Uniform magnetic properties for an ultrahigh-density lattice of noninteracting co nanostructures

Increasing Magnetic Anisotropy in Bimetallic Nanoislands Grown on fcc(111) Metal Surfaces

The magnetic properties and the atomic scale morphology of bimetallic two-dimensional nanoislands, epitaxially grown on fcc(111) metal surfaces, have been studied by means of Magneto-Optical Kerr Effect and Scanning Tunneling Microscopy. We investigate the effect on blocking temperature of one-dimensional interlines appearing in core-shell structures, of two-dimensional interfaces created by capping, and of random alloying. The islands are grown on Pt(111) and contain a Co-core, surrounded by Ag, Rh, and Pd shells, or capped by Pd. The largest effect is obtained by Pd capping, increasing the blocking temperature by a factor of three compared to pure Co islands. In addition, for Co-core Fe-shell and Co-core Fe_xCo_1−x-shell islands, self-assembled into well ordered superlattices on Au(11,12,12) vicinal surfaces, we find a strong enhancement of the blocking temperature compared to pure Co islands of the same size. These ultra-high-density (15 Tdots/in²) superlattices of CoFe nanodots, only 500 atoms in size, have blocking temperature exceeding 100 K. Our findings open new possibilities to tailor the magnetic properties of nanoislands.

Large magnetic anisotropy in an OsIr dimer anchored in defective graphene

Single-atom magnets represent the ultimate limit of magnetic data storage. The identification of substrates that anchor atom-sized magnets firmly and, thus, prevent their diffusion and large magnetic anisotropy has been at the centre of intense research efforts for a long time. Using density functional theory we show the binding of transition metal (TM) atoms in defect sites in the graphene lattice: single vacancy and double vacancy, both pristine and decorated by pyridinic nitrogen atoms, are energetically more favourable than away from the centre of defects, which could be used for engineering the position of TMs with atomic precision. Relativistic calculations revealed magnetic anisotropy energy (MAE) of ∼10 meV for Ir@NSV with an easy axis parallel to the graphene plane. MAE can be remarkably boosted to 50 meV for OsIr@NSV with the easy axis perpendicular to the graphene plane, which paves the way to the storage density of ∼490 Tb/inch²with the blocking temperature of 14 K assuming the relaxation time of 10 years. Magnetic anisotropy is discussed based on the relativistic electronic structures. The influence of an orbital-dependent on-site Coulomb repulsionUand a non-local correlation functional optB86b-vdW on MAE is also discussed.

X3 synthon geometries in two-dimensional halogen-bonded 1,3,5-tris(3,5-dibromophenyl)benzene self-assembled nanoarchitectures on Au(111)-()

The self-assembly of star-shaped 1,3,5-tris(3,5-dibromophenyl)benzene molecules on Au(111)-() in a vacuum is investigated using scanning tunneling microscopy and core-level spectroscopy. Scanning tunneling microscopy shows that the molecules self-assemble into a hexagonal porous halogen-bonded nanoarchitecture. This structure is stabilized by X_3-A synthons composed of three type-II halogen-interactions (halogen-bonds). The molecules are oriented along the same direction in this arrangement. Domain boundaries are observed in the hcp region of the herringbone gold surface reconstruction. Molecules of the neighboring domains are rotated by 180°. The domain boundaries are stabilized by the formation of X_3-B synthons composed of two type-II and one type-I halogen-interactions between molecules of the neighboring domains. Core-level spectroscopy confirms the existence of two types of halogen-interactions in the organic layer. These observations show that the gold surface reconstructions can be exploited to modify the long-range supramolecular halogen-bonded self-assemblies.

On-surface synthesis of superlattice arrays of ultra-long graphene nanoribbons

We report the on-surface synthesis of graphene nanoribbon superlattice arrays directed by the herringbone reconstruction of the Au(111) surface.

Growth of Co Nanomagnet Arrays with Enhanced Magnetic Anisotropy

A trigon structure formed by submonolayer gadolinium deposition onto Au(111) is revealed as a robust growth template for Co nanodot arrays. Scanning Tunneling Microscopy and X-Ray Magnetic Circular Dichroism measurements evidence that the Co nanoislands behave as independent magnetic entities with an out-of-plane easy axis of anisotropy and enhanced magnetic anisotropy values, as compared to other self-organized Co nanodot superlattices. The large strain induced by the lattice mismatch at the interface between Co and trigons is discussed as the main reason for the increased magnetic anisotropy of the nanoislands.

Moiré induced organization of size-selected Pt clusters soft landed on epitaxial graphene

Two-dimensional hexagonal arrays of Pt nanoparticles (1.5 nm diameter) have been obtained by deposition of preformed and size selected Pt nanoparticles on graphene. This original self-organization is induced, at room temperature, by the 2D periodic undulation (the moiré pattern) of graphene epitaxially grown on the Ir(111) surface. By means of complementary techniques (scanning tunneling microscopy, grazing incidence X ray scattering), the Pt clusters shapes and organization are characterized and the structural evolution during annealing is investigated. The soft-landed clusters remain quasi-spherical and a large proportion appears to be pinned on specific moiré sites. The quantitative determination of the proportion of organized clusters reveals that the obtained hexagonal array of the almost spherical nanoparticles is stable up to 650 K, which is an indication of a strong cluster-surface interaction.

Magnetic properties of self-organized Co dimer nanolines on Si/Ag(110)

We demonstrate the kinetically controlled growth of one-dimensional Co nanomagnets with a high lateral order on a nanopatterned Ag(110) surface. First, self-organized Si nanoribbons are formed upon submonolayer condensation of Si on the anisotropic Ag(110) surface. Depending on the growth temperature, individual or regular arrays (with a pitch of 2 nm) of Si nanoribbons can be grown. Next, the Si/Ag(110) system is used as a novel one-dimensional Si template to guide the growth of Co dimer nanolines on top of the Si nanoribbons, taking advantage of the fact that the thermally activated process of Co diffusion into the Si layer is efficiently hindered at 220 K. Magnetic characterization of the Co nanolines using X-ray magnetic circular dichroism reveals that the first atomic Co layer directly adsorbed onto the Si nanoribbons presents a weak magnetic response. However, the second Co layer exhibits an enhanced magnetization, strongly suggesting a ferromagnetic ordering with an in-plane easy axis of magnetization, which is perpendicular to the Co nanolines.

Self-organized ultrathin FePt nanowires produced by glancing-angle ion-beam codeposition on rippled alumina surfaces

Ultradense macroscopic arrays of ferromagnetic alloy nanowires exhibit unique properties that make them attractive both for basic physics studies and for prospective nanodevice applications in various areas. We report here on the production of self-organized equiatomic FePt nanowires produced by glancing-angle ion-beam codeposition on alumina nanoripple patterns at room temperature and subsequent annealing at 600 °C. This study demonstrates that periodically aligned FePt nanowires with tunable size (∼10-20 nm width and ∼0.5-10 nm height) can be successfully grown as a consequence of shadowing effects and low mobility of Fe and Pt on the rippled alumina surface. Moreover, the structure and magnetic properties of the FePt nanowires, which undergo a phase transition from a disordered A1 (soft) structure to a partially ordered L10 (hard) structure, can be modified upon annealing. We show that this behavior can be further exploited to change the effective uniaxial anisotropy of the system, which is determined by a strong interplay between the shape and magnetocrystalline anisotropies of the nanowires.

Spontaneous formation of high-index planes in gold single domain nanocrystal superlattices

Crystals of nanocrystals, also called supracrystals and nanocrystal superlattices, are expected to exhibit specific properties that differ from both the corresponding bulk material and nanosized elementary units. In particular, their surfaces have a great potential as nanoscale interaction plateforms. However, control of the symmetry, compacity, and roughness of their surfaces remains an open question. Here, we describe the spontaneous formation of upper vicinal surfaces for supracrystals of Au nanocrystals grown on a sublayer of ordered Co nanocrystals. Stepped or kinked surfaces vicinal to the {100}, {110}, and {111} planes are observed to be extended on the micrometer range. The formation of such high-index planes is explained by a heteroepitaxial relationship between both Co and Au nanocrystal superlattice.

Co nanodot arrays grown on a GdAu2 template: substrate/nanodot antiferromagnetic coupling

Controlling anisotropy and exchange coupling in patterned magnetic nanostructures is the key for developing advanced magnetic storage and spintronic devices. We report on the antiferromagnetic interaction between a Co nanodot array and its supporting GdAu2 nanotemplate that induces large anisotropy values in individual Co nanodots. In clear contrast with nonmagnetic Au substrates, GdAu2 triggers an earlier switch from out-of-plane anisotropy in monatomic high dots to in-plane when the dot height becomes biatomic.

Tuning magnetic anisotropies of Fe films on Si(111) substrate via direction variation of heating current

We adopted a novel method to tune the terrace width of Si(111) substrate by varying the direction of heating current. It was observed that the uniaxial magnetic anisotropy (UMA) of Fe films grown on the Si(111) substrate enhanced with decreasing the terrace width and superimposed on the weak six-fold magnetocrystalline anisotropy. Furthermore, on the basis of the scanning tunneling microscopy (STM) images, self-correlation function calculations confirmed that the UMA was attributed mainly from the long-range dipolar interaction between the spins on the surface. Our work opens a new avenue to manipulate the magnetic anisotropy of magnetic structures on the stepped substrate by the decoration of its atomic steps.

Atomic-scale engineering of magnetic anisotropy of nanostructures through interfaces and interlines

The central goals of nanoscale magnetic materials science are the self-assembly of the smallest structure exhibiting ferromagnetic hysteresis at room temperature, and the assembly of these structures into the highest density patterns. The focus has been on chemically ordered alloys combining magnetic 3d elements with polarizable 5d elements having high spin–orbit coupling and thus yielding the desired large magneto-crystalline anisotropy. The chemical synthesis of nanoparticles of these alloys yields disordered phases requiring annealing to transform them to the high-anisotropy L1₀ structure. Despite considerable efforts, so far only part of the nanoparticles can be transformed without coalescence. Here we present an alternative approach to homogeneous alloys, namely the creation of nanostructures with atomically sharp bimetallic interfaces and interlines. They exhibit unexpectedly high magnetization reversal energy with values and directions of the easy magnetization axes strongly depending on chemistry and texture. We find significant deviations from the expected behaviour for commonly used element combinations. Ab-initio calculations reproduce these results and unravel their origin.

The design and assembly of nanostructures exhibiting ferromagnetic hysteresis at room temperature are recognized goals for high-density data storage. Here, the authors engineer nanostructures with atomically sharp bimetallic interfaces and interlines, which exhibit large magnetic anisotropy and high temperature hysteresis.

Templating of arrays of Ru nanoclusters by monolayer graphene/Ru Moirés with different periodicities

We have studied the formation of Ru nanocluster arrays on several monolayer graphene/Ru Moiré structures with different relative orientations of the graphene and Ru lattices. Experiments and ab initio calculations clearly show that the presence of a graphene/Ru Moiré does not guarantee the ordered adsorption of Ru nanoclusters. The simultaneous deposition of Ru onto coexisting Moirés demonstrates that a structure with aligned graphene and Ru lattices templates the formation of arrays of small Ru clusters with narrow size spread and adsorption exclusively in a single site (the 'low fcc' site). The other Moirés considered here gave rise to substantially larger clusters with broader size distribution and without detectable site selectivity. Calculations explain these findings via the density of states (DOS) at different sites of the graphene/Ru Moiré. The ordered nucleation of many small clusters instead of incorporation of metal atoms into larger ones requires one Moiré site with a large DOS at the Fermi level, so that the binding of metal adatoms to this site is stronger than to competing sites in the Moiré and to existing metal clusters.

Low-resistivity 10 nm diameter magnetic sensors

Resistivities of 5.4 μΩ·cm were measured in 10-nm-diameter metallic wires. Low resistance is important for interconnections of the future to prevent heating, electromigration, high power consumption, and long RC time constants. To demonstrate application of these wires, Co/Cu/Co magnetic sensors were synthesized with 20-30 Ω and 19% magnetoresistance. Compared to conventional lithographically produced magnetic tunnel junction sensors, these structures offer facile fabrication and over 2 orders of magnitude lower resistances due to smooth sidewalls from in situ templated chemical growth.

Molecular cloisonné: multicomponent organic alternating nanostructures at vicinal surfaces with tunable length scales

By careful management of the adsorption preference of organic molecules at faceted vicinal surfaces, organic alternating structures can be extended to multilayers and multicomponent with tunable size scales ranging from several to a few tens nanometers.

ATOMISTIC MODELING OF THE FORMATION AND STABILITY OFNiANDVNANOWIRES ON A STEPPEDRh(553)SUBSTRATE

The formation and stability of Ni and V nanowires on a stepped Rh(553) substrate is investigated using the BFS method for alloys via an atom-by-atom description of the formation process. In agreement with experiment, Ni is found to form one-dimensional wires attached to the surface steps, while V does not. However, a detailed analysis of the energetics reveals that there the apparent differences leave room for the possibility of alloying in the case of Ni and the formation of structures along the step in the case of V .

Magnetic surface nanostructures

Recent trends in the emerging field of surface-supported magnetic nanostructures are reviewed. Current strategies for nanostructure synthesis are summarized, followed by a predominantly theoretical description of magnetic phenomena in surface magnetic structures and a review of experimental research in this field. Emphasis is on Fe- or Co-based nanostructures in various low-dimensional geometries, which are studied as model systems to explore the effects of dimensionality, atomic coordination, chemical bonds, alloying and, most importantly, interactions with the supporting substrate on the magnetism. This review also includes a discussion of closely related systems, such as 3d element impurities integrated into organic networks, surface-supported Fe-based molecular magnets, Kondo systems or 4d element nanostructures that exhibit emergent magnetism, thereby bridging the traditional areas of surface science, molecular physics and nanomagnetism.

Self-assembled growth of Au islands on a Mo(110) surface

The self-assembled growth of epitaxial Au(111) islands on a Mo(110) buffer layer has been investigated as a function of growth temperature and amount of deposited material by reflection high energy electron diffraction and atomic force microscopy. At the growth temperature of 385 degrees C the dendrite-shaped islands coexist with the compact ones. The uniform islands formed at 500 degrees C adopt mostly a shape of truncated pyramids with a well developed (111) top plane and {111} and {100} side facets. As the growth temperature reaches 800 degrees C the Au islands take less regular shapes due to occurrence of coalescence. The averaged area and height of the islands increase with the deposition temperature and the amount of deposited material. The surface density of the islands decreases with increasing temperature. The epitaxial relations at the interface between the Au islands and the Mo buffer determined from the angular dependence of the electron diffraction pattern favour the Nishiyama-Wassermann growth mode. Factors responsible for the island-like growth and possible mechanisms of diffusion are discussed in details.

Au111-based nanotemplates by gd alloying

A new class of nanostructured templates is obtained by submitting Au111 films to high-temperature vapor deposition of Gd in ultrahigh vacuum. In a low coverage regime, Gd atoms are embedded in the topmost Au layer, inducing a structural transformation of the herringbone reconstruction to create a network of trigons. At higher dose, the reactive deposition of Gd leads to the formation of an atomically perfect GdAu2 surface compound characterized by a long-range periodic Moire pattern. Both the trigon and Moire lattices are highly ordered nanostructures, which turned out to be robust templates to grow metal nanodots. As a test example, Co was deposited at room temperature, forming uniform dots that faithfully arrange by following the underlying trigons or Moire periodicity. For the latter, one can achieve nanodot arrays that exhibit record areal density.

Dominant role of the epitaxial strain in the magnetism of core-shell Co/Au self-organized nanodots

Self-organized Co nanodots on a Au(111) surface have been surrounded by controlled Au rings that progressively cap the entire dots. The magnetic susceptibility of these dots has been measured in situ as a function of the Au coverage. The blocking temperature increases when the Co bilayer dots are surrounded by the first Au atomic layer and decreases with the subsequent capping. This result cannot be explained by interfacial anisotropy which is generally assumed to be the dominant term in the magnetic anisotropy of nanostructures. Using molecular dynamics simulations, we evidence that the large strain inside the Co clusters is the main driving force for the anisotropy changes during the Au encapsulation.

High resolution in situ magneto-optic Kerr effect and scanning tunneling microscopy setup with all optical components in UHV

A surface magneto-optic Kerr effect (MOKE) setup fully integrated in an ultrahigh vacuum chamber is presented. The system has been designed to combine in situ MOKE and scanning tunneling microscopy. Magnetic fields up to 0.3 T can be applied at any angle in the transverse plane allowing the study of in-plane and out-of-plane magnetization. The setup performance is demonstrated for a continuous film of 0.9 monolayers (ML) Co/Rh(111) with in-plane easy axis and for a superlattice of nanometric double layer Co islands on Au(11,12,12) with out-of-plane easy axis. For Co/Au(11,12,12) we demonstrate that the magnetic anisotropy energies deduced from thermally induced magnetization reversal and from applying a torque onto the magnetization by turning the field are the same. For the presented setup we establish a coverage detection limit of 0.5 ML for transverse and 0.1 ML for polar MOKE. For island superlattices with the density of Co/Au(11,12,12), the latter limit corresponds to islands composed of about 50 atoms. The detection limit can be further reduced when optimizing the MOKE setup for either one of the two Kerr configurations.

Quantitative modelling of the surface plasmon resonances of metal nanoclusters sandwiched between dielectric layers: the influence of nanocluster size, shape and organization

The effects of size, shape and organization on the surface plasmon resonances of Ag nanoclusters sandwiched between Si(3)N(4) layers are studied by transmission electron microscopy and anisotropic spectroscopic ellipsometry. We present an easy-to-handle model that quantitatively links the nanostructure and optical response of the films, which are considered as dielectric/metal:dielectric/dielectric trilayers, with the central nanocomposite layer being an effective medium whose optical properties are described by an anisotropic dielectric tensor. The components of this tensor are calculated using a generalization of the Yamaguchi theory taking into account the real organization, size and shape distributions of ellipsoidal nanoclusters, whose electronic properties are assumed to reflect shape-dependent finite size effects. Using this model, it is shown that the optical response of the films in the visible range is dominated by the excitation of the surface plasmon resonance of the clusters along their in-plane long axis, while no surface plasmon resonance resulting from an excitation along their in-plane short axis can be observed due to damping effects. Moreover, the spectral position of this resonance appears to be mainly affected by the average shape of the clusters, and weakly by their size, their shape distribution and the electromagnetic interaction between them.

Role of hydrogen in giant spin polarization observed on magnetic nanostructures

We demonstrate that the giant spin contrast observed by scanning tunneling microscopy for double-layer Coislands on Pt(111) is caused by adsorbates at the apex of the Cr-coated W tip. The most likely candidate, in ab initio simulations, is hydrogen. Here, the electron charge is highly polarized by the adjacent Cr layers. The hydrogen adsorption site is shown to change from hollow to on top due to the electric field at the tip apex, created by the tunnel voltage.

Magnon coherent conductance via atomic nanocontacts

A calculation for the coherent scattering and conductance of magnons via atomic nanocontacts is presented. The model system is composed of two groups of semi-infinite magnetically ordered Heisenberg monatomic chains, joined together by the magnetic nanocontact, and the system is supported on a non-magnetic substrate and considered otherwise free from magnetic interactions. The coherent transmission and reflection coefficients are derived as elements of a Landauer-type scattering matrix. Transmission and reflection scattering cross sections are calculated specifically for three distinct symmetric and asymmetric geometric configurations of the nanocontact. Three cases of local magnetic exchange on the nanocontact domain are analysed for each configuration to investigate the influence of softening and hardening of the magnetic boundary conditions. In analogy with coherent electronic transport, we calculate the magnon coherent transport. The numerical results show the interference effects between the incident scattered magnons and the localized spin states on the nanocontact, with characteristic Fano resonances. The numerical results yield an understanding of the relationship between the coherent magnon conductance and the architecture of the embedded magnetic nanocontact.

Two-dimensional Ir cluster lattice on a graphene moiré on Ir(111)

Lattices of Ir clusters have been grown by vapor phase deposition on graphene moirés on Ir(111). The clusters are highly ordered, and spatially and thermally stable below 500 K. Their narrow size distribution is tunable from 4 to about 130 atoms. A model for cluster binding to the graphene is presented based on scanning tunneling microscopy and density functional theory. The proposed binding mechanism suggests that similar cluster lattices might be grown of materials other than Ir.

Chemical reactivity of Ni-Rh nanowires

The properties of bimetallic Ni-Rh nanowires, fabricated by decorating the steps of vicinal Rh(111) surfaces by stripes of self-assembled Ni adatoms, have been probed by STM, photoemission, and ab initio density functional theory calculations. These Ni-Rh nanowires have specific electronic properties that lead to a significantly enhanced chemical reactivity towards oxygen. As a result, the Ni-Rh nanowires can be oxidized exclusively, generating novel quasi-one-dimensional oxide structures.