Iron oxides are among the most abundant compounds on earth and are critical for industry. They also were the first natural ferromagnets used. They are also important in corrosion as the oxidation of Fe produces hematite, α-Fe2O3. Other forms of Fe2O3 can form naturally, but are more rare. At extreme pressures other forms of Fe2O3 have been observed, but none of them exhibit the distinct monoclinic structure. Epitaxy can be used to guide atoms to specific locations, such that their crystal structure is locked into a form that otherwise would not form. By using Molecular Beam Epitaxy, we found that a new monoclinic structure of Fe2O3 could be stabilized by depositing thin layers of Fe2O3 on to β-Ga2O3. Atomic resolution images of the Fe2O3 inserts show that they perfectly copy the β-Ga2O3 substrate on which they are deposited, i.e. they have the same crystal structure, but different composition. This new monoclinic form of Fe2O3 we call, μ for monoclinic, as β-Fe2O3 is already the label for a cubic phase of Fe2O3. Interestingly, although μ-Fe2O3 should be a perfect antiferromagnet, it turns out that the interface of β-Ga2O3 and μ-Fe2O3 generates a thin interface layer of ferromagnetic material. This may enable magnetic functionality integration within ultra wide band gap semiconductors. We are currently exploring the magneto-transport properties of this new form of rust. Its lower symmetry can lead to potential benefits in the magnetic and electronic anisotropy.
See: Jamison et al. Cry. Gro. Des. (2019), and Hettiaratchy et al. JVSTA (2021)