Carbon encapsulation of magnetite nanoparticles enhances magnetism at room-temperature due to spin-polarized charge transfer

This study fabricated amorphous-carbon-encapsulated magnetite (Fe3O4@a-C) nanoparticles (NPs) and observed that the encapsulation significantly enhanced the room-temperature magnetization of the NPs by approximately 22.5%. Generalized gradient approximation with the Hubbard-U correction involved the ionic picture hypothesis of electron acceptors for Fe3O4, namely, e2↓ t23↓ → e2↓ t23↓ e↑, which link the spin-polarized charge transfer to Fe ions at the tetrahedral site FeA of Fe3O4. Fe K-edge X-ray absorption near-edge structure (XANES) spectra suggested a formal charge reduction in the FeA of the Fe3O4@a-C NPs. Moreover, Fe L3-edge x-ray magnetic circular dichroism yielded the same conclusion as that obtained using the XANES spectra and suggested the restoration of an orbital moment in the Fe3O4@a-C NPs. Synchrotron radiation spectroscopy confirmed the charge reduction of FeA, which accounted for the magnetic enhancement.