Transition of exchange bias from the linear to oscillatory regime with the progression of surface oxidation of Ag@FeCo@Ag core@shell@shell nanoparticles

The evolution of the exchange bias field was investigated in Ag@FeCo@Ag and AgAu@FeCo@AgAu core@shell@shell nanoparticles as a function of aging time. Because of the partial oxidation of the thin (∼2.5-nm thick) ferromagnetic (FM) FeCo intermediate shell to an antiferromagnetic (AFM) Co_0.5Fe_0.5O phase, exchange bias was clearly observed at the interface between the FM and AFM layers. The exchange bias field monotonically increased with aging time when the ratio between the volumes of the AFM (v AFM) and FM (v FM) layers, v AFM / v FM, was less than 2.8. However, the exchange bias field showed an oscillatory behavior with respect to v AFM / v FM, when v AFM / v FM > 3.6. This oscillatory behavior probably results from the small interfacial area between the FM and AFM layers, which causes fluctuations of the surplus magnetization in the AFM shell. Using the exchange bias field within the linear regime, it is possible to sensitively estimate the thickness of the AFM layer (i.e., the surface oxide layer).