Development of new recyclable materials based on iron oxides for nano-devices

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To develop new spintronics materials with environment friendly, we prepared well-crystallized epitaxial Fe_<1.2>Ti_<0.8>O_3 films which had the largest magnetization within the hematite (α-Fe_2O_3)-ilmenite (FeTiO_3) solid solution system. The films were prepared by reactive sputtering technique with an FeTi alloy target. The precise control of substrate temperature and oxygen pressure during sputtering was necessary to obtain ferromagnetic films with R^^- symmetry, in which Fe-rich and Ti-rich layers alternately stacked along the c-axis.By the way it is well known that the exchange coupling between two magnetic layers influence the magnetic properties of adjacent layers. We prepared Fe_<1.2>Ti_<0.8>O_3/Fe_2O_3 bilayered films and measured the magnetic Curie temperature (T_c) of them. In spite of the higher substrate temperature of 550℃, the films had a sharp interface between layers. The T_c of bilayered films was about 220 K, which was a little higher than that (210 K) of Fe_<1.2>Ti_<0.8>O_3 single layered films. The strong magnetic coupling to the antiferromagnetic α-Fe_2O_3 layer, which had the high Neel temperature of 960 K, could increase the T_c of the Fe_<1.2>Ti_<0.8>O_3 layer.The electronic structures of α-Fe_2O_3-FeTiO_3 are generally considered as the mixed valence states between Fe^<2+> and Fe^<3+> due to the substitution of Fe^<3+> for Ti^<4+>. The nominal valence states of Ti are fixed to 4+. However high-resolution X-ray fluorescence spectra of various titanium oxides suggested that the Ti Kα and Kβ peak profiles of FeTiO_3 were slightly different form those of other Ti^<4+> compounds and were better to assign to have some Ti^<3+> characters. Moreover the valence band spectra of FeTiO_3 by X-ray photoelectron spectroscopy had a small density just below the Fermi edge, probably assigned to the localized Ti 3d electron.