The influence of graphene on the magnetism of ultrathin cobalt.

Abstract

The increasing interest in ultrathin magnetic films is driven by the recent observation of chiral magnetic domains and their current-induced motion. In this scenario films of cobalt are of particular importance in designing synthetic antiferromagnets, such as two ferromagnetic layers coupled antiferromagnetically through a nonmagnetic spacer layer. The observation of AFM-coupled Fe/Gr/Co/Ir(111) multi-stack has opened the possibility to use graphene (Gr) as a spacer layer. In this context, a key part is to investigate the effect of covering Co films with a graphene overlayer, that has shown to enhance the PMA in Co. The focus of this thesis is to study the effect of the morphology and crystalline structure of graphene on the magnetic properties of Co, trying to observe a relation between PMA and the graphene quality. For this purpose, the structural properties of graphene were be varied by modifying the growth methods for graphene overlayer. In this respect, three different methods were employed based on CVD growth by ethylene as precursor for carbon atoms. The first one was by low temperature treatment of the surface, favoring the growth a graphene layer with considerable amount of disorder. In the second method by introducing oxygen acting on the formation process a more complete graphene layer was obtained though still showing some disorder. In the last one the interaction between rhenium substrate and carbon atoms was exploited to form well-oriented graphene nuclei, which were used for further growth subsequent to Co intercalation. Synthesis and characterization were performed by LEEM/LEED for morphological and structural analysis, XPS for chemical state and XMCD, MOKE for the magnetic response of the system, using the facilities available at the Nanospectroscopy beamline at Elettra Sincrotrone Trieste. The first most important result can be summarized as the tunable quality of the graphene overlayer, which in the case of the third method consisted of a full epitaxial layer, while for the others some rotated domains are present. Consequently, an increased PMA was found in the sample with full epitaxial graphene.