Graphene-mediated exchange coupling between cobaltocene and magnetic substrates S. Marocchi, 1, 2, * P. Ferriani, 3 N. M. Caffrey, 3 F. Manghi, 1, 2 S. Heinze, 3 and V. Bellini 2, 4, † 1 Dipartimento di Fisica, Universit´ a di Modena e Reggio Emilia, Via Campi 213/A, 41125 Modena, Italy. 2 S3 - Istituto di Nanoscienze - CNR, Via Campi 213/A, 41125 Modena, Italy 3 Instit¨ ut f¨ ur Theoretische Physik und Astrophysik, Christian-Albrecht-Universit¨ at zu Kiel, Leibnizstr. 15, 24098 Kiel, Germany 4 Istituto di Struttura della Materia (ISM) - Consiglio Nazionale delle Ricerche (CNR), I-34149, Trieste, Italy (Dated: April 24, 2013) Using first-principles calculations we demonstrate sizable exchange coupling between a magnetic molecule and a magnetic substrate via a graphene layer. As a model system we consider cobal- tocene (CoCp2) adsorbed on graphene deposited on Ni(111). We find that the magnetic coupling between the molecule and the substrate is antiferromagnetic and varies considerably depending on the molecule structure, the adsorption geometry, and the stacking of graphene on Ni(111). We show how this coupling can be tuned by intercalating a magnetic monolayer, e.g. Fe or Co, be- tween graphene and Ni(111). We identify the leading mechanism responsible for the coupling to be the spatial and energy matching of the frontier orbitals of CoCp2 and graphene close to the Fermi level, and we demonstrate the role of graphene as an electronic decoupling layer, yet allowing spin communication between molecule and substrate. PACS numbers: 71.15.Mb, 75.50.Xx, 68.43.-h, 81.05.ue The emerging field of organic spintronics capitalizes on the novel functionalities achieved when organic molecules are adsorbed on magnetic substrates. The ability to ma- nipulate and tune these functionalities is an important goal. Several problems remain however, before such sys- tems can be incorporated into new technological devices. One in particular is the capability to adsorb molecules on surfaces without any detrimental effects being caused to either the structural or magnetic properties of the molecule. For this reason it is vital to choose molecules with maximum structural robustness upon adsorption [1– 3]. To this end, the phthalocyanine and porphyrin fam- ilies are popular choices due to their planar geometry [4–9]. However, in some cases, the strong interaction be- tween the metal ion of such flat molecules and the sub- strate can modify its electronic states and even quench its magnetic moment [10]. The use of non-planer molecules, such as metallocenes, can minimize this effect. Metallocenes are composed of a 3d transition-metal ion sandwiched between two cyclopentadienyls (Cp). Depending on the metal ion species, both non-magnetic and paramagnetic behavior can be found [11]. The spin of the metal ion is shielded from the surface by the cage formed by the two Cp rings, reducing the possibility that it will be modified substan- tially after adsorption. Unfortunately, the deposition of metallocenes on metallic surfaces is a difficult process [12] and, in some cases, complete dissociation of the molecule occurs [13, 14]. The intercalation of a graphene spacer layer between the reactive surface and the metallocene can reduce the possibility of molecular dissociation during deposition. Additionally, evidence of charge transfer at molecule- graphene-Ni(111) interfaces [15, 16] and the theoreti- cal prediction of large charge transfer from cobaltocene (CoCp 2 ) to graphene [17] would suggest that a magnetic coupling between cobaltocene and the Ni(111) surface through the graphene layer is still achievable. In this Letter, we predict, by first principles elec- tronic structure methods, a sizable magnetic coupling for CoCp 2 adsorbed on a graphene layer deposited on a Ni(111) substrate. Furthermore, we propose interca- lation of different ferromagnetic metal monolayers, such as Fe and Co, between graphene and the Ni substrate as a route to tailor the magnetic coupling. Due to the unique electronic properties of graphene [18, 19], metal- organic systems of this kind could serve as a basis for future spintronics devices. Density functional theory (DFT) calculations have been performed using the projector augmented wave method as implemented in the VASP code [20, 21] with the Perdew-Burke-Ernzerhof (PBE) exchange correlation functional [22]. Dispersion interactions have been in- cluded according to the DFT-D2 approach [23]. Further computational details can be found in the Supplemental Material [24]. For a comprehensive characterization of the interface geometry we consider three possible struc- tural degrees of freedom, namely the molecular conforma- tion, the graphene-substrate stacking, and the molecular adsorption site. Isolated CoCp 2 has already been stud- ied extensively by DFT [11] and several possible struc- tures have been studied. We consider here CoCp 2 in the D 5h high-symmetry configuration [24] where two possi- ble Jahn-Teller distorted structures characterized by two different electronic states occur. The probability den- sity of the highest occupied molecular orbital (HOMO) of both of these states, labeled 2 B 2 and 2 A 2 , are plotted in Fig. 1(a). In both cases, the CoCp 2 molecule at- tains a nominal S=1/2 spin. The small lattice mismatch (1.2%) of graphene and Ni(111) lattice constant results