Unraveling Finite Size Effects on Magnetic Properties of Cobalt Nanoparticles

Using first principle calculations, the problem of scaling magnetic properties in nanoparticles is addressed. To this aim, the local electronic structure is characterized in cobalt quasi-spherical magnetic nanoparticles, in a large size range, from 0.5 to 2 nm diameter. First, specific patterns of the magnitude of local spin magnetic moments are evidenced depending on the shape and the size of the nanoparticles. Then effects of local structural environment (atomic coordination, structural deformations, finite size effects, shape changes) are unraveled. In small icosahedral nanoparticles, the local spin magnetic moment is found to decrease from the surface to the center. General rules driving charge transfers are observed whereby donor atomic sites are exclusively subsurface atoms and more unexpected vertex surface atomic sites. The variation of the magnetic moment is driven by the coupling between cluster microstructure and complex hybridization effects. In larger truncated octahedral clusters, whereas some properties are still found (quasi-zero interatomic site charge transfer, the reversal from antiferromagnetic to ferromagnetic coupling of electronic sp states with d states at vertex atomic sites), others vary such as the behavior of the local spin magnetic moment which now presents weak oscillations.