Magnetic, morphological and structural properties of polycrystalline ultrathin cobalt films for organic spintronics

The opportunity of using the organic molecules in spintronic devices appeared challenging since these materials, having nominally high spin relaxation times, are suitable for coherent spin manipulation. The spin behaviour in these molecular spintronic devices has been demonstrated to strongly depend on the nature of the chemical bonds between the organic molecules and the magnetic electrodes affecting also the magnetic response of both molecular and metallic sides. In particular, the adsorption of an organic molecule on a ferromagnetic layer has been proved to change the local magnetism of a magnetic substrate. In spite of their technological interest, the investigation of such effect in the case of the polycrystalline magnetic thin film is still lacking. My work contributes to filling this gap by studying the structural, morphological and magnetic properties of ultrathin polycrystalline cobalt films covered by the well-known buckminsterfullerene organic molecule (C60). The combined investigation by AFM, TEM, SQUID magnetometry and anisotropic magnetoresistance allowed to correlate the sample microstructure with the magnetic response and to identify the main mechanism responsible for spin transport in these FM layers. Analysed films are composed of polycrystalline cobalt grains decoupled by non-crystalline amorphous regions. The volume ratio between crystalline grains and amorphous regions increases by increasing the film thickness. As expected, the values of saturation magnetisation decrease as the crystallinity decreases and a typical blocking behaviour is present. The cobalt layers are also subjected to oxidation at the interface with the single crystal oxide substrate. The presence of amorphous phase in polycrystalline cobalt ultrathin film impacts the analysis of transport properties: the anisotropic magnetoresistance slightly depends on the crystalline phase while it is mainly inherent to the amorphous component.