Structural, electrical and magnetic properties of Co/sub x/C/sub 1-x/ granular films

We report on the structural, electrical and magnetic properties of thin Co/sub x/C/sub 1-x/ granular films with metal volume fraction x=0.27 close to the percolation threshold x/sub p//spl ap/0.3. The granular films are prepared by simultaneous UHV electron beam evaporation of high purity Co and C at room temperature. The structural properties are determined from transmission electron microscopy revealing small Co clusters with mean diameters of about 3 nm randomly dispersed in an electrically insulating C matrix. As-prepared films show insulating resistance behavior. Upon annealing to T/sub a/=600/spl deg/C in-situ measurements within the electron microscope show coalescence of the Co clusters forming continuous cluster-networks. Resistance versus temperature and magnetoresistance measurements on annealed films show a gradual change from insulating to metallic behavior. Room temperature measurements of the magnetic properties show the evolution of ferromagnetic behavior from superparamagnetic behavior upon annealing with a formation of an unusual perpendicular anisotropy.