The exchange-biased electrode of the magnetic tunneling junction (MTJ), consisting of Ta/NiFe/IrMn/CoFe/AlOx/Ta layer was annealed at 300 degreesC to study the extent of interdiffusion within the electrode. The tunnel barrier, AlOx was also under- and over-oxidized in the oxygen plasma in order to elucidate the effects of plasma oxidation on the interdiffusion. A significant amount of Mn diffusion into the AlOx layer was observed in the over-oxidized electrode from the depth profiles using Auger electron spectroscopy (AES) while the under-oxidized electrode exhibited a minimal amount of Mn diffusion after annealing at 300 degreesC. X-ray photoelectron spectroscopy (XPS) proved that the Mn found at the CoFe/AlOx interface and in the CoFe layer of the over-oxidized electrode was in the form of MnO2 while Co at the CoFe/AlOx interface remained unoxidized. The XPS data suggests that the Mn diffusion was accelerated by the preferential oxidation of Mn. The results indicate that the oxidation condition of the insulator will influence the Mn diffusion and subsequent post-annealing response of MTJ as the plasma oxidation provides inherently meta-stable AlOx layer, creating a source for the unreacted oxygen radicals. (C) 2002 Elsevier Science B.V. All rights reserved.