, we successfully produced an insulating α-Fe₂O₃ thin film. In contrast, at oxygen pressures ranging from 2 to 8 × 10^-6 Torr, we achieved Fe₃O₄ thin films exhibiting the Verwey transition, with properties varying according to the oxygen amount. Additionally, at an oxygen pressure of 4 × 10 ^-7 Torr, we observed mixed phases of α-Fe and Fe₃O₄. In this talk, we will discuss the electrical and magnetic properties of the iron oxide thin films, especially how varying oxygen concentrations influence their characteristics.