Effect of Organic Coating Variation on the Electric and Magnetic Behavior of Ferrite Nanoparticles.

Abstract

Organic ligand coatings can modify the surface properties of nanoparticles. With the proper choice of the type of nanoparticles and of the ligand, a targeted modification can be achieved that is suitable for specific applications. In the present work, we employ density functional theory calculations with Hubbard corrections (DFT + U) to treat localized states in order to investigate the magnetic and electrostatic properties of ferrite nanoparticles (CoFe2O4 and Fe2O3) covered with COOH-terminated [oleic acid (OA)] and OH-terminated [diethylene glycol (DEG)] ligands by varying the ligands coverage. OA results in a decrease of the mean magnetic moment for both particles as the coating coverage increases. The magnetic anisotropy (MAE) significantly decreases for CoFe2O4, whereas for Fe2O3 a significant increase of MAE is found as the OA coverage percentage increases. For DEG, the variation of both types of nanoparticles in the magnetic moment and the magnetic anisotropy is not significant since DEG shows a weaker attachment on the surface. As COOH shows a larger percentage of covalent bonding than OH, a larger amount of charge is transferred to both particles when OA is attached on their surface. In this case, the particles possess a higher charge, and thus they can produce a larger electrostatic potential in the neighborhood independently of the screening by the coating. Thus, the repulsive Coulombic forces are enhanced mainly in the OA coating case, resulting in an enhancement of their colloidal stability.