Structural Instability of Ferromagnetic fcc Fe Films on Cu(100)

Unusual strain effect of a Pt-based L10 face-centered tetragonal core in core/shell nanoparticles for the oxygen reduction reaction

Nanoparticles with a low-Pt content core and a few-layer thick Pt skin are attractive catalysts toward the oxygen reduction reaction (ORR) not only for their low cost, but also because their activity can be enhanced by judiciously choosing the core alloy.

Realizing high magnetic moments in fcc Fe nanoparticles through atomic structure stretch

We describe the realization of a high moment state in fcc Fe nanoparticles through a controlled change in their atomic structure. Embedding Fe nanoparticles in a Cu(1-x)Au(x) matrix causes their atomic structure to switch from bcc to fcc. Extended x-ray absorption fine structure (EXAFS) measurements show that the structure in both the matrix and the Fe nanoparticles expands as the amount of Au in the matrix is increased, with the data indicating a tetragonal stretch in the Fe nanoparticles. The samples were prepared directly from the gas phase by co-deposition, using a gas aggregation source and MBE-type sources respectively for the nanoparticle and matrix materials. The structure change in the Fe nanoparticles is accompanied by a sharp increase in atomic magnetic moment, ultimately to values of ~2.5 ± 0.3 μ(B)/atom .

Strong metal adatom-substrate interaction of Gd and Fe with graphene

Graphene is a unique 2D system of confined electrons with an unusual electronic structure of two inverted Dirac cones touching at a single point, with high electron mobility and promising microelectronics applications. The clean system has been studied extensively, but metal adsorption studies in controlled experiments have been limited; such experiments are important to grow uniform metallic films, metal contacts, carrier doping, etc. Two non-free-electron-like metals (rare earth Gd and transition metal Fe) were grown epitaxially on graphene as a function of temperature T and coverage θ. By measuring the nucleated island density and its variation with growth conditions, information about the metal-graphene interaction (terrace diffusion, detachment energy) is extracted. The nucleated island densities at room temperature (RT) are stable and do not coarsen, at least up to 400 °C, which shows an unusually strong metal-graphene bond; most likely it is a result of C atom rebonding from the pure graphene sp(2) C-C configuration to one of lower energy.

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.

Extended x-ray absorption fine structure studies of the atomic structure of nanoparticles in different metallic matrices

It has been appreciated for some time that the novel properties of particles in the size range 1-10 nm are potentially exploitable in a range of applications. In order to ultimately produce commercial devices containing nanosized particles, it is necessary to develop controllable means of incorporating them into macroscopic samples. One way of doing this is to embed the nanoparticles in a matrix of a different material, by co-deposition for example, to form a nanocomposite film. The atomic structure of the embedded particles can be strongly influenced by the matrix. Since some of the key properties of materials, including magnetism, strongly depend on atomic structure, the ability to determine atomic structure in embedded nanoparticles is very important. This review focuses on nanoparticles, in particular magnetic nanoparticles, embedded in different metal matrices. Extended x-ray absorption fine structure (EXAFS) provides an excellent means of probing atomic structure in nanocomposite materials, and an overview of this technique is given. Its application in probing catalytic metal clusters is described briefly, before giving an account of the use of EXAFS in determining atomic structure in magnetic nanocomposite films. In particular, we focus on cluster-assembled films comprised of Fe and Co nanosized particles embedded in various metal matrices, and show how the crystal structure of the particles can be changed by appropriate choice of the matrix material. The work discussed here demonstrates that combining the results of structural and magnetic measurements, as well as theoretical calculations, can play a significant part in tailoring the properties of new magnetic cluster-assembled materials.

The role of surface stress in structural transitions, epitaxial growth and magnetism on the nanoscale

We review results on combined stress measurements by the crystal curvature technique and structural investigations on different nanoscale systems. It is shown that stress measurements offer highly sensitive and accurate data which identify even subtle structural changes in the sub-monolayer coverage regime. We discuss the unique potential of stress measurements to complement structural investigations of atomic layers and at surfaces. Our examples reveal that stress measurements enhance, support and clarify the interpretation of quantitative structural data. The role of surface stress and film stress for structural transitions in epitaxial growth, surfactant-mediated growth, surface reconstruction and adsorbate-induced spin reorientation transitions in monolayers is discussed.

Ferromagnetic Fe on Cu(001) throughout the fcc-like phase: arguing from the viewpoint of the electronic structure

The scientific enthusiasm for ultrathin Fe films on Cu(001) has now lasted for more than 20 years. Is there ferromagnetic iron with a face-centred cubic (fcc) structure? Does ferromagnetism in Fe hinge on the body-centred cubic (bcc) structure? In this contribution, we try to establish that the electron system gives evidence of ferromagnetic behaviour with fcc-like electronic bands. We examine a crystal-induced surface state, which is characteristic of fcc surface order. Furthermore, we compare electronic signatures of fcc and bcc: the d-band exchange splitting, image-potential-state energies and the work function. We conclude that, from the viewpoint of the electronic structure, Fe on Cu(001) is found to be ferromagnetic throughout the fcc-like phase. This result raises a new question: how much deviation from the relaxed fcc order is acceptable without losing the electronic signature of fcc?

Rheology of colloidal microphases in a model with competing interactions

We study the rheological properties of colloidal microphases in two dimensions simulating a model of colloidal particles with competing interactions. Due to the competition between short-range attraction and long-range repulsion, as a function of the density the model exhibits a variety of microphases such as clusters, stripes, or crystals with bubbles. We prepare the system in a confined microphase employing Monte Carlo simulations and then shear the resulting configurations by applying a drag force profile. We integrate numerically the equation of motion for the particles and analyze the dynamics as a function of the density and the applied strain rate. We measure the stress-strain curves and characterize the yielding of the colloidal microphases. The results depend on the type of microphase. (i) Clusters are easily sheared along layers and the relative motion is assisted by rotations. (ii) Stripes shear easily when they are parallel to the flow and tend to jam when they are perpendicular to it. Under a sufficiently strong shear rate perpendicular stripes orient in the flow direction. (iii) Crystals with bubbles yield by fracturing along the bubbles and eventually forming stripes. We discuss the role of dislocations, emitted by the bubbles, in the yielding process. Finally, we analyze the effect of thermal fluctuations on the rheological properties.

"Live" surface ferromagnetism in Fe nanodots/Cu multilayers on Cu(111)

We investigate the crossover behavior from two-dimensional (2D) to three-dimensional in multilayers of magnetic nanodots grown by stacking 2D Fe nanodot assemblies on Cu(111) single crystal substrate with a Cu spacing layer. Using an in situ magneto-optical Kerr effect, we have observed a striking ferromagnetic to spin-glass-like phase transition with an increasing number of Fe dot layers. The topmost layer of the Fe dots survives the phase transition and remains ferromagnetic. This unusual surface ferromagnetism is likely caused by a surface-state-mediated coupling which is stronger than the coupling in bulk layers. This is confirmed by the fact that the critical temperature of the surface ferromagnetism is considerably higher than that of the bulk spin-glass phase in the system.

Surface structure of ultrathin Fe films on Cu(001) revisited

The structure and magnetism of ultrathin Fe films epitaxially grown on a Cu(001) surface are investigated by grazing scattering of fast H and He atoms or ions. By making use of a new variant of ion beam triangulation based on the detection of the number of emitted electrons, we obtain direct information on the structure of the film surface. We observe for room temperature growth a dominant and defined fcc-like structure. Complex surface reconstructions as reported in recent STM and LEED studies are observed only for cooling and H2 dosing.

Vanishing magnetic interactions in ferromagnetic thin films

We have used element-specific hysteresis measurements, based on the x-ray magnetic circular dichroism technique, to investigate magnetic trilayer structures composed of Fe and Ni layers. Within a critical regime we have discovered a class of structures in which the exchange interaction, the mechanism responsible for the macroscopic magnetism, can become vanishingly small. The experimental observations are supported by first principles theory and are explained as arising from a cancellation of several competing magnetic interactions. Hence, we have discovered a system with a novel exchange interaction between magnetic layers in direct contact that replaces the conventional exchange interaction in ferromagnets.

Temperature dependence of the surface anisotropy of Fe ultrathin films on Cu(001)

We report an experimental approach to separate temperature dependent reversible and irreversible contributions to the perpendicular magnetic anisotropy of Fe films grown at low temperatures on Cu(001) substrates. The surface anisotropy K(S)(T) is found to decrease linearly with temperature, causing a thermally induced spin reorientation into the plane. The irreversible shift of the spin reorientation transition and the coercivity of the iron films are directly correlated to the increasing Fe island size during annealing. The increased coercivity is discussed in terms of domain wall energy inhomogeneities provided by the islands.

Computer simulations of a two-dimensional system with competing interactions

The results and methodology of large scale computer simulations of the two-dimensional dipolar Ising model with long-range interactions are reported. Systems as large as 117,649 particles were studied to elucidate the elementary excitations and phase diagram of two-dimensional systems, such as Langmuir monolayers, thin garnet films, and adsorbed films on solid surfaces, which spontaneously form patterns of stripes, bubbles, and intermediately shaped domains. The challenging numerical investigations of large scale systems with long-range interactions at low temperatures were made possible by combining the fast multipole method and a non-Metropolis Monte Carlo sampling technique. Our simulations provide evidence that, at sufficiently high ratios of the repulsive to the attractive coupling constant for the model, twofold stripe order in the systems of interest is lost through a defect-mediated mechanism. Heat capacity data and the excitations observed in our simulations as the system disorders indicate that it is most likely an instance of a Kosterlitz-Thouless phase transition. The results from simulations with and without external field are in excellent agreement with the predictions of an analytic scaling theory [A. D. Stoycheva and S. J. Singer, Phys. Rev. E 64, 016118 (2001)], confirming the phase diagram furnished by the analytic model. The scaling theory suggests that, under certain conditions, defect-mediated stripe melting may be supplanted by Ising like disordering within stripes for small repulsion strength. A qualitative discussion of a model that supports both disordering mechanisms is presented.

Shear instability of gamma-Fe in bulk and in ultrathin films

Using ab initio local-spin-density calculations we demonstrate that along the Bain path describing the transformation of face-centered-cubic (fcc) gamma-Fe into body-centered-cubic (bcc) alpha-Fe, tetragonal Fe is unstable against monoclinic shear deformations producing a nearly bcc structure. In the limit of a monolayer adsorbed on a fcc substrate, the epitaxial constraint suppresses the shear instability, but in ultrathin films with three to six monolayers a striped pattern of near-bcc domains develops, confirming recent observations by scanning tunneling microscopy. A strong correlation between the shear instability and the magnetic state is reported.

Crystallographic structure of ultrathin Fe films on Cu(100)

We report the observation of bcc-like crystal structures in 2-4 monolayer (ML) Fe films grown on fcc Cu(100) using scanning tunneling microscopy. The local bcc structure provides a straightforward explanation for their frequently reported outstanding magnetic properties, i.e., ferromagnetic ordering in all layers with a Curie temperature above 300 K. The nonpseudomorphic structure, which becomes pseudomorphic above 4 ML film thickness, is unexpected in terms of conventional rules of thin film growth and stresses the importance of finite thickness effects in ferromagnetic ultrathin films.

Scaling theory for two-dimensional systems with competing interactions

We derive an analytic scaling theory for a two-dimensional system in which spontaneous patterns of stripes, bubbles, and intermediately shaped domains arise due to the competition of short-range attractions and long-range dipolar repulsions. The theory predicts temperature and domain-size scaling as a function of the relative repulsion strength eta, the ratio of the repulsive to the attractive coupling constant in the system's Hamiltonian. As eta decreases, the domain size explodes exponentially and the melting temperature for a system of ordered stripes increases. Our findings shed new light on the phase diagram and critical excitations for the dipolar Ising ferromagnet or lattice gas and their continuum analogs. We show that the features described by the scaling theory are insensitive to details like cutoffs for the dipolar interactions and, therefore, should be widely applicable. Our corresponding states analysis explains the experimentally observed stripe melting upon compression in a Langmuir monolayer. A phenomenological extension of the analytic scaling theory describes how the system's behavior is modified in the presence of magnetization or density fluctuations. Fluctuations are found to suppress domain size and the stripe melting temperature. In regimes where fluctuations are important, we predict that domain size will decrease with increasing temperature.

Nucleation of bcc iron in ultrathin fcc films

Needle-shaped bcc nucleation centers in fcc films of Fe on Cu(100) are observed by scanning tunneling microscopy. They form virtually without mass transfer and nearly under conservation of volume, which causes a large strain within the nascent bcc grain. The corresponding strain energy almost equals the gain in structural energy, rendering the bcc nucleation very sensitive to any effect influencing this subtle balance. We suggest that modifying the film by straining, alloying, or surface adsorption may inhibit the bcc nucleation and lead to thick metastable fcc films.

Theoretical calculation of magnetic properties of ultrathin Fe films on Cu(100)

The temperature dependence of the magnetization in fcc Fe on Cu(100) is calculated using a self-consistent local mean-field theory. The model reproduces an experimental magnetization oscillation as a function of film thickness and supports a picture where the top two layers are ferromagnetically coupled, and the remaining layers are antiferromagnetically coupled. The origin of the puzzling linear temperature dependence in oscillation amplitude is understood as a "surface phenomena" of the antiferromagnetic layer at the Fe/Cu interface. Proximity effects between a thin antiferromagnet with a low Neel temperature and a neighboring ferromagnet with a higher Curie temperature are discussed.