Ferromagnetic Epitaxial μ-Fe$_{2}$O$_{3}$ on β-Ga$_{2}$O$_{3}$: A New Monoclinic form of Fe$_{2}$O$_{3}$

Abstract: Here we demonstrate a new monoclinic iron oxide phase ({\mu}-Fe${2}$O${3}$), epitaxially stabilized by growth on (010) {\beta}-Ga${2}$O${3}$. Density functional theory (DFT) calculations find that the lattice parameters of freestanding {\mu}-Fe${2}$O${3}$ are within ~1% of those of {\beta}-Ga${2}$O${3}$ and that its energy of formation is comparable to that of naturally abundant Fe${2}$O${3}$ polytypes. A superlattice of {\mu}-Fe${2}$O${3}$/{\beta}-Ga${2}$O${3}$ is grown by plasma assisted molecular beam epitaxy, with resulting high-resolution x-ray diffraction (XRD) measurements indicating that the {\mu}-Fe${2}$O${3}$ layers are lattice-matched to the substrate. The measured out-of-plane (b) lattice parameter of 3.12 $\pm$ 0.4 {\AA} is in agreement with the predicted lattice constants and atomic-resolution scanning transmission electron microscopy (STEM) images confirm complete registry of the {\mu}-Fe${2}$O${3}$ layers with {\beta}-Ga${2}$O${3}$. Finally, DFT modeling predicts that bulk {\mu}-Fe${2}$O${3}$ is antiferromagnetic, while the interface region between {\mu}-Fe${2}$O${3}$ and {\beta}-Ga${2}$O${3}$ leads to ferromagnetic coupling between interface Fe$^{3+}$ cations selectively occupying tetrahedral positions. Magnetic hysteresis persisting to room temperature is observed via SQUID measurements, consistent with the computationally predicted interface magnetism.