Cobalt cluster-assembled thin films deposited by low energy cluster beam deposition : structural and magnetic investigations of deposited layers
Résumé
Cobalt cluster-assembled thin films were deposited on amorphous carbon coated copper grids and on silicon substrates at room temperature by Low Energy Cluster Beam Deposition (LECBD). Characterisations using High-Resolution Transmission Electronic Microscopy (HRTEM) and Atomic Force Microscopy (AFM) reveal randomly stacked agglomerates of 9 to 11 nm diameter, which are themselves composed of small 3.6 nm diameter fcc cobalt clusters. The magnetic response of these films was analysed using Vibrating Sample Magnetometer (VSM) measurements, performed between 15 K to 300 K, and Magnetic Force Microscopy (MFM) investigations, realized at room temperature. The films are ferromagnetic up to room temperature and above, which implies that the clusters are exchange coupled. The approach to saturation is analysed within the Random Anisotropy Model (RAM). The values of the exchange coefficient A and the anisotropy constant K then derived are discussed. The temperature dependence of the coercivity below 100 K is discussed in terms of thermal activation effects. All results indicate that the fundamental entity governing the magnetic behaviours is constituted by the 9-11 nm diameter agglomerates rather than by the clusters themselves.