Abstract
Exchange coupling (EC) in as-deposited and annealed Co80Fe20 (CoFe)/MgO/Co80Fe20 (CoFe) for MgO thickness spanning 0.45 to 4.5 nm has been studied. Unlike relevant existing results and theories, antiferromagnetic EC behaviors were observed in annealed CoFe/MgO/CoFe with 2.7-nm MgO insulating barrier. Interestingly, a transition from antiferromagnetic to nearly paramagnetic response to applied magnetic field was found around 520 °C corresponding to Néel temperature of CoFe2O4. Based on grazing incidence x-ray diffraction study, oxide phases of CoO, Co3O4, and CoFe2O4 starts to be built in the interface annealed at 150 °C and then those of CoO, Fe3O4, Fe2O3, and CoFe2O4 are mostly distributed over the interface annealed at 250 °C. This indicates CoO and CoFe2O4 formed at the Co-rich interface act as antiferromagnetic sources giving rise to the exchange-coupled antiferromagnetic/ferromagnetic (AFM/FM) interface and then AFM EC into the CoFe/MgO/CoFe.
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