Fe Adatoms Research Articles

Single 3d transition metal atoms on multi-layer graphene systems: electronic configurations, bonding mechanisms and role of the substrate

The electronic configurations of Fe, Co, Ni and Cu adatoms on graphene and graphite have been studied by x-ray magnetic circular dichroism and charge transfer multiplet theory. A delicate interplay between long-range interactions and local chemical bonding is found to influence the adatom equilibrium distance and magnetic moment. The results for Fe and Co are consistent with purely physisorbed species having, however, different 3d-shell occupations on graphene and graphite ( and , respectively). On the other hand, for the late 3d metals Ni and Cu a trend towards chemisorption is found, which strongly quenches the magnetic moment on both substrates.

Complex trend of magnetic order in Fe clusters on4dtransition-metal surfaces. I. Experimental evidence and Monte Carlo simulations

We demonstrate the occurrence of compensated spin configurations in Fe clusters and monolayers on Ru(0001) and Rh(111) by a combination of x-ray magnetic circular dichroism experiments, first-principles calculations, and Monte Carlo simulations. Our results reveal complex intracluster exchange interactions which depend strongly on the substrate $4d$-band filling, the cluster geometry, as well as lateral and vertical structural relaxations. The importance of substrate $4d$-band filling manifests itself also in small nearest-neighbor exchange interactions in Fe dimers and in a nearly inverted trend of the Ruderman-Kittel-Kasuya-Yosida coupling constants for Fe adatoms on the Ru and Rh surface.

Strong out-of-plane magnetic anisotropy of Fe adatoms onBi2Te3

The electronic and magnetic properties of individual Fe atoms adsorbed on the surface of the topological insulator ${\mathrm{Bi}}_{2}{\mathrm{Te}}_{3}$(111) are investigated. Scanning tunneling microscopy and spectroscopy prove the existence of two distinct types of Fe species, while our first-principles calculations assign them to Fe adatoms in the hcp and fcc hollow sites. The combination of x-ray magnetic circular dichroism measurements and angular dependent magnetization curves reveals out-of-plane anisotropies for both species with anisotropy constants of ${K}_{\text{fcc}}=(10\ifmmode\pm\else\textpm\fi{}4)$ meV/atom and ${K}_{\text{hcp}}=(8\ifmmode\pm\else\textpm\fi{}4)$ meV/atom. These values are well in line with the results of calculations.

Electric field as a novel switch for magnetization of Fe/graphene system

The magnetic property of a graphene-adsorbed Fe adatom was observed under an external electric field effect (−0.3–0.3eV/Å) using density functional theory (DFT) calculations. In this study, it was demonstrated that the magnetic moment of Fe on graphene was changed linearly according to the electric field. The density of states and differential planar-averaged charge-density indicated that the changing electronic structure was due to a redistribution of valence electrons under external electric field that induces a continuous change in the localized magnetic moment of the Fe adatom. This research suggests that the magnetic property of the adatom on graphene is tunable by an electric field. Furthermore, these results may be applicable to the spintronic memory device industry.

Spin-induced band modifications of graphene through intercalation of magnetic iron atoms

Intercalation of magnetic iron atoms through graphene formed on the SiC(0001) surface is found to induce significant changes in the electronic properties of graphene due mainly to the Fe-induced asymmetries in charge as well as spin distribution. From our synchrotron-based photoelectron spectroscopy data together with ab initio calculations, we observe that the Fe-induced charge asymmetry results in the formation of a quasi-free-standing bilayer graphene while the spin asymmetry drives multiple spin-split bands. We find that Fe adatoms are best intercalated upon annealing at 600 °C, exhibiting split linear π-bands, characteristic of a bilayer graphene, but much diffused. Subsequent changes in the C 1s, Si 2p, and Fe 3p core levels are consistently described in terms of Fe-intercalation. Our calculations together with a spin-dependent tight binding model ascribe the diffuse nature of the π-bands to the multiple spin-split bands originated from the spin-injected carbon atoms residing only in the lower graphene layer.

Magnetism of an adatom on bilayer graphene and its control: A first-principles perspective

We present a first-principles investigation of the electronic and magnetic properties of an adatom on bilayer graphene within the framework of density functional theory. In particular, we study the influence of an applied gate voltage which modifies the electronic states of the bilayer graphene as well as shifts the adatom energy states relative to that of the graphene energy states. Our study carried out for a choice of three different adatoms, Na, Cu, and Fe, shows that the nature of adatom-graphene bonding evolves from ionic to covalent, in moving from an alkali metal, Na, to a transition metal, Cu or Fe. This leads to the formation of magnetic moments in the latter cases (Cu, Fe) and the absence of a magnetic moment in the former (Na). Application of an external electric field to bilayer graphene completely changes the scenario, switching on a magnetic moment for the Na adatom, and switching off the magnetic moments for Cu and Fe adatoms. Our results have important implications for fundamental studies of controlled adatom magnetism and spintronics applications in nanotechnology.

Complex magnetic structure of clusters and chains of Ni and Fe on Pt(111)

We present an approach to control the magnetic structure of adatoms adsorbed on a substrate having a high magnetic susceptibility. Using finite Ni-Pt and Fe-Pt nanowires and nanostructures on Pt(111) surfaces, our ab initio results show that it is possible to tune the exchange interaction and magnetic configuration of magnetic adatoms (Fe or Ni) by introducing different numbers of Pt atoms to link them, or by including edge effects. The exchange interaction between Ni (or Fe) adatoms on Pt(111) can be considerably increased by introducing Pt chains to link them. The magnetic ordering can be regulated allowing for ferromagnetic or antiferromagnetic configurations. Noncollinear magnetic alignments can also be stabilized by changing the number of Pt-mediated atoms. An Fe-Pt triangularly-shaped nanostructure adsorbed on Pt(111) shows the most complex magnetic structure of the systems considered here: a spin-spiral type of magnetic order that changes its propagation direction at the triangle vertices.

Designing Fe Nanostructures at Graphene/h-BN Interfaces

Tailor-made magnetic nanostructures offer a variety of functionalities useful for technological applications. In this work, we explore the possibilities of realizing Fe nanostructures at the interfaces of 2D graphene and h-BN by ab initio density functional calculations. With the aid of ab initio Born–Oppenheimer molecular dynamics simulations and diffusion barriers calculated by the nudged elastic band method, we find that (i) diffusion barriers of Fe on BN are much smaller than those on graphene, (ii) the Fe adatoms form clusters within a short time interval (∼2.1 ps), and (iii) Fe clusters diffuse easily across the C–N interface but become immobile at the C–B interface. The calculated magnetic exchange coupling between Fe clusters at C–B interfaces varies nonmonotonically as a function of the width of BN separating the graphene parts. One may envisage design of magnetic nanostructures at the C–B interface of 2D graphene/h-BN hybrids to realize interesting applications related to spintronics.

Modulating the electronic properties of graphdiyne nanoribbons

By ab initio calculation, Au, Cu, Fe, Ni, and Pt adatoms were proposed for modulating the electronic property of graphdiyne naoribbons (GDNRs). GDNRs of 1–4nm in width were found to be stable at room temperature, and the thermal rates of Au, Cu, Fe, Ni, and Pt adatoms escaping from GDNR are slower than 0.003atoms per hour even at 900K. According to the calculation, Au and Cu-decorated GDNRs are metallic with carrier concentrations close to that of graphene at room temperature, while Fe, Ni, and Pt-decorated GDNRs are n-type semiconductors with impurity states below Fermi energy. Heterojunction composed by doping Au, Cu, or Fe atom on one side of GDNR was proposed as metal–semiconductor rectifier with rectification ratio of 2.8, 1.5, or 2.5 at 1.0V, respectively.

Metal desorption from Fe(1 1 0) and its alloyed surfaces

We have performed first-principles calculations to study the surface metal–metal bonding and energetics that control desorption on metal and alloy surfaces. The adsorbed and desorbed states of Fe, Cr, and Mo adatoms on the Fe(110) surface were examined. Their formation and extraction were investigated both as neutral particles and as metal complexes. In addition, the roles which alloying elements play on adatom-surface binding is also a critical question that needs to be addressed. The alloying effect was found to relate to the nature of charge transfer between alloying elements and host metals.

Density functional theory study of Fe adatoms adsorbed monolayer and bilayer MoS2 sheets

Functionalization of MoS2 sheet (monolayer and bilayer) by the adsorption of transition metal Fe adatom to its surface and interlayer has been investigated computationally using first-principles calculations based on the density functional theory. We found that the systems with absorption of Fe adatoms on the surfaces of both monolayer and bilayer MoS2 sheets are still semiconductors, without spin polarization at the Fermi level. However, for the system with absorption of Fe adatom in the interlayer of bilayer MoS2 sheet, its electronic structure exhibits half-metal behavior, with 100% spin polarization at the Femi level, which provides a promising material for spintronic devices.

Correlation effects in insulating surface nanostructures

We study the role of static and dynamical Coulomb correlation effects on the electronic and magnetic properties of individual Mn, Fe and Co adatoms deposited on the CuN surface. For these purposes, we construct a realistic Anderson model, solve it by using finite-temperature exact diagonalization method and compare the calculated one-particle spectral functions with the LDA+$U$ densities of states. In contrast to Mn/CuN and Fe/CuN, the cobalt system tends to form the electronic excitations at the Fermi level. Based on the calculated magnetic response functions, the relative relaxation times for the magnetic moments of impurity orbitals are estimated. To study the effect of the dynamical correlations on the exchange interaction in nanoclusters, we solve the two-impurity Anderson model for the Mn dimer on the CuN surface. It is found that the experimental exchange interaction can be well reproduced by employing $U$=3 eV, which is two times smaller than the value used in static mean-field LDA+$U$ calculations. This suggests on important role of dynamical correlations in the interaction between adatoms on a surface.

Perpendicular magnetic anisotropy with enhanced orbital moments of Fe adatoms on a topological surface of Bi2Se3

We have found a perpendicular magnetic anisotropy of iron adatoms on a surface of the prototypical three-dimensional topological insulator Bi2Se3 by using x-ray magnetic circular dichroism measurements. The orbital magnetic moment of Fe is strongly enhanced at lower coverage, where angle-resolved photoemission spectroscopy shows coexistence of non-trivial topological states at the surface.

Probing the surface phase diagram of Fe3O4(001) towards the Fe-rich limit: Evidence for progressive reduction of the surface

Reduced terminations of the Fe${}_{3}$O${}_{4}$(001) surface were studied using scanning tunneling microscopy, x-ray photoelectron spectroscopy (XPS), and density functional theory (DFT). Fe atoms, deposited onto the thermodynamically stable, distorted B-layer termination at room temperature (RT), occupy one of two available tetrahedrally coordinated sites per ($\sqrt{2}\ifmmode\times\else\texttimes\fi{}\sqrt{2}$)$R$45\ifmmode^\circ\else\textdegree\fi{} unit cell. Further RT deposition results in Fe clusters. With mild annealing, a second Fe adatom per unit cell is accommodated, though not in the second tetrahedral site. Rather both Fe atoms reside in octahedral coordinated sites, leading to a ``Fe-dimer'' termination. At four additional Fe atoms per unit cell, all surface octahedral sites are occupied, resulting in a FeO(001)-like phase. The observed configurations are consistent with the calculated surface phase diagram. Both XPS and DFT+$U$ results indicate a progressive reduction of surface iron from Fe${}^{3+}$ to Fe${}^{2+}$ upon Fe deposition. The antiferromagnetic FeO layer on top of ferromagnetic Fe${}_{3}$O${}_{4}$(001) suggests possible exchange bias in this system.

Transition metal atoms pathways on rutile TiO2 (110) surface: Distribution of Ti3+ states and evidence of enhanced peripheral charge accumulation

Charge transfer between metal nanoparticles and the supported TiO2 surface is primarily important for catalytic applications as it greatly affects the catalytic activity and the thermal stability of the deposited nanoparticles on the surface. Herein, systematic spin-polarized density functional and HSE06 calculations are performed to evaluate the adsorption, diffusion, and charge state of several transition metal monomers on both stoichiometric and reduced rutile TiO2 (110) surface. Although the presence of oxygen vacancy (Ov) increases the binding of Au, Pt and Pd on the surface, it weakens the interaction thus enhancing the diffusion for Fe, Co, Ni, Ag, and Cu adatoms on the surface. For pristine reduced surface, only a small portion (around 5%) of the excess electrons occupy the topmost surface, which are mainly delocalized at the second nearest and third nearest fivefold coordinated Ti (Ti5c) atoms. Excess electrons populating at the Ti5c atoms on the basal plane can be transferred to strongly electronegative adsorbates like Au and Pt thus enabling a moderate adsorption at this site, whereas no stable adsorption is found for other less electronegative transition metal adatoms (Ag, Cu, Fe, Co, Ni, and Pd) on the reduced surface and for all the adatoms on stoichiometric surface. This result clarifies the origin of the experimental observation of the adsorption of O2 and CO molecules at Ti5c sites in connection with charge transfer. In addition, the spatial redistribution of the excess electrons around the Ov upon the adsorption of the monomers is thoroughly examined. Our finding of an accumulation of excess electrons at the Ti5c sites around the monomers explains the critical role of the perimeter interface of the deposited nanoparticles in promoting the adsorption and activation of reactants observed in experiments.

The formation of FecoreAlshell and FeshellAlcore nanoparticles, a molecular dynamics simulation

According to the surface segregation theory, bimetallic nanoparticles can easily form core–shell configuration. An atom with low surface energy and large atomic size distributes on the surface of the nanoparticle, and another atom occupies the internal position. In the present work, the diffusion and growth configuration of Fe–Al nanoparticles are examined. The calculated energy barriers for the interfacet diffusion via the hopping or the exchange mechanisms are larger than that for the intrafacet diffusion. The result suggests that both FecoreAlshell and FeshellAlcore nanoparticles can be obtained at proper temperatures. The growths of the Fe and Al adatoms on the icosahedron of Al and the rhombohedron of Fe are simulated by molecular dynamics along with embedded atom methods in the temperature range of 100–300K. FeshellAlcore, a reversed core–shell configuration, is obtained at low temperatures.

Adatoms and Clusters of3dTransition Metals on Graphene: Electronic and Magnetic Configurations

We investigate the electronic and magnetic properties of single Fe, Co, and Ni atoms and clusters on monolayer graphene (MLG) on SiC(0001) by means of scanning tunneling microscopy (STM), x-ray absorption spectroscopy, x-ray magnetic circular dichroism (XMCD), and abinitio calculations. STM reveals different adsorption sites for Ni and Co adatoms. XMCD proves Fe and Co adatoms to be paramagnetic and to exhibit an out-of-plane easy axis in agreement with theory. In contrast, we experimentally find a nonmagnetic ground state for Ni monomers while an increasing cluster size leads to sizeable magnetic moments. These observations are well reproduced by our calculations and reveal the importance of hybridization effects and intra-atomic charge transfer for the properties of adatoms and clusters on MLG.

Controllable magnetic doping of the surface state of a topological insulator.

A combined experimental and theoretical study of doping individual Fe atoms into Bi(2)Se(3) is presented. It is shown through a scanning tunneling microscopy study that single Fe atoms initially located at hollow sites on top of the surface (adatoms) can be incorporated into subsurface layers by thermally activated diffusion. Angle-resolved photoemission spectroscopy in combination with ab initio calculations suggest that the doping behavior changes from electron donation for the Fe adatom to neutral or electron acceptance for Fe incorporated into substitutional Bi sites. According to first principles calculations within density functional theory, these Fe substitutional impurities retain a large magnetic moment, thus presenting an alternative scheme for magnetically doping the topological surface state. For both types of Fe doping, we see no indication of a gap at the Dirac point.

Formation of Fe Cluster Superlattice in a Metal-Organic Quantum-Box Network

We report on the self-assembly of Fe adatoms on a Cu(111) surface that is patterned by a metal-organic honeycomb network, formed by coordination of dicarbonitrile pentaphenyl molecules with Cu adatoms. Fe atoms landing on the metal surface are mobile and steered by the quantum confinement of the surface state electrons towards the center of the network hexagonal cavities. In cavities hosting more than one Fe, preferential interatomic distances are observed. The adatoms in each hexagon aggregate into a single cluster upon gentle annealing. These clusters are again centered in the cavities and their size is discerned by their distinct apparent heights.

First-principles studies of Fe atoms adsorption on hydrogen-terminated boron nitride nanoribbons

Abstract By using density functional theory (DFT), we have studied the adsorption of Fe atoms on boron nitride nanoribbons (BNNRs). The stabilities of Fe atoms adsorption on BNNRs for the inner sites are very weakly dependent on the adsorption sites and Fe atoms energetically prefer to be adsorbed on the edge boron sites. The analysis of electronic structures shows that the most of the stable systems are the interesting spin gapless semiconductors. The magnetism of the Fe adatom is preserved well on the BNNRs, very close to that of the corresponding isolated atom for all the considered adsorption sites.