The thermally induced changes of the magnetic coupling in an artificial antiferromagnet exchange biased by an antiferromagnetic layer in a magnetic tunnel junction (Mn–Ir/Co–Fe/Ru/Co–Fe/AlOx/Ni–Fe) is investigated for annealing temperatures up to 450 °C. Beside the usual increase of the tunneling magnetoresistance (TMR) by annealing (maximum TMR 43.9% at 325 °C) the degradation of the artificial antiferromagnet causes a well-defined 90° coupling of the Co–Fe layers in a narrow temperature window around 325 °C. At higher temperature, both Co–Fe layers are coupled ferromagnetically but the exchange biasing by the Mn–Ir is still present. This behavior results from an interplay of different coupling mechanisms of the two Co–Fe layers: (1) indirect antiferromagnetic interlayer exchange coupling across the Ru spacer, (2) ferromagnetic coupling by pinholes in the Ru spacer, and (3) exchange coupling between the lower Co–Fe layer and the antiferromagnet Mn–Ir. The importance of each contribution is discussed with respect to thermally activated diffusion processes.
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Schmalhorst et al. (2003) studied this question.
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