Dark Field Observation of Fe₃O₄-γFe₂O₃ Multiphase Films

Abstract
We investigated the phase separation of cubic and tetragonal systems in Fe₃O₄-γFe₂O₃ intermediate thin films. An Fe thin film of 50 nm was deposited on a SiO₂ substrate using an electron beam evaporator at room temperature. The film was oxidized to αFe₂O₃ at 400°C in air, then the film was reduced to Fe₃O₄ at 320°C in methanol vapor. The gradual oxidation of the Fe₃O₄ film to γFe₂O₃ film at 240°C was terminated when the coercive force of the film increased more than 1000 Oe at an intermediate state. The intermediate film was examined by dark-field TEM observation with tetragonal diffraction spots. The result revealed that crystallites of about 10 nm of tetragonal structure are distributed in a matrix of cubic phase.