Magnetic tunnel junctions with an Fe{sub}3O{sub}4 top electrode have been fabricated. High resolution transmission electron microscopy (HRTEM) shows that in as-deposited state, Fe and FeO phases exist at the interface. These two phases are believed to have formed as a result of the unstable plasma condition at the start of the Fe{sub}3O{sub}4 deposition. A relatively low magnetoresistance ratio (MR) (3.5) and low switching field (H{sub}(c2)) (40 Oe) is observed which is associated with the fact that the FeO phase magnetically isolates the Fe phase from the Fe{sub}3O{sub}4 phase at the interface. After annealing at 150 ℃ for 5 h, the MR as well as the switching field (H{sub}(C2)) increases by a factor of two. HRTEM shows that the FeO phase at the interface has transformed into Fe and Fe{sub}3O{sub}4 in the annealed sample, resulting in the increased MR (7.0) and higher switching field (H{sub}(c2)) (300 Oe). However, annealing does not completely remove Fe at the interface, thereby limiting the MR.
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