Graphene-mediated exchange coupling between cobaltocene and magnetic substrates (original) (raw)
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Complex Magnetic Exchange Coupling between Co Nanostructures and Ni(111) across Epitaxial Graphene
ACS Nano, 2015
We report on the magnetic coupling between isolated Co atoms as well as small Co islands and Ni(111) mediated by an epitaxial graphene layer. X-ray magnetic circular dichroism and scanning tunneling microscopy combined with density functional theory calculations reveal that Co atoms occupy two distinct adsorption sites, with different magnetic coupling to the underlying Ni(111) surface. We further report a transition from an antiferromagnetic to a ferromagnetic coupling with increasing Co cluster size. Our results highlight the extreme sensitivity of the exchange interaction mediated by graphene to the adsorption site and to the in-plane coordination of the magnetic atoms.
Physical Review B
We investigate the magnetic response of a spin interface constituted by MnPc molecules adsorbed on graphene/Co and its robustness against thermal fluctuations by x-ray magnetic circular dichroism. Elementselective hysteresis loops reveal a remarkable antiferromagnetic coupling between MnPc and Co that is strong both in perpendicular and in-plane magnetic orientations, thanks to the magnetic anisotropy properties and electronic configuration of MnPc. The magnetic interaction between MnPc and Co is mediated by the molecular states and the graphene π orbitals in a superexchange mechanism that allows a strong exchange coupling while the molecular orbitals symmetries are preserved by the graphene decoupling layer. Our results show that the strength and stability of the magnetic coupling between MnPc molecules and Co layer(s), intercalated at the graphene/Ir(111) interface, is further optimized by the open 3d shell of the central Mn ion. The magnetic properties are compared with analogous molecular spin interfaces with high thermal stability, paradigmatic examples to exploit in surface-supported molecular spin electronics.
Atomic-scale magnetism of cobalt-intercalated graphene
Physical Review B, 2013
Using spin-polarized scanning tunneling microscopy and density functional theory, we have studied the structural and magnetic properties of cobalt-intercalated graphene on Ir(111). The cobalt forms monolayer islands being pseudomorphic with the Ir(111) beneath the graphene. The strong bonding between graphene and cobalt leads to a high corrugation within the Moiré pattern which arises due to the lattice mismatch between the graphene and the Co on Ir(111). The intercalation regions exhibit an out-of-plane easy axis with an extremely high switching field, which surpasses the significant values reported for uncovered cobalt islands on Ir(111). Within the Moiré unit cell of the intercalation regions, we observe a site-dependent variation of the local effective spin polarization. State-of-the-art first-principles calculations show that the origin of this variation is a site-dependent magnetization of the graphene: At top sites the graphene is coupled ferromagnetically to the cobalt underneath, while it is antiferromagnetically coupled at fcc and hcp sites.
Tailoring the magnetism of Co atoms on graphene through substrate hybridization
Physical review letters, 2014
We determine the magnetic properties of individual Co atoms adsorbed on graphene (G) with x-ray absorption spectroscopy and magnetic circular dichroism. The magnetic ground state of Co adatoms strongly depends on the choice of the metal substrate on which graphene is grown. Cobalt atoms on G/Ru(0001) feature exceptionally large orbital and spin moments, as well as an out-of-plane easy axis with large magnetic anisotropy. Conversely, the magnetic moments are strongly reduced for Co/G/Ir(111), and the magnetization is of the easy-plane type. We demonstrate how the Co magnetic properties, which ultimately depend on the degree of hybridization between the Co 3d orbitals and graphene π bands, can be tailored through the strength of the graphene-substrate coupling.
The interfacial spin modulation of graphene on Fe(111)
arXiv (Cornell University), 2018
When Fe, which is a typical ferromagnet using dor f-orbital states, is combined with 2D materials such as graphene, it offers many opportunities for spintronics. The origin of 2D magnetism is from magnetic insulating behaviors, which could result in magnetic excitations and also proximity effects. However, the phenomena were only observed at extremely low temperatures. Fe and graphene interfaces could control spin structures in which they show a unique atomic spin modulation and magnetic coupling through the interface. Another reason for covering graphene on Fe is to prevent oxidation under ambient conditions. We investigated the engineering of spin configurations by growing monolayer graphene on an Fe(111) single crystal surface and observed the presence of sharply branched, 3D tree-like domain structures. Magnetization by a sweeping magnetic field (m-H) revealed that the interface showed canted magnetization in the in-plane (IP) orientation. Moreover, graphene could completely prevent the oxidation of the Fe surface. The results indicate possible control of the spin structures at the atomic scale and the interface phenomena in the 2D structure. The study introduces a new approach for room temperature 2D magnetism. state. Next, the Kohn-Sham equations are solved with no spin-orbit coupling (SOC) taken into account to determine the charge distribution of the system's ground state. Finally, the SOC is included, and the non-self-consistent total energy of the system is determined when the orientations of the magnetic moments are set both IP and OOP.
Unraveling Dzyaloshinskii–Moriya Interaction and Chiral Nature of Graphene/Cobalt Interface
A major challenge for future spintronics is to develop suitable spin transport channels with long spin lifetime and propagation length. Graphene can meet these requirements, even at room temperature. On the other side, taking advantage of the fast motion of chiral textures, i.e., Néel-type domain walls and magnetic skyrmions, can satisfy the demands for high-density data storage, low power consumption and high processing speed. We have engineered epitaxial structures where an epitaxial ferromagnetic Co layer is sandwiched between an epitaxial Pt(111) buffer grown in turn onto MgO(111) substrates and a graphene layer. We provide evidence of a graphene-induced enhancement of the perpendicular magnetic anisotropy up to 4 nm thick Co films, and of the existence of chiral left-handed Néel-type domain walls stabilized by the effective Dzyaloshinskii-Moriya interaction (DMI) in the stack. The experiments show evidence of a sizeable DMI at the gr/Co interface, which is described in terms of a conduction electron mediated Rashba-DMI mechanism and points opposite to the Spin Orbit Coupling-induced DMI at the Co/Pt interface. In addition, the presence of graphene results in: i) a surfactant action for the Co growth, producing an intercalated, flat, highly perfect fcc film, pseudomorphic with Pt and ii) an efficient protection from oxidation. The magnetic chiral texture is stable at room temperature and grown on insulating substrate. Our findings open new routes to control chiral spin structures using interfacial engineering in graphene-based systems for future spin-orbitronics devices fully integrated on oxide substrates.
Adsorption of cobalt on graphene: Electron correlation effects from a quantum chemical perspective
Physical Review B, 2012
In this work, we investigate the adsorption of a single cobalt atom (Co) on graphene by means of the complete active space self-consistent field approach, additionally corrected by the second-order perturbation theory. The local structure of graphene is modeled by a planar hydrocarbon cluster (C24H12). Systematic treatment of the electron correlations and the possibility to study excited states allow us to reproduce the potential energy curves for different electronic configurations of Co. We find that upon approaching the surface, the ground-state configuration of Co undergoes several transitions, giving rise to two stable states. The first corresponds to the physisorption of the adatom in the high-spin 3d 7 4s 2 (S = 3/2) configuration, while the second results from the chemical bonding formed by strong orbital hybridization, leading to the low-spin 3d 9 (S = 1/2) state. Due to the instability of the 3d 9 configuration, the adsorption energy of Co is small in both cases and does not exceed 0.35 eV. We analyze the obtained results in terms of a simple model Hamiltonian that involves Coulomb repulsion (U) and exchange coupling (J) parameters for the 3d shell of Co, which we estimate from first-principles calculations. We show that while the exchange interaction remains constant upon adsorption (≃ 1.1 eV), the Coulomb repulsion significantly reduces for decreasing distances (from 5.3 to 2.6±0.2 eV). The screening of U favors higher occupations of the 3d shell and thus is largely responsible for the interconfigurational transitions of Co. Finally, we discuss the limitations of the approaches that are based on density functional theory with respect to transition metal atoms on graphene, and we conclude that a proper account of the electron correlations is crucial for the description of adsorption in such systems.
The Journal of Physical Chemistry C, 2014
Inspired by Lie symmetries, we study the electronic and magnetic properties of cobalt (Co) and nickel (Ni) adatom adsorption on the graphene material using density functional theory calculations. The system we consider here consists of a static single layer of graphene interacting with transition-metal (TM) atoms. This system shows a nice geometrical shape having a double hexagonal structure appearing in the G 2 Lie algebra. This structure is associated with 25% concentration corresponding to a coverage of 0.666 monolayers placed at H sites. This new symmetry forces the derived Co material to behave like a ferromagnetic metal with a strong spin polarization. However, the derived Ni material remains a nonmagnetic metal. For the Co case, we show that the magnetic mechanism responsible for such behavior is the interaction between the Co atoms. In fact, there are two interaction types. The first one is associated with the direct interaction between the Co atoms, while the second one corresponds to the indirect interaction via the carbon atoms. Using Monte Carlo simulation, the Curie temperature for the Co material is estimated to be around 438 K. This value could be explored in nanomagnetic applications.
Tuning the Magnetic Coupling of a Molecular Spin Interface via Electron Doping
Mastering the magnetic response of molecular spin interfaces by tuning the occupancy of the molecular orbitals, which carry the spin magnetic moment, can be accomplished by electron doping. We propose a viable route to control the magnetization direction and magnitude of a molecular spin network, in a graphene-mediated architecture, achieved via alkali doping of manganese phthalocyanine (MnPc) molecules assembled on cobalt intercalated under a graphene membrane. The antiparallel magnetic alignment of the MnPc molecules with the underlying Co layer can be switched to a ferromagnetic state by electron doping. Multiplet calculations unveil an enhanced magnetic state of the Mn centers with a 3/2 to 5/2 spin transition induced by alkali doping, as confirmed by the steepening of the hysteresis loops, with higher saturation magnetization values. This new molecular spin configuration can be aligned by an external field, almost independently from the hardmagnet substrate effectively behaving as a free magnetic layer.