Inhomogeneous oxidation behaviors of FeCoNiTiCu high-entropy alloy nanoparticles confined within graphite shells

The oxidation of magnetic high-entropy alloy nanoparticles (HEA-NPs) is an inevitable process in service environments, which can induce unexpected properties and result in the failure of devices. Graphite shells coating is an effective approach to prevent the oxidation of nanoparticles, and the defects on graphite shells can also have a significant impact on the physical and chemical properties of nanoparticles. Thus, it is significant to uncover the oxidation process and understand the change of magnetic properties of HEA-NPs encapsulated in graphite shells (HEA@C-NPs). Herein, we demonstrate the microstructure evolution of HEA@C-NPs under different annealing temperatures. With the increase of annealing temperatures, the graphite shell is broken gradually and then oxygen atoms enter into the HEA core across the defects in graphite shell. An inhomogeneous partial-empty structure is observed at 300 celcius, which can result from the heterogeneous diffusion of different elements across graphite shell with ununiform defects. Accompanied with the formation of the oxide, the saturation magnetization decreases linearly and the exchange bias effect increases gradually. Our results give some understanding for the oxidation process and thus provide a guidance for designing HEA-NPs on complex service environments.