Preparation and characterization of ilmenite-hematite thin films

Ilmenite-hematite solid solution (Fe_2-xTi_xO₃) is one of the candidates for practical magnetic semiconductors with the high Curie temperature (T_C). We have successfully prepared well-crystallized and epitaxial Fe_1.4Ti_0.6O₃ films by using a reactive helicon plasma sputtering technique. The structural characterizations indicated that the films had the order structure (R over(3, -) symmetry), where Ti- and Fe-rich layers were stacked alternately along the c-axis, without any secondary phase. The subsequent annealing of the films in vacuum at 700 °C promoted the R over(3, -) symmetry. Only the films having the good R over(3, -) symmetry had large ferrimagnetic moments at room temperature. The resistivities of the films clearly showed semiconducting behavior, i.e., logarithmic resistivity showed a monotonous increase with increasing the inverse temperature. The minimum room temperature resistivity of the films was about 10^-1 Ω cm. From the Hall effect measurement all films had n-type conduction. The carrier concentration was estimated to be about 10¹⁹-10²¹ cm^-3, depending on the annealing conditions.