Structures and magnetic properties of Co films on rubrene/Si(100) have been investigated by employing atomic force microscopy, X-ray diffraction and magneto-optic Kerr effect techniques. For Co/rubrene/Si(100) films, separated Co clusters are found as the Co thickness increases. For 20 nm Co overlayer, the Co clusters are uniform with an average diameter around 90 nm. After annealing treatments at 550 K for rubrene/Si(100), rubrene molecules nucleate to form clusters and pinholes develop in the rubrene layer. For Co/annealed rubrene/Si(100) films, the formation of a Co/Si(100) interface is important for the occurrence of the pyramid-like nanostructure with an hcp stacking of the Co layer. Comparative studies of thickness dependencies of the magnetic properties for x nm Co on rubrene/Si(100) prepared at 300 and 550 K have been performed. With increasing the cobalt thickness, the Kerr intensities approach a saturation value for thicker Co due to an attenuation of the laser intensity in a condensed material. For Co deposited on annealed rubrene/Si(100), the enhancement of the coercive force is attributed to the imperfection introduced by rough interface to impede the magnetization reversal. As the Co thickness increases from 10 to 40 nm on rubrene/Si(100), the squareness increases from 0.9 to near unity. For Co deposited on annealed rubrene/Si(100), the squareness of magnetization curve is close to unity for all the Co thicknesses. The formation of Co islands in triangular shapes plays an important role on the enhancement of the squareness of magnetization curve for Co overlayers.
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