Size analysis and magnetic structure of nickel nanoparticles

The size distribution of an assembly of fcc nickel nanoparticles is studied by measuring the temperature dependent magnetization curves fitted by a uniform model and a core-shell model, both based on the Langevin function for superparamagnetism with a log-normal particle volume distribution. The uniform model fits lead to a spontaneous magnetization M_s much smaller than the M_s for bulk nickel and to particle sizes larger than the ones evaluated by transmission electron microscopy; the core-shell model fits can result in a correct size distribution but the M_s in the core becomes significantly greater than the M_s for bulk nickel. It is concluded that there is a core-shell magnetic structure in nickel particles. Although the enhanced M_s in the core may be related to the narrowing of the energy bands of 3d electrons in small fcc nickel particles, the modeling values of M_s are over large compared with previous calculations on nickel clusters of different structures, which implies possible existence of an exchange interaction between the core and the shell, which is not considered in the simple core-shell model.