Substrate-Controlled Magnetism: Fe Nanowires on Vicinal Cu Surfaces.

Here we present a novel approach to controlling magnetic interactions between atomic-scale nanowires. Our ab initio calculations demonstrate the possibility to tune magnetic properties of Fe nanowires formed on vicinal Cu surfaces. Both intrawire and interwire magnetic exchange parameters are extracted from density functional theory (DFT) calculations. This study suggests that the effective interwire magnetic exchange parameters exhibit Ruderman-Kittel-Kasuya-Yosida-like (RKKY) oscillations as a function of Fe interwire separation. The choice of the vicinal Cu surface offers possibilities for controlling the magnetic coupling. Furthermore, an anisotropic Heisenberg model was used in Monte Carlo simulations to examine the stability of these magnetic configurations at finite temperatures. The predicted critical temperatures of the Fe nanowires on Cu(422) and Cu(533) surfaces are well above room temperature.

Engineering magnetic anisotropy and magnetization switching in multilayers by strain.

The effect of the strain on the magnetic properties of metallic multilayers has been investigated by ab initio studies. Our results indicate that the magnetic anisotropy energy (MAE) of an Fe(001) surface can be drastically enhanced by capping with 5d elements. By choosing Ir-Fe multilayers as a model system, we demonstrate that the MAE which depends on the composition and the structure of the multilayers can be tuned in a large range by strain. Furthermore, our results show that not only the amplitude of the MAE but also the easy axis of Pt-Fe multilayers can be engineered by strain. Magnetization switching by strain is also investigated.

Electric-field-modulated exchange coupling within and between magnetic clusters on metal surfaces: Mn dimers on Cu(1 1 1).

The effects of external electric fields (EFs) on the magnetic state and substrate-mediated magnetic coupling between Mn dimers on Cu(1 1 1) have been studied using a first-principles theoretical method. The calculations show that a change in the ground-state magnetic order, from antiferromagnetic (AF) to ferromagnetic (FM), can be induced within an isolated Mn2 on Cu(1 1 1) by applying a moderately strong EF of about 1 V Å(-1). The magnetic exchange coupling between pairs of dimers displays Ruderman-Kittel-Kasuya-Yosida-like oscillations as a function of the interdimer distance, which depend significantly on the magnetic order within the dimers (FM or AF) and on their relative orientation on the surface. Moreover, it is observed that applying EFs allows modulation of the exchange coupling within and between the clusters as a function of the intercluster distance. At short distances, AF order within the dimers is favoured even in the presence of EFs, while for large distances the EF can induce a FM order. EFs pointing outwards and inwards with respect to the surface favour parallel and antiparallel magnetic alignment between the dimers, resspectively. The dependence of the substrate-mediated interaction on the magnetic state of Mn2 is qualitatively interpreted in terms of the differences in the scattering of spin-polarized surface electrons.

Tuning magnetic anisotropy in metallic multilayers by surface charging: an ab initio study.

Our ab initio studies show clear evidence that magnetic anisotropy (MA) and the direction of magnetization in metallic magnetic multilayers can be tailored at once by surface charging. By taking Fe-Pt multilayers as a representative example, we demonstrate that surface charging has a deep effect on the magnitude of the MA, which is composition dependent, achieving remarkably large values for systems featuring a single Fe layer capped with Pt. More intriguing is the behavior of the multilayers capped with iron bilayers, for which surface charging not only affects the value of the anisotropy but an easy-axis switching is also revealed. By analyzing the electronic structure of the magnetic layers and relating the MA to the orbital moment anisotropy, some insights about the origin of the MA from a local perspective can be inferred.

Misfit-induced modification of structure and magnetism in O/Fe(001)-p(1×1).

The geometry of oxygen atoms in hollow sites of Fe nanoislands (⊘≈1-2 nm) on Fe(001) is modified by mesoscopic misfit-induced relaxations of the island atoms. Surface x-ray diffraction, scanning tunneling microscopy, and ab initio calculations indicate a 0.3 Å increased adsorption height [0.7 Å versus 0.4 Å in O/Fe(001)-p(1×1)] of O atoms going in parallel with a reduced Fe-Fe layer spacing inducing a reduction of the surface magnetic moment (2.85µ(B) versus 3.2µ(B)). Our results demonstrate the importance of the mesoscopic misfit for surface physical properties in general.

Elementary excitations at magnetic surfaces and their spin dependence.

The elementary surface excitations are studied by spin-polarized electron energy loss spectroscopy on a prototype oxide surface [an oxygen passivated Fe(001)-p(1×1) surface], where the various excitations coexist. For the first time, the surface phonons and magnons are measured simultaneously and are distinguished based on their different spin nature. The dispersion relation of all excitations is probed over the entire Brillouin zone. The different phonon modes observed in our experiment are described by means of ab initio calculations.

Electric field as a switching tool for magnetic states in atomic-scale nanostructures.

One of the most promising candidates for the construction of ultrahigh-density storage media is low-dimensional atomic-scale magnetic nanostructures exhibiting magnetic bi- or multistability. Here we propose a novel route of locally controlling and switching magnetism in such nanostructures. Our ab initio studies reveal that externally applied electric field can be used for this purpose.

Spin-dependent quantum interference within a single magnetic nanostructure.

Quantum interference is a coherent quantum phenomenon that takes place in confined geometries. Using spin-polarized scanning tunneling microscopy, we found that quantum interference of electrons causes spatial modulation of spin polarization within a single magnetic nanostructure. We observed changes in both the sign and magnitude of the spin polarization on a subnanometer scale. A comparison of our experimental results with ab initio calculations shows that at a given energy, the modulation of the spin polarization can be ascribed to the difference between the spatially modulated local density of states of the majority spin and the nonmodulated minority spin contribution.

Switching a single spin on metal surfaces by a STM Tip: Ab Initio studies.

The exchange coupling between single 3d magnetic adatoms (Cr, Mn, Fe, and Co) adsorbed on a Cu(001) surface and a Cr STM tip is studied with ab initio calculations. We demonstrate that the spin direction of single adatoms can be controlled by varying the tip-substrate distance, and the sign of the exchange energy is determined by the competition of the direct and the indirect interactions between the tip and the adatom. Based on the spin-dependent transport calculations, we find a magnetoresistance of about 70% at short tip-substrate distances.

Melting of two-dimensional adatom superlattices stabilized by long-range electronic interactions.

The melting transition of Ce adatom superlattices stabilized by long-range substrate-mediated electronic interactions on Cu(111) and Ag(111) noble metal surfaces has been investigated by low-temperature scanning tunneling microscopy, density functional theory calculations, and kinetic Monte Carlo simulations. Intriguingly, owing to the interaction between Ce adatoms and substrate, these superlattices undergo two-dimensional melting to a liquid without transition through the hexatic phase. The crucial parameters for this direct solid to liquid transition are identified.

Strain relief guided growth of atomic nanowires in a Cu3N-Cu(110) molecular network.

A self-corrugated Cu(3)N-Cu(110) molecular network shows the potential to overcome the element dependence barrier as demonstrated by epitaxial growth of atomic nanowires (approximately 1 nm in width) among various 3d, 4d, and 5d elements. Scanning tunneling microscopy shows that all of the investigated atomic nanowires share an identical structure, featuring uniform width, height, orientation and the same minimum separation distance. Ab initio study reveals that the formation mechanism of atomic nanowires can be directly attributed to a strain relief guided asymmetric occupation of atoms on the originally symmetric crest zone of the corrugated network.

Direct evidence for the effect of quantum confinement of surface-state electrons on atomic diffusion.

We report on the direct observations of the effect of quantum confinement of surface-state electrons on atomic diffusion. Confined electronic states induced by open nanoscale resonators [consisting of two parallel monatomic Cu chains on Cu(111)] are studied by means of scanning tunneling microscope measurements and first-principles calculations. Strongly anisotropic diffusion of adatoms around and inside resonators is revealed at low temperatures. The formation of diffusion channels and empty zones is demonstrated. We show that it is possible to engineer atomic diffusion by varying the distance between the resonator walls.

Tailoring exchange interactions in engineered nanostructures: an ab initio study.

We present a novel approach to spin manipulation in atomic-scale nanostructures. Our ab initio calculations clearly demonstrate that it is possible to tune magnetic properties of subnanometer structures by adjusting the geometry of the system. By the example of two surface-based systems we demonstrate the following. (i) The magnetic moment of a single adatom coupled to a buried magnetic Co layer can be stabilized in either a ferromagnetic or an antiferromagnetic configuration depending on the spacer thickness. It is found that a buried Co layer has a profound effect on the exchange interaction between two magnetic impurities on the surface. (ii) The exchange interaction between magnetic adatoms can be manipulated by introducing artificial nonmagnetic Cu chains to link them.

Buried Ni/Cu(001) interface at the atomic scale.

We present a quantitative surface x-ray analysis of the buried Ni/Cu(001) interface structure after deposition of 3 and 5 monolayers of Ni at room temperature. Interface mixing is found where 27+/-10% of top layer Cu atoms are exchanged by Ni. Atomic scale simulations reveal a kinetic pathway for the Ni/Cu-exchange process and explain the observed limited degree of intermixing. A disperse distribution of Ni within the Cu surface with a preferential Ni-Ni separation of 3-4 nearest neighbor distances is determined.

Direct evidence for mesoscopic relaxations in cobalt nanoislands on Cu(001).

Surface x-ray diffraction in combination with scanning tunneling microscopy and molecular dynamics calculations provide first quantitative evidence for unusually large relaxations in nanometer-sized Co islands deposited on Cu(001) at 170 K. These lead to sharply reduced interatomic Co distances as low as 2.36 A as compared to bulk Co (2.51 A) involving low symmetry Co adsorption sites. Our results prove the validity of the concept of mesoscopic mismatch which governs the strain relaxation of nanosized islands in general.

Exchange interaction between single magnetic adatoms.

The magnetic coupling between single Co atoms adsorbed on a copper surface is determined by probing the Kondo resonance using low-temperature scanning tunneling spectroscopy. The Kondo resonance, which is due to magnetic correlation effects between the spin of a magnetic adatom and the conduction electrons of the substrate, is modified in a characteristic way by the coupling of the neighboring adatom spins. Increasing the interatomic distance of a Cobalt dimer from 2.56 to 8.1 A we follow the oscillatory transition from ferromagnetic to antiferromagnetic coupling. Adding a third atom to the antiferromagnetically coupled dimer results in the formation of a collective correlated state.

Size-dependent surface states of strained cobalt nanoislands on Cu(111).

Low-temperature scanning tunneling spectroscopy over Co nanoislands on Cu(111) showed that the surface states of the islands vary with their size. Occupied states exhibit a sizable downward energy shift as the island size decreases. The position of the occupied states also significantly changes across the islands. Atomic-scale simulations and ab initio calculations demonstrate that the driving force for the observed shift is related to size-dependent mesoscopic relaxations in the nanoislands.

Adatom self-organization induced by quantum confinement of surface electrons.

We present a novel mechanism of nanostructure growth based on quantum confinement of surface-state electrons. Ab initio calculations and the kinetic Monte Carlo simulations reveal the phenomenon of confinement-induced adatom self-organization in quantum corrals. Our studies indicate that new atomic-scale nanostructures can be engineered exploiting the quantum confinement of surface electrons.

Controlling the spin polarization of nanostructures on magnetic substrates.

It is shown that, by utilizing spin-selective quantum interference, the spin polarization of nanostructures deposited on a magnetic substrate with a surface state can be strongly modulated locally and energetically by an appropriate structural design. This finding is deduced from state-of-the-art ab initio calculations and interpreted within an analytical model. We present results for hexagonal Cu corrals and mesoscopic triangular Co islands on Co-covered Cu(111). These systems are experimentally feasible, and the effect should be detectable with current technology.

Interaction of scanning tunneling microscopy tip with mesoscopic islands at the atomic-scale.

Performing atomic-scale simulations, we study the interaction of the scanning tunneling microscopy (STM) tip with mesoscopic islands at zero bias voltage. Our calculations reveal tip-induced shape transitions in Co islands on Cu(100) as the tip approaches the surface. The structure of the islands and of the tip are found to depend strongly on the tip-substrate distance. A significant influence of the tip on atomic diffusion on the top and at the edges of the islands is demonstrated. The size-dependent strain relief in the islands caused by the tip and by the substrate is found to play a key role in atomistic processes on islands. Our results show that, for certain tip-surface separations, the hopping diffusion of Co adatoms on the top of Co islands and the upward mass transport at the edge of the islands can be strongly enhanced. Our findings point out the possibility of manipulating atomic motion on mesoscopic islands using the STM tip.