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The short-range order in a CoPt alloy was determined at 1203 and 1423 K using neutron diffuse scattering measurements. The effective pair interactions provided by data analysis reproduce well the experimental order-disorder transition temperature in Monte Carlo simulations. They complete previous results reported for the Co-Pt system and are compared to those obtained within tight-binding and ab initio formalisms. Our results show that the important dependence of the nearest-neighbor pair interactions with composition is not related to the sample magnetic state at the measured temperatures. Interactions measured in the paramagnetic domain for the CoPt alloy behave like those in the ferromagnetic domain for the ${\mathrm{Co}}_{3}\mathrm{Pt}$ and ${\mathrm{Co}}_{0.65}{\mathrm{Pt}}_{0.35}$ alloys. The effective pair interactions related to the tight-binding Ising model provide a relatively good description of the CoPt alloy thermodynamics close to the ordering temperature (short-range order and temperature of phase transformation), even if they strongly differ from those measured in this study. The average magnetic moment of Co atoms at high temperatures was determined from the analysis of the intensity contribution that is not dependent on the scattering vector. The obtained value is very close to the moment measured at room temperature or determined from ab initio calculations. This confirms the Curie-Weiss behavior of the CoPt alloy. Finally, transmission electron microscope observations carried out on samples annealed for about 30 days confirmed that the order-disorder transition takes place in the 830--843 K temperature interval at the ${\mathrm{Co}}_{3}\mathrm{Pt}$ composition.

We study the magnetic short range order (MSRO) in ${\mathrm{Fe}}_{1\ensuremath{-}x}{\mathrm{Cr}}_{x}\phantom{\rule{4pt}{0ex}}(0\ensuremath{\le}x\ensuremath{\le}0.15)$ where an inversion of atomic short range order (ASRO) occurs at ${x}_{C}=0.11(1)$. Our combination of neutron diffuse scattering and bulk magnetization measurements offers a comprehensive description of these local orders at a microscopic level. In the dilute alloys $(xl0.04)$, the Cr atoms bear a large moment ${\ensuremath{\mu}}_{\mathrm{Cr}}=\ensuremath{-}1.0(1)\phantom{\rule{4pt}{0ex}}{\ensuremath{\mu}}_{\mathrm{B}}$, antiparallel to the Fe ones $({\ensuremath{\mu}}_{\mathrm{Fe}})$. They fully repel their Cr first and second neighbors, and perturb the surrounding Fe moments. With increasing $x$, near neighbor Cr-Cr pairs start to appear and the Cr moment magnitude decreases, while ${\ensuremath{\mu}}_{\mathrm{Fe}}$ shows a rounded maximum for ${x}_{1}=0.06(1)l{x}_{C}$. Above ${x}_{C}$, ASRO turns to local Cr segregation, thereby increasing magnetic frustration. First-principles calculations reproduce the observed moment variations but overestimate the magnitude of the Cr moment. In order to reconcile theory with experiment quantitatively, we propose that the magnetic moments start canting locally, already above ${x}_{1}$. This picture actually anticipates the spin glasslike behavior of Cr-rich alloys. The whole study points out the subtle interplay of MSRO and ASRO, yielding an increasing frustration as $x$ increases, due to competing Fe-Cr and Cr-Cr interactions and Cr clustering tendency.

International audience; The magnetization state of one-dimensional magnetic nanoparticle chains plays a key role for a wide range of applications ranging from diagnosis and therapy in medicine to actuators, sensors and quantum recording media. The interplay between the exact particle orientation and the magnetic anisotropy is in turn crucial for controlling the overall magnetization state with high precision. Here, we report on a three-dimensional description of the magnetic structure of one-NP-wide chains. In this aim, we combined two complementary experimental techniques, magnetic force microscopy (MFM) and electronic holography (EH) which are sensitive to out-of-plane and in-plane magnetization components, respectively. We extended our approach to micromagnetic simulations which provided results in good agreement with MFM and EH. The findings are at variance with the known results on unidirectional nanoparticle assemblies, and show that magnetization is rarely strictly collinear to the chain axis. The magnetic structure of one-NP-wide chains can be interpreted as head-to-head magnetic domain structures with off-axis magnetization components, which is very sensitive to morphological defects in the chain structure such as minute size variation of NPs, tiny misalignment of NPs and/or crystal orientation with respect to easy magnetization axis.

Triple-defect formation in B2-ordered binary A-B intermetallic compounds results fromthe asymmetry between the formation energies of A- and B-antisite defects. Chemical disorderingin such systems is strictly correlated with vacancy formation, which is the reason for usually veryhigh vacancy concentration. Consequently, Kinetic Monte Carlo (KMC) simulation of processes occurringin the triple-defect systems and controlled by atomic migration via vacancy mechanism mustinvolve complete vacancy thermodynamics – i.e. the simulated system must contain the equilibriumtemperature-dependent number of vacancies. The fully consistent approach based on two differentMonte Carlo techniques has been applied in the present study. The AB intermetallic was modelled withan Ising-type Hamiltonian and KMC simulated for “order-order” kinetics with temperature-dependentequilibrium number of vacancies previously determined by means of Semi Grand Canonical MonteCarlo (SGCMC) simulations. The procedure required in addition the determination of saddle -pointenergies assigned to particular atomic jumps to nn vacancies. Their values were estimated in relationto the nn pair-interaction energies with reference to Molecular Statics simulations performed for NiAlsystem with EAM energetics. The results elucidated the role of triple-defect formation as the atomisticscaleorigin of the experimentally observed surprisingly low rate of the “order-order” kinetics in bulkNiAl.

Self-diffusion of component atoms and ‘order–order’ relaxations in a B2-ordering binary system AB showing a tendency for triple-defect formation were consistently simulated by means of two Monte Carlo techniques. In view of a strict correlation between antisite-defect and vacancy concentrations the Kinetic Monte Carlo (KMC) simulations were implemented with a temperature-dependent vacancy concentration determined by means of Semi-Grand Canonical Monte Carlo (SGCMC) simulations. The Ising model of the system was completed with local-configuration-dependent saddle-point energy parameters related to vacancy mediated atomic jumps. The simulations elucidated the atomistic origin of the experimentally observed low rate of ‘order–order’ relaxations in NiAl, as well as reproduced the experimental relation between the activation energies for ‘order–order’ kinetics and Ni self-diffusion in NiAl. Higher value of the deduced activation energy for atomic migration with respect to the effective energy barriers related to individual atomic jumps indicated their high correlation.

Lattice-gas decomposition model for equilibrium vacancy concentration in B2-ordered triple defect A–B binary intermetallic systems previously formulated and solved within the Bragg–Williams approximation was now simulated by means of Semi-Grand Canonical Monte Carlo (SGCMC) technique. Simulations of a ternary Ising lattice gas A–B–V with pair-interaction energies promoting the generation of A-antisites and A-vacancies revealed a miscibility gap and a coexistence of vacancy-poor and vacancy-rich phases within a wide range of temperature. The B2-ordered vacancy-poor phases were identified with intermetallic compounds containing equilibrium number of vacancies, which, at sufficiently low temperature, resulted proportional to the number of antisite defects. The latter effect additionally accompanied by the presence of constitutional vacancies in B-rich systems was interpreted as a signature of triple defect formation. The reliability of the model was checked by its implementation with Ni–Al EAM potential and by comparing the calculated vacancy concentrations in NiAl with the available experimental data. The modelled temperature dependence of the equilibrium vacancy concentration and configuration in triple defect B2-ordering binary intermetallics will be applied in Kinetic Monte Carlo (KMC) simulations of “order–order” kinetics in these systems.

Thermal vacancy formation correlated with atomic ordering was modelled in B2-ordering A–B binary intermetallics. Ising Hamiltonian was implemented with a specific Bragg–Williams-type thermodynamic formalism for thermal vacancy formation based on the phase equilibria in a lattice gas composed of atoms and vacancies. It has been demonstrated that for pair-interaction energetics favouring vacancy formation on A-atom sublattice, equilibrium concentrations of vacancies and antisite defects result mutually proportional in well defined temperature ranges. The effect observed in both stoichiometric and non-stoichiometric binary alloys was interpreted as a tendency for triple defect formation. In B-rich non-stoichiometric binary alloys vacancy concentration did not extrapolate to zero at temperature approaching zero, which indicated the formation of constitutional vacancies. Energetic conditions for the occurrence of the effects were analysed in detail.

International audience; Among nanoalloys, Co–Pt type (CoPt or FePt) supported nanostructures are very interesting systems due to the direct link between atom arrangement and magnetic behavior. In addition, these alloys become model systems in the field of nanoalloys, due to the diversity of atom arrangements either present in the bulk state or specific to the nanoscale (chemically ordered L10, L12, or disordered fcc structures, core–shell, five-fold structures – icosahedral or decahedral, etc.). The synergy between experimental and modeling efforts has allowed the emergence of an overview of the structural, morphological and chemical behaviors of CoPt-based supported nanoparticles in terms of phase diagrams (temperature, composition, size effect), kinetic behavior (growth, annealing, ordering), and also in terms of environment effects (substrate, capping, matrix, gas) and of magnetic properties. All aspects of this complexity are reviewed: synthesis strategies (physical deposition, cluster beam deposition and wet chemical methods), magnetic behavior (atomic magnetic moment, magnetic anisotropy energy), structural transitions (non-crystalline/crystalline structures, order/disorder, surface/interface segregation), etc. In this field, the investigation techniques, such as electron microscopy and X-ray scattering or absorption techniques, are generally used at their ultimate limit due the small size of the studied objects. Finally, several aspects of the annealing process, which is a key phenomenon to achieve the chemical order, have been discussed in both thermodynamic and kinetic points of view (size effect, critical temperature, annealing time, twinning, coalescence, etc.).

“Order–order” kinetics in L10-ordered FePt nano-layers have been studied by Monte Carlo (MC) simulation implemented with Glauber dynamics and vacancy atomic-jump mechanism. In the presented preliminary approach the effects of the substrate character and of the tetragonal distortion of the superstructure were neglected. Atomic pair-interaction energies in two co-ordination shells were evaluated on the basis of Cluster-Expansion “ab initio” calculations carried out for Fe–Pt binary system. Nano-layers limited by two (0 0 1)-type free surfaces were simulated by imposing two-dimensional (x–y) periodic boundary conditions upon a sample of FePt with different variants of L10 superlattice (different orientations of monoatomic planes). Selective effect of free surfaces on the stability of L10 variants was observed. While no effect was detected in the case of the variants with (1 0 0)- and (0 1 0)-type Fe and Pt planes (x- and y-L10 variants, respectively), the L10 stability was strongly reduced in the case of z-L10 variant ((0 0 1)-type monoatomic planes). As a result, the initially homogeneous FePt films ordered in z-L10 variant showed a complex “order–order” process resulting in a microstructure of x- and y-L10 variant domains. Kinetics of the process was analysed in atomistic scale.

Double perovskite Sr2FeMoO6 with cation disorder was investigated by means of the X-ray diffractometry, atomic resolution transmission electron microscopy, AC susceptibility, and 57Fe Mossbauer spectroscopy. Atomic resolution TEM measurements revealed the existence of domains with modulated crystal structure inside regions of the ideal double perovskite. 57Fe Mossbauer spectroscopy revealed that cation disorder leads to a suppression of delocalization of the 4d1 electron of Mo5+, and consequently regions with cation disorder form insulating domains in the half-metallic matrix of the ordered double perovskite structure.

Diffuse neutron scattering measurements were carried out in a FePd single crystal at 1020 K in the (100) and (110) reciprocal planes. The diffuse intensity is nearly symmetric, showing that the static displacements are small. A set of displacement and short-range order parameters have been fitted to the measured data using a least square procedure. The short-range order parameters have been used in conjunction with an inverse cluster variation method to deduce the pair interaction energies. A strong attractive interaction between second nearest-neighbour like atoms is found. PACS. 61.12.Ex Neutron scattering - 64.60.Cn Order-disorder transformations - 61.66.Dk Alloys

The frequencies of the normal modes of vibration of FePd single crystal have been measured using inelastic neutron scattering. The measurements were performed for the L10 ordered structure at 300 and 860 K and for the fcc disordered state at 1020 K. A general lowering of phonon frequencies with increasing temperature is found in FePd, the dependence being the strongest for acoustical transverse [ξ ξ ξ] branches. The migration energies as well as several thermodynamic quantities have been calculated from the densities of states. The contribution of ordering to the migration energy, as experimentally determined in this work, agrees well with the obtained one by Monte Carlo simulations in the L10 structure.

Intermetallic compounds showing high stability of superstructure are promising technological materials. As their attractive properties follow mostly from a high degree of long-range atomic order (LRO), knowledge of ordering kinetics in these systems is crucial. Recently, kinetics of “order–order” processes in L12 systems such as Ni3Al have been extensively investigated experimentally, as well as by means of Monte Carlo (MC) and molecular dynamics simulations. Computer simulations yield an information about the atomistic mechanism of ordering in terms of a correlation of atomic jumps between particular sublattice sites. The present contribution reports new Monte Carlo simulation results of “order–order” processes in B2 and L10 superstructures. The results reveal an effect of the sublattice geometry; specifically, the availability of easy-diffusion channels, on the structure of “order–order” relaxations.

We present molecular dynamics simulations of ordering kinetics in the Ni3Al alloy performed within the embedded atom method (EAM) as a scheme for interatomic potentials. The simulation cell containing 1372 atoms was initially perfectly L12-long-range ordered. After having artificially created one vacancy by removing at random one Ni-atom, the dynamics of the system was simulated at constant temperature and pressure. Atoms are found to migrate predominantly via jumps to nearest-neighbour (nn) vacancies. The number of antisites is low in comparison to the total number of atomic jumps. Therefore most of the jumps are ineffective for ordering kinetics causing only temporary change of the chemical order. Accordingly, the jumps creating antisites are most often followed by reverse jumps. Antisite defects are created as nn antisite pairs. This result is in agreement with predictions based on a model originally formulated within Monte Carlo simulations. 2002 Published by Elsevier Science B.V.

Diffraction anomalous fine structure on both Fe and Co K edges and transmission electron microscopy are used to study the crystallographic structure and morphology of thin ternary silicides films grown on Si(1 1 1). Co 1-x Fe x Si 2 layers (about 10 nm thick) have been grown by UHV-MBE in epitaxy on silicon. On either oriented or slightly mis-oriented (1 1 1) surfaces, and in situ annealed at about 925 K. In the Fe-rich part of composition, annealing at 925 K results in a phase separation into grains of two different structures, namely a CaF 2 -type (as CoSi 2 ) and α-FeSi 2 -type structures. In contrast, for Si surfaces with a very low miscut and in the Co-rich part of composition, annealing at 925 K results in an almost single phase with a CaF 2 -type structure. Transmission electron microscopy images show that, even in the Co-rich part, the stress gradient generated at the step edges acts as a nucleation centre for the α-FeSi 2 phase, which is thus probably more stable than the CaF 2 -type phase, stabilised by the stress on the terraces.

Structural information on ultrathin films of Fe x Co 1− x alloys grown epitaxially by coevaporation on Si(0 0 1) at room temperature is obtained by X-ray photoelectron diffraction, low-energy electron diffraction, inelastic medium-energy electron diffraction and transmission electron microscopy. The body-centered-cubic structure is found to be stabilized for alloy thin films up to very high concentration of Co beyond the thermodynamically stable bcc range (0.25⩽ x ⩽1). For alloy films with thickness up to 10 monolayers, the bcc structure with (0 0 1)[1 0 0]Fe x Co 1− x //(0 0 1)[1 0 0]Si is actually observed over the entire range of alloy concentrations that includes elemental bcc Co. All films are ferromagnetic with in-plane magnetization. They show clear in-plane fourfold magnetic anisotropy for all alloy concentrations and the relevant cubic anisotropy constant shows a strong composition and thickness dependence.

The results of previous extensive experiments on γ′-Ni 3 Al systems are interpreted and explained by means of Monte Carlo computer simulations. The complex character of the experimental ‘order–order’ relaxation curves, as well as the theoretically predicted effect of vacancy ordering have been reproduced and analysed in detail in terms of the dynamics of particular kinds of atomic jumps. The Monte Carlo simulations of ‘order–order’ kinetics were performed both for an Ising A 3 B L1 2 -ordered system and for Ni 3 Al with lattice energetics considered within the ‘embedded-atom-method’ (EAM). The EAM yielded estimations of both atomic pair-interactions and ‘effective’ saddle-point energies for vacancy jumps in Ni 3 Al. The proposed model scenario for the creation and elimination of anti-site atoms in the relaxing L1 2 -type superstructure shows that the experimentally observed features of the ‘order–order’ processes in Ni 3 Al follow from an interplay between short- and long-range ordering effects resulting from the system energetics and the superlattice geometry.

A structural study has been performed on the La0.8Sr0.2FexCo1-xO3 (x = 0.025 to 0.3) system displaying large magnetoresistance (MR) at room temperature. A detailed analysis of the crystal structure and microstructure was done by X-ray diffraction (XRD), transmission and scanning electron microscopy (TEM and SEM). The atomic resolution TEM images and the appearing superreflections in the corresponding SAED patterns revealed that a superstructure is formed due to the presence of iron. The correlation between the ordered microstructure and the observed large MR ratio is discussed. 57Fe Mossbauer spectroscopy was utilized to gain information on the valence state of iron in the sample with x = 0.3. The lattice parameters of Fe- doped La0.8Sr0.2FexCo1-xO3 compounds were found to increase monotonously with increasing Fe content. The valence state of iron was found to be Fe3+.

Epitaxial Co/Mn multilayers (0.75 to 6 nm Co, 0.4 nm Mn layer thickness) have been grown on mica substrates covered by a (0002) Ru buffer layer. The structural properties of these layers have been studied using X-ray diffraction, nuclear magnetic resonance (NMR), and high resolution transmission electron microscopy (HRTEM). The Co layers, grown as face centred cubic (fcc), were found to be stabilised by the very thin Mn layers. Data obtained using X-ray diffraction and NMR were analysed and found to be in good agreement, while Monte-Carlo simulations were used to interpret the data and calculate the expected diffracted intensity and NMR spectra. The HRTEM data show that the Mn layers give rise to a large strain contrast extending, in the growth direction, over a distance which exceeds the thickness of the Mn layers. The superlattices could be described as having an fcc structure containing randomly located stacking faults with varying densities. The results verify the presence of a dominant, almost perfect phase of fcc stacking, and of a faulted hcp phase, while the number of defects increases with the Co layer thickness.

The magnetic anisotropy of an ultradense array of crystalline cobalt nanowires is investigated by means of broadband ferromagnetic resonance and magnetic torque measurements. The array is grown epitaxially in solution on a Pt(111) film and consists of single crystalline metallic wires with a diameter of 6.2 nm and a center-to-center interwire distance of 9.6 nm. The shape anisotropy and the Co hexagonal compact structure with the c-axis along the wire axis combine with each other to impose a perpendicular magnetic anisotropy despite the high density of 8 x 10 (1)2 wires/in.(2). The intrinsic uniaxial magnetocrystalline anisotropy constants K-1 and K-2 are extracted from the ferromagnetic resonance and torque measurements using a mean field approach accounting for the interwire dipolar interactions. At room temperature, and despite air exposure, an unexpected increase of K-1 and K-2 of more than 40% with respect to the bulk is evidenced. Published by AIP Publishing.

The thermodynamic parameters that drive the atomic migration in 2D and 3D binary alloys are studied using Monte-Carlo simulations. The model is based on a vacancy–atom jump mechanism between nearest neighbour sites, with a constant vacancy concentration, so that only the migration counterpart ( E M ) of the total activation energy ( E A = E M + E F , where E F is the vacancy formation energy) is determined. The ordering kinetics are well described by exponential like behaviours with one (respectively two) relaxation time in the 2D (respectively 3D) cases. The temperature dependence of the long relaxation time is an Arrhenius law yielding effective migration energies. Slowing down of the relaxation is observed at the Order–Disorder transitions. In both cases, we observe a linear variation of the effective migration energy with the Order–Disorder critical temperature.

We demonstrate the possibility of growing good-quality epitaxial Fe films on a Si(001) substrate, opening up new prospects to prepare ferromagnetic superlattices on this substrate. A template technique prevents the formation of disordered interfacial iron silicides. Transmission electron microscopy reveals that the Fe layers are fairly uniform in thickness with abrupt interfaces and in majority epitaxial relationship Fe(001)[100]//Si(001)[100]. Both the diffraction data and the observation of Moire fringes indicate that the Fe lattice is almost relaxed towards its bulk bcc phase in thick layers [≥40 monolayers (ML)] but is still strained by the substrate in the thinnest films. Magneto-optical Kerr effect measurements show that the films evaporated at normal incidence with a thickness above 4 ML are ferromagnetic at room temperature and exhibit in-plane biaxial anisotropy.

Ultrathin Fe films, in the thickness range 0\char21{}40 monolayers (ML), have been grown on Si(001) by molecular-beam epitaxy and characterized by low-energy electron diffraction, inelastic medium-energy electron diffraction, x-ray photoelectron spectroscopy, angular-resolved ultraviolet spectroscopy, x-ray photoelectron diffraction, ion scattering spectroscopy, and transmission electron microscopy. For Fe depositions onto Si(001) at room temperature, a disordered layer is obtained due to a high degree of intermixing between the Fe deposit and the Si substrate. Successful epitaxial growth of Fe at room temperature is achieved by use of a thin (\ensuremath{\sim}10 \AA{}) ${\mathrm{CoSi}}_{2}$ silicide interlayer epitaxially grown on the Si(001) substrate prior to the Fe deposition, which prevents the intermixing of the Si substrate atoms into the Fe overlayer. Below a coverage of \ensuremath{\sim}2 ML, a reacted ordered iron-rich phase forms at the surface. At higher coverages, there is growth of an epitaxial essentially body-centered cubic (bcc) Fe(001) overlayer with the orientational relationships $\mathrm{Fe}(001)\mathrm{〈}001\mathrm{〉}\ensuremath{\parallel}{\mathrm{CoSi}}_{2}(001)\mathrm{〈}001\mathrm{〉}\ensuremath{\parallel}\mathrm{Si}(001)\mathrm{〈}001\mathrm{〉}.$ Finally, a well-ordered ${\mathrm{F}\mathrm{e}/\mathrm{C}\mathrm{o}\mathrm{S}\mathrm{i}}_{2}$ interface is formed even at room temperature.

The experimental investigations of "order-order" kinetics in Ni3 Al-based L1 2-ordered intermetallic compounds revealed the relaxation curves composed of two parallel processes considerably differing in relaxation rates. A simple Ising-type model based on a vacancy mechanism of atomic jumps was used to carry the Monte Carlo simulations of long-range-order relaxations m a binary A3 B system with L1 2 -type superstructure. The simulated relaxation curves fitted weighted sums of two exponentials with significantly different relaxation times. It was found out that the fast relaxation process is controlled by the dynamics of the minority B-atom jumps. PACS numbers: 05.50.+q, 05.20.Dd, 66.30.—h, 81.30.Hd

The Monte Carlo model previously developed to simulate the ordering kinetics in B2 phases is now applied to the calculation of phase diagrams that display DO 3 and B2 structures and to that of the relaxation of the long range order parameters in DO 3 and B2 phases and at the DO 3 –B2 and DO 3 –A2 transitions. The model, which is based on a vacancy jump mechanism, lies on an Ising Hamiltonian with interaction energies between first and second nearest neighbors. The simulations reproduce experimental trends for the activation energy of the self-diffusion in equiatomic FeCo and for the migration energy in Fe 3 Al and Fe 3 Si compounds. Critical slowing-down of the relaxation times are observed at the order–disorder transitions, in qualitative agreement with experimental results in Fe 3 Al.

The alloying of isomorphic L12 ferromagnetic CoPt3, MnPt3, and antiferromagnetic FePt3 yields pseudobinary ~M 12xMx 8Pt3! compounds whose magnetic phase diagrams display paramagnetic, ferromagnetic, antiferromagnetic, spin glass, and reentrant spin glass regions, depending on concentration and temperature. We show that it is possible to reproduce many features of these phase diagrams, namely, the Curie and Neel temperatures, using a simple Ising Hamiltonian, treated in the pair approximation of a square cluster-variation method, with only three dominant magnetic interactions between 3d elements, the magnetic role of Pt being neglected. The homoatomic interactions ~JMM , JM8M8! are given by the magnetic transitions of the binary compounds, whereas the heteroatomic JMM8 interaction is either parametrized or fitted and left free to vary with concentration. @S0163-1829~97!01225-3#

PtCo alloys have long been known to possess higher activity in the electrochemical oxygen reduction reaction compared to pure Pt. This work addresses the influence of the chemical order on the electrocatalytic activity of PtCo alloy, the phenomenon, which has been already documented in the literature but yet poorly understood. Single crystalline PtCo(0 01) alloy films were epitaxially grown on Mg(0 0 1) substrates and used as model electrodes to study the influence of the chemical order on the electrochemical oxygen reduction reaction. Electrodes with close Pt:Co atomic ratios in the bulk and near surface region show grossly different electrocatalytic activities against the chemical order, the ordered L1(0) phase being ca. 7 times more active than the disordered fcc A1 structure. Electronic structure calculations and quantum mechanical theory of electron transfer are utilized to provide a rational for this remarkable phenomenon. (C) 2013 Elsevier Ltd. All rights reserved.

The short-range order in a ${\mathrm{Fe}}_{0.804}$${\mathrm{V}}_{0.196}$ single crystal at 1133 and 1473 K was measured using neutron diffuse scattering in the [100] and [110] planes. The data were used in conjunction with the inverse cluster-variation method in order to extract the first five effective-pair-interaction potentials ${\mathit{V}}_{1}$ to ${\mathit{V}}_{5}$. The interactions obtained from the experimental short-range-order intensities are compared with the results of different theoretical calculations.

The thermodynamic parameters that drive the atomic migration in B2 alloys are studied using Monte-Carlo simulations. The model is based on a vacancy jump mechanism between nearest neighbor sites, with a constant vacancy concentration. The ordering energy is described through an Ising Hamiltonian with interaction potentials between first and second nearest neighbors. Different migration barriers are introduced fur A and B atoms. The results of the simulations compare very well with those of experiments. The ordering kinetics are well described by exponential-like behaviors with two relaxation times whose temperature dependences are Arrhenius laws yielding effective migration energies. The ordering energy contributes significantly to the total migration energy.

In situ observations of reflection high‐energy electron diffraction (RHEED) are used to study the structure and growth of epitaxial Co/Mn superlattices on hcp (0001) Ru buffer layers. Mn deposited on fcc (111) or hcp (0001) Co presents a singular growth behavior which can be interpreted as an incoherent growth with compressive stresses perpendicular to the surface. Moreover, the Mn structure is found to switch from a sixfold in‐plane symmetric 1×1 structure, which probably corresponds to strained fcc γ‐Mn, to a sixfold in‐plane symmetric √3×√3−30° structure with in‐plane lattice parameter a=4.69 A. This structural change occurs at a critical thickness of 5–6 monolayers, after the partial relaxation of the Mn in‐plane lattice parameter. An analysis of the three‐dimensional contribution to the RHEED patterns shows that the √3×√3−30° structure is probably identical to the Cu2Mg Laves phase which in turn closely resembles the α‐Mn phase.

The influence of magnetism on atomic short range order is shown in a single crystalline FeAl{sub 19.5} at % alloy in the disordered phase. The short range order has been measured in the paramagnetic state between 973 and 1 573 K on D7 at ILL through neutron diffuse scattering in the (110) plane. The effective pair interaction potentials V{sub 1}, V{sub 2}, V{sub 3} and V{sub 5} are calculated through an invers cluster variation method. The results are compared to those obtained through X-ray diffuse scattering on the same alloy frozen in the ferromagnetic order state at 772 K. The effect of magnetic coupling on the atomic order is the most important on the 1st effective potential, which is smaller in the ferromagnetic phase than in the paramagnetic one by about a factor of 2. 16 refs.; 9 figs.; 2 tabs.

Magnetic measurements on electrodeposited Ni–Cu/Cu multilayers indicated a continuous composition change at the magnetic-nonmagnetic interfaces. This is confirmed by the observation of a giant magnetoresistance (GMR) component due to a superparamagnetic region of these interfaces in the GMR curves for multilayers with d Ni–Cu =3 nm and d Cu ⩾0.75 nm.

The local atomic arrangement in FeAl 19.5% has been investigated by X-ray diffuse scattering on a single crystal quenched from 772 K in the ferromagnetic range. We find a very short ranged order, the largest SRO parameters being the first one (repulsive AlAl interaction) and the third one (attractive one). The displacement amplitude also rapidly decreases with the atomic shell radius. SRO pair interaction potentials have been deduced through an inverse cluster variation method.

The results of previous extensive experiments on ${\ensuremath{\gamma}}^{\ensuremath{'}}\ensuremath{-}{\mathrm{Ni}}_{3}\mathrm{Al}$ systems are interpreted and explained by means of Monte Carlo computer simulations based on the Glauber algorithm for vacancy mechanism of atomic migration in a superstructure. The complex character of the experimental ``order-order'' relaxation curves, as well as the theoretically predicted effect of vacancy ordering have been perfectly reproduced and analyzed in detail in terms of the dynamics of particular kinds of atomic jumps. The relaxations corresponding to an increase and decrease of the degree of long-range order showed up as processes only partially symmetric in time. The proposed model scenario for the creation and elimination of antisite atoms in the relaxing ${L1}_{2}$-type superstructure shows that the experimentally observed features of the ``order-order'' processes in ${\mathrm{Ni}}_{3}\mathrm{Al}$ follow from an interplay between two dominating and coupled modes of long- and short-range ordering.

Neutron diffuse scattering measurements have been achieved in two FeAl19.5 at% and FeV20 at% single crystals at high temperature in the paramagnetic range. Pair interaction potentials have been deduced through an inverse cluster variation method. In the FeAl crystal, the effect of ferromagnetism on the local order has been shown.

“Order-order” relaxations in γ-Ni3Al previously extensively studied by means of resistometry, are simulated within a model of vacancy mechanism of atomic migration in a superstructure implemented with Monte Carlo technique and the Glauber algorithm. The observed operation of two simultaneous relaxation processes showing different rates, as well as the theoretically predicted effect of vacancy ordering have been definitely reproduced and analysed in detail in terms of the dynamics of particular kinds of atomic jumps. The proposed model scenario for the creation and elimination of antisite atoms in the relaxing L12-type superstructure shows that the experimentally observed features of the “order-order” processes in Ni3Al follow from an interplay between two dominating and coupled modes of long- and short-range ordering: the creation/elimination of nn pairs of antisites (SRO) and the change of the “overall” number of antisites (LRO). High profile of the first process results in a high contribution of the fast component of LRO kinetics.

Monte Carlo simulations of the isothermal long-range order (LRO) relaxation in A3B system with Ll2 superstructure have been performed within a model based on a vacancy jump mechanism between nearest neighbour lattice sites. The studies aimed at the explanation of the origin of two simultaneous LRO relaxation processes experimentally observed in Ni3Al. An effect of pair interaction energies and saddle-point energies (assigned to jumping atoms) was studied. The preference of vacancies for the A-sublattice (face centres), commonly postulated for Ni3Al, occurred only for a narrow range of pair-interaction energies within the domain corresponding to Ll2 ordering. It was found that the appearance of the fast relaxation process is definitely correlated with the efficiency of B-atom jumps, as well as with the values of the saddle-point energies. The results lead to a microscopic model of the “order-order” relaxation in Ll2 superstructure and yield new indications for the choice of pair-interaction energy parameters applied in Monte Carlo simulations of atomic migration in Ll2 intermetallic compounds.

A reciprocal space formulation of the Cluster Variation is used in order to extract effective pair interactions in alloys from experimental short-range order diffuse intensities. The method is applied to the analysis of the short range order contribution to the neutron diffuse scattering of a $Fe-19.5 {\%} Al$ single crystal. A detailed comparison with real space methods is carried out for three different levels of approximations. For the highest level of approximation used in this study, effective pair interactions up to fifth neighbors are obtained., Comment: 23 pages, Revtex, 6 figures

The investigation of ordering-ordering kinetics (LRO or SRO) in inter-metallic compounds yields informations on the activation energy of the ordering process that can be compared to the energy of the interdiffusion. A systematic study of aluminide phases built either on a fcc (L12) or bcc (DO3) lattice shows that quite different behaviours are observed in the two types of structure. In L12 phases, the role played by the ordering energy has been tested in the pseudo-binary Ni3Al1-xMx (M = Fe, Mn, Cr) system which displays a strong decrease of the L12 phase stability when the transition metal M is increasingly substituted to aluminium. The activation energy (EA) of the ordering process is found to increase with the phase stability, in agreement with theoretical predictions. In binary bcc Fe1-xAlx phases, the result is different since the activation energy strongly decreases correlatively to the formation of long range ordered DO3 structure. Such an unexpected behaviour can be attributed to a large decrease of the migration energy through a lattice dynamics effect, as recently proposed in models which explain the enhanced diffusion observed in some bcc transition metals and alloys. Such a comparative experimental study shows that, depending on basic lattices, either thermodynamical or vibrational properties are determinant factors of the activation energy for the ordering kinetics.

According to theoretical models, the value of ordering energy determining the “order-disorder” transition temperature Tt should affect the kinetics of long-range ordering (LRO). The kinetics of L12 ordering was studied by means of in situ resistometry within the γ’-phase of a quasi-binary system Ni75Al25-xFex, where Tt drops drammatically with x.

We report on the interplay between the structural and the magnetic properties of different size dots patterned in two structurally different CoPt thin films with L10 order. The significant changes observed in the magnetic domain configuration and coercive field between the samples are interpreted in the light of crystalline structure. The magnetic hysteresis curves measured for single/several dots are compared to those of the dot assembly and discussed in view of structure and edge defects. In particular, the concentration fluctuations are found to enlarge the magnetic switching field distribution despite the presence of a single variant of the L10 phase. In turn, the distribution is narrow in samples that are homogeneous in concentration and contain well defined magnetization nucleation sites, namely, the x and y variants of the L10 phase formed during annealing.

The contributions of the atomic and spin disorder to the resistivity of two intermetallic Co1−x Ptx (x=0.5 and 0.7) compounds have been previously determined from experimental studies and separated from the phonon contribution. Now, the atomic and magnetic contributions and their variation with temperature through the Curie and order-disorder transitions are analyzed using a statistical model. The components of the resistivity are described as sums of resistivities of individual tetrahedral clusters. The cluster probabilities are calculated in a magnetic (up and down spin model) CVM approximation, the parameters of which are those that reproduce the experimental Co-Pt phase diagram. The contribution of chemically (magnetically) ordered clusters to the resistivity due to the chemical (magnetic) disorder are taken equal to zero. It is the temperature dependence of all other cluster probabilities which determine the resistivity behavior. Although there are severe approximations in both the experimental analysis and the statistical model, the agreement between theory and experiment is very satisfactory, underscoring the strong interplay between chemical and magnetic order in such ferromagnetic compounds.

The influence of magnetism on atomic short range order is shown in a single crystalline FeAl19.sat% alloy in the disordered phase. The short range order has been measured in the paramagnetic state between 973 and 1573 K on D7 at ILL through neutron diffuse scattering in the [110] plane. The effective pair interaction potentials V1 , V2, V3 and V5 are calculated through an inverse cluster variation method. The results are compared to those obtained through X-ray diffuse scattering on the same alloy frozen in the ferromagnetic order state at 772 K. The effect of magnetic coupling on the atomic order is the most important on the 1st effective potential, which is smaller by about a factor 2 in the ferromagnetic phase than in the paramagnetic one.

Single domain uniformly magnetized CoPt dots are used to probe magnetic force microscopy (MFM) tip characteristics. Magnetic contrasts obtained in MFM experiments are exploited to get the tip hysteresis loop in the influence of the dot. Coercivity, remnant magnetization, as well as details about the magnetization reversal of the tips are obtained. Well defined dot geometries allow a quantitative determination of the stray field of the dots acting on the tip. The tip’s hysteresis loop refers to an effective tip height, involved in the imaging, which is much larger than the extension of a magnetic domain of the CoCr tip coating. When MFM tips and sample dots have widely different characteristics, it is possible to get separate magnetic information, such as hysteresis loops, on both tip and sample, depending on the magnitude of the out-of-plane external field.

In multilayers [Co ×/Mn 0.4 nm] × N (0.75 < × < 6 nm), an fcc epitaxial, but incoherent growth of Co and Mn has been revealed using X-ray diffraction and high resolution transmission electron microscopy. The stacking faults, numerous in the Co, are located mainly near the Mn layers.