Size effects in the temperature-dependent magnetization of iron clusters

The magnetic behavior of silver-capped iron clusters on tungsten (110) and iron clusters in a silver matrix on Si (111) has been investigated with respect to the temperature by means of synchrotron-based Mِssbauer spectroscopy and the magneto-optical Kerr-effect. Fe clusters on W (110) of monoatomic height exhibit a perpendicular magnetic anisotropy at temperatures below 100 K. At a higher Fe coverage the strong surface anisotropy favours an in-plane magnetization. Well separated and mass-selected Fe nanoparticles with diameters of about 8.1 and 11.7 nm embedded in a silver matrix show a ferromagnetic order below 77 K. A temperature rise to 300 K leads to superparamagnetism of the 8.1 nm particles.