Band offset and magnetic property engineering for epitaxial interfaces:: A monolayer of M2O3 (M=Al,Ga,Sc,Ti,Ni) at the α-Fe2O3/α-Cr2O3 (0001) interface

We have used density-functional theory with the gradient corrected exchange-correlation functional PW91 to study the effect of an interfactant layer, where Fe and Cr are replaced by a different metal, on electronic and magnetic properties of an epitaxial interface between alpha-Fe2O3 and alpha-Cr2O3 in the hexagonal (0001) basal plane. We studied a monolayer of M2O3 (M=Al,Ga,Sc,Ti,Ni) sandwiched with five layers of chromia and five layers of hematite through epitaxial interfaces of two types, termed "oxygen divided" or "split metal." We found that both the electronic and magnetic properties of the superlattice are modified by the interfactant monolayer. For the split-metal interface, which is favored through the growth pattern of chromia and hematite, the valence-band offset can be changed from 0.62 eV (no interfactant) up to 0.90 eV with the Sc2O3 interfactant, and down to -0.51 eV (i.e., the alpha-Fe2O3/alpha-Cr2O3 heterojunction changes from type II to type I) with the Ti2O3 interfactant, due to a massive interfacial charge transfer. The band gap of the system as a whole remains open for the interfactant monolayers based on Al, Ga, and Sc, but it closes for Ti. For Ni, the split-metal interface has a negative band offset and a small band gap. Thus, nanoscale engineering through layer-by-layer growth will strongly affect the macroscopic properties of this system.