Electronic and magnetic properties of sub-unit cell α-Fe2O3 films on the α-Al2O3 (0001) substrate

Electronic and magnetic properties of sub-unit cell Fe2O3 (0001) films with variant thicknesses (N = 1 ∼ 6, the number of Fe-O octahedral slabs) on the Al2O3 (0001) substrate are investigated by first-principles calculations. Five different magnetic configurations are comparatively studied:(i) Type I (++|−−) with spins parallel within each Fe-O octahedral slab and antiparallel between two neighboring Fe-O octahedral slabs; (ii) Type II (+−|+−) with spins antiparallel within each Fe-O octahedral slab and between two neighboring Fe-O octahedral slabs; (iii) Type III (++|++) with spins parallel for all Fe; (iv) Type IV (++|+−) with spins parallel and antiparallel within each Fe bilayer but parallel between two neighboring Fe-O octahedral slabs; and (v) Type V (−+|+−) with spins antiparallel within each Fe-O octahedral slab but parallel between two neighboring Fe-O octahedral slabs. It is found that ferromagnetic configuration (++) is stabilized for monolayer (N = 1) Fe2O3 with the lowest formation energy. The magnetic property of sub-unit cell Fe2O3 (0001) films depends on the film thickness. The Fe2O3 thin films comprising of odd number (N = 1, 3, 5) of Fe-O octahedral slabs are ferromagnetic, while those of even number (N = 2, 4, 6) of Fe-O octahedral slabs are antiferromagnetic. The band gap of sub-unit cell Fe2O3 films is obviously lower than that of the bulk. These findings can not only reveal the magnetic coupling nature in sub-unit cell antiferromagnetic crystals, but also promote the applications of Fe2O3 ultrathin films in magnetic and electronic devices.