Magneto-optics of thin magnetic films composed of Co nanoparticles

The magneto-optic properties of 8, 10, and 12 nm diameter Co particles deposited on Si and Al substrates are investigated at room temperature in the photon-energy range from 0.8 to 4.8 eV using the magneto-optic Kerr effect. The 10 and 12 nm particles give rise to a magneto-optic signal, while the 8 nm particles show virtually no magneto-optic response at room temperature. The spectral features of the polar Kerr rotation depend on particle size and are at variance from those of a reference spectrum of a thick Co film. For the 10 nm particles an aging effect on a time scale of months is observed through the magneto-optic signature. The magneto-optic signal also depends on the arrangement of the particles within the film of nanoparticles. The magnetization of the 10 and 12 nm particle films is analyzed in terms of a Langevin function. From this analysis, the effective magnetic core of the 10 nm particles is estimated as 4.5 nm, while the 12 nm particles appear to show ferromagnetic coupling. (C) 2002 American Institute of Physics.