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Spintronics, which aims to exploit the electrons spin for the development of novel information storage or logic devices, is nowadays a major and competitive research field in physics. For its purpose a huge effort is currently made by the scientific community to develop novel material suitable for applications. In this respect, graphene, a planar sheet of carbon atoms packed in a honeycomb lattice, has attracted a considerable attention due to its outstanding electrons mobility and long spin relaxation time. The first property arises from the peculiar graphene crystal structure resulting in a linear dispersion (Dirac Cones) of the low energy electrons which behave like mass-less particles. Being a light element, carbon has almost negligible spin-orbit coupling, the interaction usually responsible for spin relaxation. The use of graphene as a building block for spintronic application requires a modification of its band structure, for example band gap opening, while keeping the peculiar dispersion of the Dirac Cones. One very efficient way to achieve this purpose consists in the use of interfaces between graphene and other elements. For examples the Dirac Cones spin degeneracy can be lifted by the graphene epitaxial interface with a high spin-orbit coupled metal which can lead to novel quantum phases such as topological quantum spin Hall effect. In these hybrid systems, not only one can tailor the graphene electronic properties, but also the graphene layer can have a tremendous effect of the second material. Astonishing examples are the effects of a graphene/ferromagnet interface. On the one hand, when graphene is brought in contact with a ferromagnetic material, a magnetic moment can raise in carbon atoms together with the appearance of a spin polarized mini-Dirac Cone at the Fermi level. On the other hand, the ferromagnet experiences a huge interfacial magnetic anisotropy and its magnetic configuration changes dramatically. The study of such graphene/ferromagnetic hybrid system has a huge implications form both the basic scientific and technological standpoints. Scientifically the graphene does not consist of d or f electrons, so the magnetic moment formation is non-trivial. Technologically those systems could potentially give rise to high-Curie temperature magnets with peculiar spin-texture and meet the demand of increasing magnetic information storage density by engineering ultimately thin, quasi two-dimensional, magnetic material.
(1) Being magnetically polarizable (such as Pt or Pd), the HM material will acquire a magnetic moment through its interface with the underlying magnetic film while conserving its natural high spin-orbit coupling. Via its interface with the HM element, the graphene layer will therefore experience, at the same timeboth, exchange and spin-orbit interaction. Such a situation has been little studied so far due to the difficulties of the system preparation; as a result and the two interactions are often usually studied in a separate waysseparately. While producing a net magnetic moment in carbon atoms, a graphene/FM interface strongly modify modifies the linear electron linear dispersions, resulting in the suppression of the Dirac Cone at the Fermi level, and in with the appearance of a low band-width “mini”-Cone. On the other hand, the interaction of graphene with some high spin-orbit coupling element reliefs the spin degeneracy by Rashba effect without significantly altering the Dirac Cones linear dispersion. With the chosen system we will be able to study the effect on the graphene band structure of simultaneous exchange and spin-orbit interactions while keeping intact, as much as possible, the pristine linear electron linear dispersion. The mutual coexistence of exchange and spin-orbit interaction in the graphene layer it is therefore expected to opens the access to new physical properties. For example, new topological phases in graphene were proposed, including topological quantum spin Hall effect and quantum anomalous Hall effect.
(2) The effect of the graphene/FM interface on the magnetic properties of the ferromagnet has been put in evidence only very recently. The interface interfacial magnetic anisotropy energy that develops raises helps to stabilize the out out-of of-plane magnetization of the thin film and, for given specific thicknesses, gives rise to a peculiar spin-texture with canted magnetization. In the proposed hybrid systems, the HM/FM interface will induce a further enhancement of the anisotropy allowing the fabrication of an ultra-thin stable ferromagnet. Moreover, the insertion of the HM element in between graphene and the FM material provides a new parameter which can be exploited to modify the overall magnetic properties. Depending on how the coupling between electrons of the three constituents will behave one can expect, beside the enhancement of the magnetic anisotropy energy, even non-trivial spin textures.
All the effects mentioned above strongly depend on the cleanness and the crystallographic structure of the interfaces involved, the experimental part of this study requires therefore an epitaxial system. The low number of experimental studies on graphene/thin film epitaxial hybrids is mainly due to fabrications difficulties. The deposition of a metal on top of graphene often leads to clustered films. On the other hand the growth of graphene on top of a metallic thin film (below 1-2 nm thickness) usually requires a high temperature that breaks the film crystal structure. For this reasons graphene epitaxial hybrids have been mainly studied when graphene has been grown on metallic crystals or mechanically transferred in air on top of the film. In the first case the number of possible studies is small, in the second case this transfer often leads to a poor quality of the interface that strongly reduces the effect of the interactions. Therefore the best way to investigate the interface effects mentioned above is by the in-situ insertion of the foreign spices (FM and HM) between the graphene layer and its non-magnetic metallic substrate, which allows a layer-by-layer growth of the film and results in atomically sharp interfaces between the constituents. This insertion process of atoms between graphene and a substrate it is called intercalation and it has been extensively studied in the last few years. Although quite some evidences of the process have been already reported it is far to be completely understood. In particular the preparation of the systems proposed in this project has never been reported so far in the film thickness range at interest here. Therefore the realization of the bimetallic epitaxial hybrids proposed here would be already an important achievement and the extensive experience of the investigators in the preparation of such systems together with the preliminary results shown in the next section are guarantees of success.