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Magnetic tunnel junctions (MTJ) with a plasma-oxidized Fe electrode have been fabricated on oxidized silicon wafers with standard photolithography. High-resolution transmission electron microscopy (HRTEM) and diffraction patterns show that a thin Fe layer can be oxidized by a controlled oxygen plasma into pure Fe3O4 without other crystallographic phases such as FeO and Fe2O3. To grow Fe3O4 directly in contact with an AlOx barrier, we began with Fe layers that varied from 1.8 to 5 nm. It was found that complete oxidation only occurred for the 1.8-nm thickness. Magnetic and electrical transport properties on the MTJs were measured at room temperature and low temperature (110 K). When the layer adjacent to the AlOx barrier was Fe3O4, inverse magnetoresistance (MR) behavior was observed, which is what is expected from the band structure of Fe3O4. However, when free Fe exists due to incomplete oxidation, positive MR behavior is observed.