Ilmenite–hematite solid solution films for novel electronic devices

Abstract

Unique magnetic and electric properties of solid solutions of ilmenite (FeTiO 3) and hematite (α-Fe 2O 3) 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-Al 2O 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 (R 3 ̄ ) 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 R 3 ̄ symmetry had large ferrimagnetic moments at low temperature (78 K), although the magnetization values were depending on not only the order parameter between the R 3 ̄ and the R 3 ̄ c 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/cm 3 at 78 K after the subsequent annealing at 600 °C. However, highly oxidized nonstoichiometirc Fe 1.23Ti 0.77O 3+δ films with the R 3 ̄ c symmetry could not give the ordered structure with the R 3 ̄ symmetry.