Ilmenite-hematite solid solution films for novel electronic devices

Unique magnetic and electric properties of solid solutions of ilmenite (FeTiO₃) and hematite (α-Fe₂O₃) can be expected for new electronics, optics and spintronics applications. We have successfully prepared well-crystallized and epitaxial Fe_1.23Ti _0.77O_3+δ (001) films on α-Al₂O ₃ (001) substrates by reactive helicon plasma-sputtering technique. Structure, magnetic and electric properties of the films were seriously dependent on both the oxygen pressure and the substrate temperature during the sputtering deposition. The subsequent annealing of the films in vacuum at high temperature (600 °C) promoted the order (R3̄) structure of Fe _1.23Ti_0.77O_3+δ, where the Ti and Fe ions occupied the cation sites alternatively along the c-axis. Only the films having the R3̄ symmetry had large ferrimagnetic moments at low temperature (78 K), although the magnetization values were depending on not only the order parameter between the R3̄ and the R3̄c structures but also the oxidization states of Fe ions in Fe_1.23Ti_0.77O _3+δ. The film prepared at an oxygen partial pressure ratio of 1.4% and substrate temperature of 400 °C had considerably large spontaneous magnetization of 275 emu/cm³ at 78 K after the subsequent annealing at 600 °C. However, highly oxidized nonstoichiometirc Fe _1.23Ti_0.77O_3+δ films with the R3̄c symmetry could not give the ordered structure with the R3̄ symmetry.