Layer resolved structural relaxation at the surface of magnetic FePt icosahedral nanoparticles

Atomic-Scale Distribution and Evolution of Strain in Pt Nanoparticles Grown on MoS2 Nanosheet

Interface strain significantly affects the band structure and electronic states of metal-nanocrystal-2D-semiconductor heterostructures, impacting system performance. While transmission electron microscopy (TEM) is a powerful tool for studying interface strain, its accuracy may be compromised by sample overlap in high-resolution images due to the unique nature of the metal-nanocrystals-2D-semiconductors heterostructure. Utilizing digital dark-field technology, the substrate influence on metal atomic column contrasts is eliminated, improving the accuracy of quantitative analysis in high-resolution TEM images. Applying this method to investigate Pt on MoS2 surfaces reveals that the heterostructure introduces a tensile strain of ≈3% in Pt nanocrystal. The x-directional linear strain in Pt nanocrystals has a periodic distribution that matches the semi-coherent interface between Pt nanocrystals and MoS2, while the remaining strain components localize mainly on edge atomic steps. These results demonstrate an accurate and efficient method for studying interface strain and provide a theoretical foundation for precise heterostructure fabrication.

Chiral Magnetic Interactions in Small Fe Clusters Triggered by Symmetry-Breaking Adatoms

The chirality of the interaction between the local magnetic moments in small transition-metal alloy clusters is investigated in the framework of density-functional theory. The Dzyaloshinskii–Moriya (DM) coupling vectors Dij between the Fe atoms in Fe2X and Fe3X with X = Cu, Pd, Pt, and Ir are derived from independent ground-state energy calculations for different noncollinear orientations of the local magnetic moments. The local-environment dependence of Dij and the resulting relative stability of different chiral magnetic orders are analyzed by contrasting the results for different adatoms X and by systematically varying the distance between the adatom X and the Fe clusters. One observes that the adatoms trigger most significant DM couplings in Fe2X, often in the range of 10–30 meV. Thus, the consequences of breaking the inversion symmetry of the Fe dimer are quantified. Comparison between the symmetric and antisymmetric Fe-Fe couplings shows that the DM couplings are about two orders of magnitude weaker than the isotropic Heisenberg interactions. However, they are in general stronger than the anisotropy of the symmetric couplings. In Fe3X, alloying induces interesting changes in both the direction and strength of the DM couplings, which are the consequence of breaking the reflection symmetry of the Fe trimer and which depend significantly on the adatom-trimer distance. A local analysis of the chirality of the electronic energy shows that the DM interactions are dominated by the spin-orbit coupling at the adatoms and that the contribution of the Fe atoms is small but not negligible.

Entropically engineered formation of fivefold and icosahedral twinned clusters of colloidal shapes

Fivefold and icosahedral symmetries induced by multiply twinned crystal structures have been studied extensively for their role in influencing the shape of synthetic nanoparticles, and solution chemistry or geometric confinement are widely considered to be essential. Here we report the purely entropy-driven formation of fivefold and icosahedral twinned clusters of particles in molecular simulation without geometric confinement or chemistry. Hard truncated tetrahedra self-assemble into cubic or hexagonal diamond colloidal crystals depending on the amount of edge and vertex truncation. By engineering particle shape to achieve a negligible entropy difference between the two diamond phases, we show that the formation of the multiply twinned clusters is easily induced. The twinned clusters are entropically stabilized within a dense fluid by a strong fluid-crystal interfacial tension arising from strong entropic bonding. Our findings provide a strategy for engineering twinning behavior in colloidal systems with and without explicit bonding elements between particles.

Atomic-Scale Structure Dynamics of Nanocrystals Revealed By In Situ and Environmental Transmission Electron Microscopy

Nanocrystals are of great importance in material sciences and industry. Engineering nanocrystals with desired structures and properties is no doubt one of the most important challenges in the field, which requires deep insight into atomic-scale dynamics of nanocrystals during the process. The rapid developments of in situ transmission electron microscopy (TEM), especially environmental TEM, reveal insights into nanocrystals to digest. According to the considerable progress based on in situ electron microscopy, a comprehensive review on nanocrystal dynamics from three aspects: nucleation and growth, structure evolution, and dynamics in reaction conditions are given. In the nucleation and growth part, existing nucleation theories and growth pathways are organized based on liquid and gas-solid phases. In the structure evolution part, the focus is on in-depth mechanistic understanding of the evolution, including defects, phase, and disorder/order transitions. In the part of dynamics in reaction conditions, solid-solid and gas-solid interfaces of nanocrystals in atmosphere are discussed and the structure-property relationship is correlated. Even though impressive progress is made, additional efforts are required to develop the integrated and operando TEM methodologies for unveiling nanocrystal dynamics with high spatial, energy, and temporal resolutions.

Kinetic diffusion-controlled synthesis of twinned intermetallic nanocrystals for CO-resistant catalysis

Intermetallic catalysts are of immense interest, but how heterometals diffuse and related interface structure remain unclear when there exists a strong metal-support interaction. Here, we developed a kinetic diffusion-controlled method and synthesized intermetallic Pt₂Mo nanocrystals with twin boundaries on mesoporous carbon (Pt₂Mo/C). The formation of small-sized twinned intermetallic nanocrystals is associated with the strong Mo-C interaction-induced slow Mo diffusion and the heterogeneity of alloying, which is revealed by an in situ aberration-corrected transmission electron microscope (TEM) at high temperature. The twinned Pt₂Mo/C constitutes a promising CO-resistant catalyst for highly selective hydrogenation of nitroarenes. Theoretical calculations and environmental TEM suggest that the weakened CO adsorption over Pt sites of Pt₂Mo twin boundaries and their local region endows them with high CO resistance, selectivity, and reusability. The present strategy paves the way for tailoring the interface structure of high-melting point Mo/W-based intermetallic nanocrystals that proved to be important for the industrially viable reactions.

Atom-Resolved Investigation on Dynamic Nucleation and Growth of Platinum Nanocrystals

Understanding the mechanism of nucleation and growth of nanocrystals is crucial for designing and regulating the structure and properties of nanocrystals. However, the process from molecules to nanocrystals remains unclear because of the rapid and complicated dynamics of evolution under reaction conditions. Here, the complete evolution process of solid-phase chloroplatinic acid during the electron beam irradiation triggered reduction and nucleation of platinum nanocrystals is recorded. Aberration-corrected environmental transmission electron microscopy is used for direct visualization of the dynamic evolution from H₂ PtCl₆ to Pt nanocrystals at the atomic scale, including the formation and growth of amorphous clusters, crystallization, and growth of clusters, and the ripening of Pt nanocrystals. At the first two stages, there exists a critical size of ≈2.0 nm, which represents the start of crystallization. Crystallization from the center and density fluctuation are observed in the second stage of the crystallization of a few clusters with a size obviously larger than the critical size. The work provides valuable information to understand the kinetics of the early stage of nanocrystal nucleation and crystallization at atomic scale.

Nanocomposite structure of two-line ferrihydrite powder from total scattering

Ferrihydrite is one of the most important iron-containing minerals on Earth. Yet determination of its atomic-scale structure has been frustrated by its intrinsically poor crystallinity. The key difficulty is that physically-different models can appear consistent with the same experimental data. Using X-ray total scattering and a nancomposite reverse Monte Carlo approach, we evaluate the two principal contending models-one a multi-phase system without tetrahedral iron(III), and the other a single phase with tetrahedral iron(III). Our methodology is unique in considering explicitly the complex nanocomposite structure the material adopts: namely, crystalline domains embedded in a poorly-ordered matrix. The multi-phase model requires unphysical structural rearrangements to fit the data, whereas the single-phase model accounts for the data straightforwardly. Hence the latter provides the more accurate description of the short- and intermediate-range order of ferrihydrite. We discuss how this approach might allow experiment-driven (in)validation of complex models for important nanostructured phases beyond ferrihydrite.

Understanding the formation of multiply twinned structure in decahedral intermetallic nanoparticles

The structure of monometallic decahedral multiply twinned nanoparticles (MTPs) has been extensively studied, whereas less is known about intermetallic MTPs, especially the mechanism of formation of multiply twinned structures, which remains to be understood. Here, by using aberration-corrected scanning transmission electron microscopy, a detailed structural study of AuCu decahedral intermetallic MTPs is presented. Surface segregation has been revealed on the atomic level and the multiply twinned structure was studied systematically. Significantly different from Au and Cu, the intermetallic AuCu MTP adopts a solid-angle deficiency of -13.35°, which represents an overlap instead of a gap (+7.35° gap for Au and Cu). By analysing and summarizing the differences and similarities among AuCu and other existing monometallic/intermetallic MTPs, the formation mechanism has been investigated from both energetic and geometric perspectives. Finally, a general framework for decahedral MTPs has been proposed and unknown MTPs could be predicted on this basis.

Intrinsic strain-induced segregation in multiply twinned Cu–Pt icosahedra

We present an atomistic simulation study on the compositional arrangements throughout Cu–Pt icosahedra, with a specific focus on the effects of inherent strain on general segregation trends.

Nanostructure Optimization of Platinum-Based Nanomaterials for Catalytic Applications

Platinum-based nanomaterials have attracted much interest for their promising potentials in fields of energy-related and environmental catalysis. Designing and controlling the surface/interface structure of platinum-based nanomaterials at the atomic scale and understanding the structure-property relationship have great significance for optimizing the performances in practical catalytic applications. In this review, the strategies to obtain platinum-based catalysts with fantastic activity and great stability by composition regulation, shape control, three-dimension structure construction, and anchoring onto supports, are presented in detail. Moreover, the structure-property relationship of platinum-based nanomaterials are also exhibited, and a brief outlook are given on the challenges and possible solutions in future development of platinum-based nanomaterials towards catalytic reactions.

First-principles calculations on Fe-Pt nanoclusters of various morphologies

Bimetallic FePt nanoparticles with L1₀ structure are attracting a lot of attention due to their high magnetocrystalline anisotropy and high coercivity what makes them potential material for storage of ultra-high density magnetic data. FePt nanoclusters are considered also as nanocatalysts for growth of carbon nanotubes of different chiralities. Using the DFT-LCAO CRYSTAL14 code, we have performed large-scale spin-polarized calculations on 19 different polyhedral structures of FePt nanoparticles in order to estimate which icosahedral or hcp-structured morphology is the energetically more preferable. Surface energy calculations of all aforementioned nanoparticles indicate that the global minimum corresponds to the nanocluster possessing the icosahedron “onion-like” structure and Fe₄₃Pt₁₀₄ morphology where the outer layer consists of Pt atoms. The presence of the Pt-enriched layer around FePt core explains high oxidation resistance and environmental stability, both observed experimentally.

First-principles computation of surface segregation in L10 CoPt magnetic nanoparticles

In this study, we have employed the first-principles density functional theory (DFT) computational method to predict the influence of surface segregation on the magnetic properties of small L10 CoPt nanoparticles. For both the modelled cuboidal (with a chemical formula of Co26Pt12) and cuboctahedral (with a chemical formula of Co18Pt20) CoPt nanoparticles, the DFT calculations predict that Pt surface segregation should occur thermodynamically. Associated with this Pt surface segregation, the surface-segregated CoPt magnetic nanoparticles are predicted to have significantly reduced magnetic moments and magnetic anisotropy energies than those of the corresponding bulk-terminated (i.e. non-segregated) nanoparticles. Hence, our study suggests that surface segregation could deteriorate the magnetic properties of CoPt nanoparticles.

Strain Field in Ultrasmall Gold Nanoparticles Supported on Cerium-Based Mixed Oxides. Key Influence of the Support Redox State

Using a method that combines experimental and simulated Aberration-Corrected High Resolution Electron Microscopy images with digital image processing and structure modeling, strain distribution maps within gold nanoparticles relevant to real powder type catalysts, i.e., smaller than 3 nm, and supported on a ceria-based mixed oxide have been determined. The influence of the reduction state of the support and particle size has been examined. In this respect, it has been proven that reduction even at low temperatures induces a much larger compressive strain on the first {111} planes at the interface. This increase in compression fully explains, in accordance with previous DFT calculations, the loss of CO adsorption capacity of the interface area previously reported for Au supported on ceria-based oxides.

Orientation of FePt nanoparticles on top of a-SiO2/Si(001), MgO(001) and sapphire(0001): effect of thermal treatments and influence of substrate and particle size

Texture formation and epitaxy of thin metal films and oriented growth of nanoparticles (NPs) on single crystal supports are of general interest for improved physical and chemical properties especially of anisotropic materials. In the case of FePt, the main focus lies on its highly anisotropic magnetic behavior and its catalytic activity, both due to the chemically ordered face-centered tetragonal (fct) L10 phase. If the c-axis of the tetragonal system can be aligned normal to the substrate plane, perpendicular magnetic recording could be achieved. Here, we study the orientation of FePt NPs and films on a-SiO2/Si(001), i.e., Si(001) with an amorphous (a-) native oxide layer on top, on MgO(001), and on sapphire(0001) substrates. For the NPs of an approximately equiatomic composition, two different sizes were chosen: "small" NPs with diameters in the range of 2-3 nm and "large" ones in the range of 5-8 nm. The 3 nm thick FePt films, deposited by pulsed laser deposition (PLD), served as reference samples. The structural properties were probed in situ, particularly texture formation and epitaxy of the specimens by reflection high-energy electron diffraction (RHEED) and, in case of 3 nm nanoparticles, additionally by high-resolution transmission electron microscopy (HRTEM) after different annealing steps between 200 and 650 °C. The L10 phase is obtained at annealing temperatures above 550 °C for films and 600 °C for nanoparticles in accordance with previous reports. On the amorphous surface of a-SiO2/Si substrates we find no preferential orientation neither for FePt films nor nanoparticles even after annealing at 630 °C. On sapphire(0001) supports, however, FePt nanoparticles exhibit a clearly preferred (111) orientation even in the as-prepared state, which can be slightly improved by annealing at 600-650 °C. This improvement depends on the size of NPs: Only the smaller NPs approach a fully developed (111) orientation. On top of MgO(001) the effect of annealing on particle orientation was found to be strongest. From a random orientation in the as-prepared state observed for both, small and large FePt NPs, annealing at 650 °C for 30 min reorients the small particles towards a cube-on-cube epitaxial orientation with a minor fraction of (111)-oriented particles. In contrast, large FePt NPs keep their as-prepared random orientation even after doubling the annealing period at 650 °C to 60 min.

Nanoparticle shape, thermodynamics and kinetics

Nanoparticles can be beautiful, as in stained glass windows, or they can be ugly as in wear and corrosion debris from implants. We estimate that there will be about 70,000 papers in 2015 with nanoparticles as a keyword, but only one in thirteen uses the nanoparticle shape as an additional keyword and research focus, and only one in two hundred has thermodynamics. Methods for synthesizing nanoparticles have exploded over the last decade, but our understanding of how and why they take their forms has not progressed as fast. This topical review attempts to take a critical snapshot of the current understanding, focusing more on methods to predict than a purely synthetic or descriptive approach. We look at models and themes which are largely independent of the exact synthetic method whether it is deposition, gas-phase condensation, solution based or hydrothermal synthesis. Elements are old dating back to the beginning of the 20th century-some of the pioneering models developed then are still relevant today. Others are newer, a merging of older concepts such as kinetic-Wulff constructions with methods to understand minimum energy shapes for particles with twins. Overall we find that while there are still many unknowns, the broad framework of understanding and predicting the structure of nanoparticles via diverse Wulff constructions, either thermodynamic, local minima or kinetic has been exceedingly successful. However, the field is still developing and there remain many unknowns and new avenues for research, a few of these being suggested towards the end of the review.

Strain-Induced Segregation in Bimetallic Multiply Twinned Particles

We analyze the possibility of strain-induced segregation in bimetallic multiply twinned particles by an analytic first-order expansion within a continuum model. The results indicate that while the change in free energy may be small, there will be a noticeable segregation of larger atoms to the external surface and smaller ones to the core, which could have interesting effects when such nanoparticles are used as heterogeneous catalysts.

Dynamics and stability of icosahedral Fe–Pt nanoparticles

An ab initio theoretical study on icosahedral Fe–Pt clusters – one of the most interesting nanoalloys with high application potential.

Structural evolution of NiAu nanoparticles under ambient conditions directly revealed by atom-resolved imaging combined with DFT simulation

From an economic point of view, the structural stability of noble-transition bimetallic catalysts is as significant as their well-studied catalytic efficiency. The structural evolution and corresponding dynamics of NiAu bimetallic nanoparticles under ambient conditions are investigated using in situ Cs-corrected STEM and DFT calculations. During oxidization, the Au component promotes dissociation of oxygen and initiates Ni oxidization, which simultaneously drives the migration of Au atoms, thus yielding multi-shell structures (denoted by Ni@Au@NiO). The subsequent hydrogen reduction induces surface reconstruction, forming fcc-NiAu clusters. After several cycles of catalyzing CO oxidization, both inverse Au segregation and Ni recrystallization occur, which are ascribed to exothermic excitation. The results of this study can help researchers understand the evolutionary behaviors of the bimetallic nanoparticles under ambient conditions as well as optimize the structural design of bimetallic catalysts.

Cross-sectional analysis of W-cored Ni nanoparticle via focused ion beam milling with impregnation

Tungsten and nickel bimetallic nanoparticle is synthesized by radio frequency thermal plasma process which belongs to the vapor phase condensation technology. The morphology and chemical composition of the synthesized particle were investigated using the conventional nanoparticle transmission electron microscopy (TEM) sample. A few part of them looked like core/shell structured particle, but ambiguities were caused by either TEM sample preparation or TEM analysis. In order to clarify whether a core/shell structure is developed for the particle, various methodologies were tried to prepare a cross-sectional TEM sample. Focused ion beam (FIB) milling was conducted for cold-compacted particles, dispersed particles on silicon wafer, and impregnated particles with epoxy which is compatible with electron beam. A sound cross-sectional sample was just obtained from cyanoacrylate impregnation and FIB milling procedure. A tungsten-cored nickel shell structure was precisely confirmed with aid of cross-sectional sample preparation method.

Near-surface strain in icosahedra of binary metallic alloys: segregational versus intrinsic effects

A systematic structural analysis of FePt, CuAu, and Au icosahedral nanoparticles is presented. The uncovered particles are prepared by inert gas condensation and thermally equilibrated through in-flight optical annealing. Aberration-corrected high-resolution transmission electron microscopy reveals that the crystal lattice is significantly expanded near the particle surface. These experimental findings are corroborated by molecular statics simulations that show that this near-surface strain originates from both intrinsic strain due to the icosahedral structure and a partial segregation of the larger of the two alloy constituents to the particle surface.

Surface lattice-engineered bimetallic nanoparticles and their catalytic properties

When nanoparticles become small (ca. <5 nm), surface stress becomes significant and generates strain that results in a change of surface structures. In this regard, the surface lattice of nanoparticles can be engineered to create strains or other structural changes with atomic positions away from the normal lattice points. Such changes impact the electronic and catalytic properties of nanoparticles. Recently, several groups have reported the change of catalytic and electrocatalytic properties of bimetallic nanoparticles. In this tutorial review, we discuss the principles related to lattice strain and other distorted structures, and the catalytic properties of bimetallic nanostructures.

Room-temperature ferromagnetism in ZnO-encapsulated 1.9 nm FePt3 nanoparticle-composite thin films with giant interfacial anisotropy

As synthesized 1.9-nm FePt3 nanoparticles are superparamagnetic at room temperature. Coating those nanoparticles with ZnO renders them permanently ferromagnetic with coercivity field of 650 Oe at room temperature. First-principles calculations indicate that giant interfacial anisotropy, induced by the strong spin-orbit interaction of enhanced orbit momentum of Fe, overcomes the superparamagnetic limit, leading to exceptional room-temperature permanent ferromagnetism. The findings are important for the understanding of the origin of permanent ferromagnetism at ultrasmall size and critical for ultrahigh density recording and information processing.

Multi-L1(0) domain CoPt and FePt nanoparticles revealed by electron microscopy

The atomic structure of CoPt and FePt nanoparticles (with a diameter between 2 and 5 nm) has been studied by transmission electron microscopy. The particles have been produced by a laser vaporization cluster source and annealed under vacuum in order to promote chemical ordering. For both alloys, we observe a coexistence of crystalline and multiply twinned particles with decahedral or icosahedral shapes. In addition to particles corresponding to a single L1(0) ordered domain, we put into evidence that even small particles can display several L1(0) domains. In particular, the chemical order can be preserved across twin boundaries which can give rise to spectacular chemically ordered decahedral particles made of five L1(0) domains. The stability of such structures, which had been recently predicted from theoretical simulations, is thus unambiguously experimentally confirmed.

Understanding the metal-carbon interface in FePt catalyzed carbon nanotubes

Any tip functionalization of carbon nanotubes, for which the relative orientation between their (metallic) catalyst particle and the nanotube axis is essential, requires a detailed knowledge of the nature of the internal interface between the particle and the outgrown tube. In the present work, this interface is characterized with atomic precision using state-of-the-art low-voltage aberration-corrected transmission electron microscopy in combination with molecular dynamics simulations for the case of hard-magnetically terminated carbon nanotubes. Our results indicate that the physical principle based upon which the interfacial metal facet is chosen is a reduction of the desorption energy for carbon.

Sinter-free phase conversion and scanning transmission electron microscopy of FePt nanoparticle monolayers

Thermally robust monolayers of 4-6 nm diameter FePt nanoparticles (NPs) were fabricated by combining chemical synthesis and atomic layer deposition. Spin-cast monolayers of FePt NPs were coated with thin, 11 nm-thick layers of amorphous Al(2)O(3), followed by annealing to convert the FePt NPs from an alloy (A1) into intermetallic FePt (L1(0)) and FePt(3) (L1(2)) phases. The Al(2)O(3) layer serves as a barrier that prevents sintering between NPs during annealing at temperatures up to 730 °C. Electron and X-ray diffraction in conjunction with high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) show that as-synthesized A1 FePt NPs convert into L1(0) and L1(2) phase NPs through annealing. HAADF-STEM measurements of individual NPs reveal imperfect ordering and show that the NP composition determines which intermetallic phase is obtained. Mixed-phase NPs with L1(0) cores and FePt(3) L1(2) shells were also observed, as well as a smaller number of unconverted A1 NPs. These results highlight the need for improved control over the compositional uniformity of FePt NPs for their use in bit-patterned magnetic recording.

Atom-resolved evidence of anisotropic growth in ZnS nanotetrapods

ZnS nanotetrapods were investigated by atom-resolved microscopy characterization and quantitative simulation. The octahedron core enclosed with Zn- and S-terminated surfaces was verified. Four hexaprism-shaped arms were selectively grown from Zn-terminated surfaces of the core by alternately stacking zinc blende and wurtzite structures. The stacking order change at the core/arm interface is significant to activate the arm growth. The anisotropic growth mechanism was proposed and further proved by the synthesis of ZnS nanoparticles and nanobelts.

Extended X-ray absorption fine structure of bimetallic nanoparticles

Electronic and magnetic properties strongly depend on the structure of the material, especially on the crystal symmetry and chemical environment. In nanoparticles, the break of symmetry at the surface may yield different physical properties with respect to the corresponding bulk material. A useful tool to investigate the electronic structure, magnetic behaviour and local crystallographic structure is X-ray absorption spectroscopy. In this review, recent developments in the field of extended X-ray absorption fine structure measurements and in the analysis methods for structural investigations of bimetallic nanoparticles are highlighted. The standard analysis based on Fourier transforms is compared to the relatively new field of wavelet transforms that have the potential to outperform traditional analysis, especially in bimetallic alloys. As an example, the lattice expansion and inhomogeneous alloying found in FePt nanoparticles is presented, and this is discussed below in terms of the influence of employed density functional theory calculations on the magnetic properties.

Element-specific magnetic hysteresis of individual 18 nm Fe nanocubes

Correlating the electronic structure and magnetic response with the morphology and crystal structure of the same single ferromagnetic nanoparticle has been up to now an unresolved challenge. Here, we present measurements of the element-specific electronic structure and magnetic response as a function of magnetic field amplitude and orientation for chemically synthesized single Fe nanocubes with 18 nm edge length. Magnetic states and interactions of monomers, dimers, and trimers are analyzed by X-ray photoemission electron microscopy for different particle arrangements. The element-specific electronic structure can be probed and correlated with the changes of magnetic properties. This approach opens new possibilities for a deeper understanding of the collective response of magnetic nanohybrids in multifunctional materials and in nanomagnetic colloidal suspensions used in biomedical and engineering technologies.

Structural and magnetic properties of ternary Fe(1-)MnPt nanoalloys from first principles

BACKGROUND

Structural and magnetic properties of binary Mn-Pt and ternary Fe(1-) (x)Mn(x)Pt nanoparticles in the size range of up to 2.5 nm (561 atoms) have been explored systematically by means of large scale first principles calculations in the framework of density functional theory. For each composition several magnetic and structural configurations have been compared.

RESULTS

The concentration dependence of magnetization and structural properties of the ternary systems are in good agreement with previous bulk and thin film measurements. At an intermediate Mn-content around x = 0.25 a crossover between several phases with magnetic and structural properties is encountered, which may be interesting for exploitation in functional devices.

CONCLUSION

Addition of Mn effectively increases the stability of single crystalline L1(0) particles over multiply twinned morphologies. This, however, compromises the stability of the ferromagnetic phase due to an increased number of antiferromagnetic interactions. The consequence is that only small additions of Mn can be tolerated for data recording applications.

Resolving the structure of active sites on platinum catalytic nanoparticles

Accurate understanding of the structure of active sites is fundamentally important in predicting catalytic properties of heterogeneous nanocatalysts. We present an accurate determination of both experimental and theoretical atomic structures of surface monatomic steps on industrial platinum nanoparticles. This comparison reveals that the edges of nanoparticles can significantly alter the atomic positions of monatomic steps in their proximity, which can lead to substantial deviations in the catalytic properties compared with the extended surfaces.

Preparation and characterization of supported magnetic nanoparticles prepared by reverse micelles

Monatomic (Fe, Co) and bimetallic (FePt and CoPt) nanoparticles were prepared by exploiting the self-organization of precursor loaded reverse micelles. Achievements and limitations of the preparation approach are critically discussed. We show that self-assembled metallic nanoparticles can be prepared with diameters d = 2-12 nm and interparticle distances D = 20-140 nm on various substrates. Structural, electronic and magnetic properties of the particle arrays were characterized by several techniques to give a comprehensive view of the high quality of the method. For Co nanoparticles, it is demonstrated that magnetostatic interactions can be neglected for distances which are at least 6 times larger than the particle diameter. Focus is placed on FePt alloy nanoparticles which show a huge magnetic anisotropy in the L1(0) phase, however, this is still less by a factor of 3-4 when compared to the anisotropy of the bulk counterpart. A similar observation was also found for CoPt nanoparticles (NPs). These results are related to imperfect crystal structures as revealed by HRTEM as well as to compositional distributions of the prepared particles. Interestingly, the results demonstrate that the averaged effective magnetic anisotropy of FePt nanoparticles does not strongly depend on size. Consequently, magnetization stability should scale linearly with the volume of the NPs and give rise to a critical value for stability at ambient temperature. Indeed, for diameters above 6 nm such stability is observed for the current FePt and CoPt NPs. Finally, the long-term conservation of nanoparticles by Au photoseeding is presented.

Well-aligned Nickel Nanochains Synthesized by a Template-free Route

Highly uniform and well-aligned one-dimensional Ni nanochains with controllable diameters, including 33, 78, and 120 nm, have been synthesized by applying an external magnetic field without any surface modifying agent. The formation can be explained by the interactions of magnetic dipoles in the presence of applied magnetic field. Magnetic measurements demonstrate that the shape anisotropy dominates the magnetic anisotropy. The demagnetization factor, ∆N, is in the range of 0.23-0.36.

Structural stability of icosahedral FePt nanoparticles

The structural stability of FePt nanoparticles of about 5-6 nm diameter was investigated by dynamic high resolution transmission electron microscopy. The FePt icosahedra were very stable under an electron beam flux of approximately 20 A/cm(2) at 300 kV. Surface sputtering was suppressed due to the large sputtering threshold energy of a Pt-rich shell. Under a flux of approximately 50 A/cm(2), the trapping potential well of the FePt particle on the supporting carbon film was lowered by the magnetic interaction between the electron beam and the particle, which leads to rotational and translational motions of the particle. A large dose of electrons (approximately 200 A/cm(2)) initiated melting and recrystallization of the FePt particle. The structure of the FePt nanoparticle, a Pt enriched shell around an Fe/Pt magnetic core, is believed to be responsible for its dynamic behaviour under different beam conditions.

Direct observation of a surface induced disordering process in magnetic nanoparticles

We present experimental evidence of surface induced disordering at magnetic FeCoPd nanoparticles during the L1(0)-A1 phase transition using high-resolution aberration-corrected electron microscopy and strain mapping. In situ electron diffraction studies show a narrow temperature range of fully ordered L1(0) structure. The order-disorder transition is size dependent and induces strong lattice deformation in outer part of the nanocrystals. The formation of unusually large strain of 20% is discussed in terms of core-shell structure formation with surface disordered layer and ordered core.

The route to single magnetic particle detection: a carbon nanotube decorated with a finite number of nanocubes

We propose an experimental technique that allows us a straightforward and reliable fabrication of magnetic and electromechanical nanodevices for single particle detection and characterization. We demonstrate three different experimental methods for fabrication of nanoscale devices consisting of either single or multiwall carbon nanotubes bridging metallic electrodes and decorated with magnetic iron/iron oxide nanocubes with a size of 18 nm. Electrical characterization of the devices as well as structural and magnetic investigations of nanoparticles are reported. In particular, the proposed method based on measurements of the magnetic resonance in a single nanoparticle will enable dynamic magnetic resonance measurements on a single magnetic nanoparticle with a moment of approximately 10(5) mu(B) that are not feasible using conventional experimental techniques.