Abstract
Energy-filtered transmission electron microscopy and electron holography were used to study changes in the MgO tunnel barrier of CoFe/MgO/CoFe magnetic tunnel junctions (MTJs) as a function of annealing and in situ applied electrical bias. Annealing was found to increase the homogeneity and crystallinity of the MgO tunnel barrier. Cobalt, oxygen, and trace amounts of iron diffused into the MgO upon annealing. Annealing also resulted in a reduction of the tunneling barrier height, and decreased the resistance of the annealed MTJ relative to that of the as-grown sample. In situ off-axis electron holography was employed to image the barrier potential profile of a MTJ directly, with the specimen under electrical bias. Varying the bias voltage from \ensuremath{-}1.5 to $+$1.5 V was found to change the asymmetry of the barrier potential and decrease the effective barrier width as a result of charge accumulation at the MgO-CoFe interface.
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