Superparamagnetic property and high microwave absorption performance of FeAlat(Al, Fe)₂O₃ nanoparticles induced by surface oxidation

Surface modification is an effective way to induce new magnetic phenomena in nanostructured materials. Herein, FeAl nanoparticles (NPs) with a mean diameter of 38 nm are produced by the hydrogen plasma-metal reaction (HPMR) approach. Via the subsequent passivation process, an oxide layer is generated outside the FeAl NPs as the result of surface oxidation. The 3 nm-thick amorphous-like oxide layer consists mainly of Al₂O₃ together with a small amount of Fe₂O₃. An Fe-enriched zone is created between the oxide layer and the FeAl core due to the much higher diffusion rate of Al than Fe towards the particle surface during the passivation process, which can be explained by the Kirkendall effect. The FeAlat(Al, Fe)₂O₃ NPs surprisingly display a superparamagnetic property with a blocking temperature (T_B) of 250 K and a saturation magnetization of 36 emu g^-1 at 4.2 K. They also exhibit high microwave absorption performance with a minimum reflection loss (RL) value of -22.6 dB at a thickness of 1.7 mm, and a broad absorption bandwidth of 8.3 GHz corresponding to the RL below -10 dB. Formation of the oxide layer plays a dominant role in inducing the superparamagnetic property and high microwave absorption performance in FeAlat(Al, Fe)₂O₃ NPs. Specific core@shell NPs may open a new way to tune the magnetic and electromagnetic properties of metallic nanomaterials through surface modification.