Organic materials typically offer small spin–orbit and hyperfine interactions, which are prerequisites for spintronic applications, because they allow long spin lifetimes. Thus there is a great interest in organic building blocks for molecular spintronics [1–4], and a thorough understanding of spin transport and magnetism in these systems is called for. It is therefore important to establish model molecular spintronic systems where spin transport and magnetic interactions can be examined experimentally. Recently, it has been demonstrated in low temperature scanning tunneling microscopy (STM) experiments how C₆₀ molecules can be picked up by the STM-tip, and how they could controllably be used to contact structures such as adatoms, clusters, and molecules placed on a substrate surface [5–7]. The C₆₀ molecule is considered as an attractive anchoring group for molecular electronics due to its mechanical robustness [8]. Moreover, the lowest unoccupied molecular orbital (LUMO) of C₆₀ is close to the Fermi level of ferromagnetic elements which makes spin injection relatively easy [9], and characterizes C₆₀ as a promising building block in molecular spintronics. The high symmetry of the C₆₀ allows detailed characterization of the bonding geometries in STM. In particular, one can determine which part of C₆₀ is pointing towards the tip/surface prior to contact formation, and also after contact formation, while the C₆₀ is placed on the tip [7]. Subsequently, it is possible to investigate the interactions between tip and sample via electronic transport measurements as tip and sample are brought into contact. STM also provides a powerful tool for investigating spin-transport in magnetic nanostructures [10–17]. Direct magnetic interactions between STM tip and magnetic materials on a substrate have been studied in a number of works [18–20], and STM has been used to probe spin in organic molecules [21]. In the case of a magnetic tip and magnetic surfaces, this method may be used to study spin transport and interactions through organic molecular systems bound to the surface and gain insight into single-molecular magnetic properties. Among organic compounds, carbonic rings which are combined with transition metals are interesting for molecular spintronics purposes [22]. The interaction of the π-electron system of such rings with the d-orbitals of the transition metals, is a key to electron and subsequently to spin transport. One example of such systems is presented in a theoretical study, where calculations have been used to examine spin transport in a benzene–Co system on a Cu(001) surface contacted by a Cr tip [23]. The magnetic properties and spin transport have also been calculated for organometallic “multidecker” wires, where magnetic atoms are sandwiched between organic parts [24]. Multidecker systems involving vanadium are very promising due to their half-metallic behavior resulting in high spin polarization of the transport [24–26]. Interestingly, due to the different symmetries of the C₆₀, it might be possible to vary the electronic and magnetic properties depending on whether pentagon, hexagon or edge sites of C₆₀ are in contact with the magnetic ligand atoms.

In order to mimic a concrete STM experiment, we investigated the specific setup shown in Fig. 1. The bulk regions of the contacts (i.e., STM tip and substrate) have been chosen to be nonmagnetic copper, which has previously been employed in manipulation experiments [5–7]. We imagine that magnetic atoms are deposited on the Cu surface prior to deposition [27] of the C₆₀ molecules, and that the tip-electrode is prepared prior to the contact by either indenting a Cu tip into a cluster of these atoms in order to cover its outermost part with these, or by creating the tip from from the bulk magnetic material. We model the outermost part of the tip by a pyramid-like structure consisting of four atoms on one electrode, simulating the magnetic STM tip which is used to pick up C₆₀ for subsequent contact formation to an isolated adatom on the copper(111) substrate. In the following we will show that the case of vanadium is remarkable. We used spin-polarized pseudopotential DFT calculations with the SIESTA code [28]. Electronic structures have been calculated within a GGA-PBE approximation to the exchange and correlation functional [29]. Double-ζ polarized basis sets with grid cutoff of 250 Ry have been used. Spin-polarized transport was subsequently calculated using the non-equilibrium Green’s function (NEGF) formalism [30] in the limit of zero voltage. In order to eliminate basis set superposition error (BSSE) present in methods with atomic orbitals basis sets, we have used plane-wave (PW) formalism as implemented in [31] for total energy calculations. In these calculations, we have been using ultrasoft pseudopotentials, with 30/300 Ry cutoff for wavefunction/charge density.

As an initial rough guide in our search for interesting magnetic metals to contact the C₆₀, and to select relevant bonding sites, we first performed calculations of the binding energy of a single adatom from the first row transition metals (from vanadium to copper) with C₆₀. This will clearly overestimate the binding of the C₆₀ to a higher coordinated tip-atom but we are here focussing on the trends in binding energy depending on tip-atom species. Based on the simple adatom calculations we seek high magnetization and a high binding energy to get a stable contact. The results are summarized in Table 1 for different sites on the C₆₀ molecule. It can be seen that nickel has the strongest binding energy but with zero total magnetization (M _T), and thus, is probably not interesting for investigations of spin transport. On the other hand, chromium enjoys the largest M _T, due to its largest unpaired electronic configuration [Ar]3d⁵4s¹ but with the least binding energy strength. It has already been shown that copper STM tips can pick up C₆₀ [5]. Noting that the maximum of binding energy for copper to C₆₀ is ≈0.8 eV, we conclude that the same action should be possible with vanadium while enjoying a decent M _T.