Ferrimagnetism in Ti and Cr doped $\alpha$-Fe$_2$O$_3$ due to heterostructuring

Based on DFT calculations including an on-site Coulomb repulsion term (GGA+$U$) we perform a comparative study of Ti and Cr substitution in $\alpha$-Fe$_2$O$_3$ by varying the concentration, distribution and magnetic alignment of the cations in the Fe$_2$O$_3$ host. In the FeTiO$_3$-Fe$_2$O$_3$ system the charge mismatch at the interface between the room temperature paramagnet ilmenite and the canted antiferrromagnet hematite is identified as the driving force towards ferrimagnetism as it leads to the formation of uncompensated spins in a mixed Fe$^{2+}$, Fe$^{3+}$ layer [1-3]. In contrast, in Cr$_2$O$_3$-Fe$_2$O$_3$ there is no valence discontinuity, but the formation of superlattices opens a path to stabilize a different magnetic order than in the Cr$_2$O$_3$ end member leading to a net magnetization in the system. The magnetic properties are discussed based on the magnetic interaction parameters for the interfaces extracted from the DFT calculations. Funding by the DFG (Pe883/4-1) and ESF are gratefully acknowledged. \\[4pt] [1] P. Robinson, et al., Nature {\bf 418}, 517 (2002).\\[0pt] [2] R. Pentcheva and H. Sadat Nabi, Phys. Rev. B {\bf 77}, 172405 (2008).\\[0pt] [3] H. Sadat Nabi and R. Pentcheva, J. Appl. Phys. {\bf 105}, 053905 (2009).