Polarized Hole Injection-induced Magnetic Enhancement in Carbon-Encapsulated Cobalt Ferrite Nanoparticles

Abstract

Magnetic enhancement was observed in carbon-encapsulated cobalt ferrite (CoFe2O4) nanoparticles. The induced magnetic moment of the nanoparticles was estimated using a vibrating-sample magnetometer and corrected based on thermogravimetric loss. Notably, up to 25% magnetic enhancement was achieved via carbon encapsulation by low-temperature hydrothermal reactions. A systematic variation on Co L-edge absorption spectra revealed the introduction of valence holes owing to carbon encapsulation, and the hole injection was attributed to carbon–sp contact. X-ray magnetic circular dichroism indicates holes injected upon both Fe and Co ions. Specifically, it is observed that the injection of polarized holes into Co2+ ions at the octahedral site results in magnetic enhancement. A spin–split electronic structure analysis of CoFe2O4 based on density functional theory with Hubbard correction demonstrated that octahedral Co, which served as a minority hole reservoir, controlled the magnetic properties of the hybrid system.