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

摘要

This study fabricated amorphous-carbon-encapsulated magnetite (Fe_3O_4@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 Fe_3O_4, namely, e~(2↓) t_2~(3↓)→e(2↓) t_2~(3↓)e~↑, which link the spin-polarized charge transfer to Fe ions at the tetrahedral site FeA of Fe_3O_4. Fe K-edge X-ray absorption near-edge structure (XANES) spectra suggested a formal charge reduction in the Fe_A of the Fe_3O_4@a-C NPs. Moreover, Fe L_3-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 Fe_3O_4@a-C NPs. Synchrotron radiation spectroscopy confirmed the charge reduction of Fe_A, which accounted for the magnetic enhancement.