Structural, electrical and magnetic properties of CoxC 1-x granular films

We report on the structural, electrical and magnetic properties of thin Co_xC_1-x granular films with metal volume fraction x=0.27 close to the percolation threshold x_p≈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_a=600°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