The thermal stability of the MnIr/NiFe exchange couple in the pinned electrode of a magnetic tunnel junction was analyzed using Auger electron spectroscopy (AES) and x-ray photoelectron microscopy (XPS). When the Ta/AlOx/CoFe/MnIr/NiFe/Ta/SiO2/Si electrode was annealed at 350 °C, the magnetic properties of the electrode quickly deteriorated. AES analysis indicated that there was a large diffusion of Ni into the MnIr layer accompanied by Ir migration into the NiFe layer above 350 °C. XPS of the annealed electrode revealed that the massive migration of Ni resulted in formation of antiferromagnetic NixMn1−x within the MnIr layer. The presence of the AlOx tunnel barrier, which promoted the preferential migration of Mn, appears to be the cause of the massive migration of Ni through formation of vacancies in the MnIr lattice.