Short-range Order State Research Articles

Influence of long- and short-range chemical order on spontaneous magnetization in single-crystalline Fe0.6Al0.4 compound thin films

We systematically investigate the long- and short-range chemical order, lattice volume, and spontaneous magnetization in single-crystalline Fe0.6Al0.4 compound thin films. The vapor-quenching method based on a molecular beam epitaxy technique is utilized to fabricate the single-crystalline Fe0.6Al0.4 compound with the different B2 long-range order parameter S. S was varied by the deposition temperature T d, and it increases with increasing T d. The lattice volume V decreased with increasing T d, while the tetragonal distortion, ∼4%, due to epitaxial strain were observed. The changes in S and V were accompanied with the change in the magnetic moment per Fe, μ Fe. μ Fe showed the monotonic decrease as a function of S whereas μ Fe monotonically increases with V. With considering tetragonal distortion, μ Fe–V relationship has a good agreement with the previous reports. The μ Fe–S relationship showed the steep decrease of μ Fe around S∼ 0.6. In contrast to μ Fe–V relationship, μ Fe–S relationship does not match only from ours to previous studies but also among other reports. It implies the statistical number of the nearest-neighbor Fe–Fe bonds, i.e. S, cannot be an enough explanatory parameter. To clarify the structural origin of change in μ Fe, the short-range order (SRO) parameter inferred from the analysis of superlattice diffractions were introduced. They showed the clear difference for the films with high and low μ Fe. The results suggest that the transition from the long- to the SRO state plays the significant role on μ Fe.

Element-dependent evolution of chemical short-range ordering tendency of NiCoFeCrMn under irradiation

The evolution of short-range order (SRO) structures under irradiation has a great impact on the mechanical properties of high-entropy alloys. In this study, the atomistic mechanism of the evolution of SRO during and after cascade collisions was investigated in NiCoFeCrMn by multiscale modeling using molecular dynamics and lattice kinetic Monte Carlo simulations. SRO structures could be destructed by cascade collisions in short time and recovered by atomic diffusion in a much longer time. The destruction rate depends on the primary knock-on atom energies in cascade collisions and shows a universal law with respect to the number of replacement-per-atom. The vacancy diffusion simulations reveal that the SRO recovery rates of different element pairs vary significantly due to the distinct diffusion rates. Consequently, the SRO state under irradiation differs from that in thermodynamic equilibrium due to the difference of destruction and recovery rate for each element pair. The evolution of SRO is a result of the competition between the destruction and recovery mechanisms and depends heavily on the irradiation conditions.

Strontium ferrite SrFeO3-δ (2.50 ≤ 3-δ ≤ 2.87) studied by Raman and Mössbauer spectroscopy

Crystal structure, short range order, lattice dynamics and valence state of polycrystalline single-phase strontium ferrites SrFeO3-δ (with 3–δ = 2.87 ÷ 2.5) were studied by Raman and Mössbauer spectroscopy at normal conditions. Valence states of Fe ions and their fractions were determined from Mössbauer spectroscopy and concentrations of oxygen in all phases were estimated. For all studied SrFeO3-δ phases only five (or less) bands were observed above 200 cm−1 in spite of numerous Raman active phonon modes predicted by a factor group analysis. For the antiferromagnetic Brownmillerite phase (3–δ = 2.5), a broad band observed at ∼ 1350-1400 cm−1, was attributed to the two-magnon scattering. This band was found also for the SrFeO3-δ samples with 3–δ = 2.87 ÷ 2.725 which are in paramagnetic state at room temperature. This fact indicates an existence of the unusual magnetic correlations in all SrFeO3-δ perovskites.

Antiferromagnetic short-range order and cluster spin-glass state in diluted spinel ZnTiCoO4

The nature of magnetism in the doubly-diluted spinel ZnTiCoO4 = (Zn2+) A [Ti4+Co2+] B O4 is reported here employing the temperature and magnetic field (H) dependence of dc susceptibility (χ), ac susceptibilities (χ′ and χ″), and heat capacity (C p) measurements. Whereas antiferromagnetic (AFM) Néel temperature T N = 13.9 K is determined from the peak in the ∂(χT)/∂T vs T plot, the fit of the relaxation time τ (determined from the peak in the χ″ vs T data at different frequencies) to the Power law: τ = τ 0 [(T − T SG)/T SG]−zν yields the spin glass freezing temperature T SG = 12.9 K, z ν ∼ 11.75, and τ 0 ∼ 10−12 s. Since the magnitudes of τ 0 and z ν depend on the magnitude of T SG, a procedure is developed to find the optimum value of T SG = 12.9 K. A similar procedure is used to determine the optimum T 0 = 10.9 K in the Vogel–Fulcher law: τ = τ 0 exp[E a/k B(T − T 0)] yielding E a/k B = 95 K, and τ 0 = 1.6 × 10−13 s. It is argued that the comparatively large magnitude of the Mydosh parameter Ω = 0.026 and k B T 0/E a = 0.115 (≪1) suggests cluster spin-glass state in ZnTiCoO4 below TSG. In the C p vs T data from 1.9 K to 50 K, only a broad peak near 20 K is observed. This and absence of λ-type anomaly near T N or T SG combined with the reduced value of change in magnetic entropy from 50 K to 1.9 K suggests only short-range AFM ordering in the system, consistent with spin-glass state. The field dependence of T SG shows slight departure (ϕ ∼ 4.0) from the non-mean-field Almeida–Thouless line T SG(H) = T SG(0) (1 − AH 2/ϕ ). Strong temperature dependence of magnetic viscosity S and coercivity H C without exchange bias, both tending to zero on approach to T SG from below, further support the spin-glass state which results from magnetic dilution driven by diamagnetic Zn2+ and Ti4+ ions leading to magnetic frustration. Magnetic phase diagram in the H–T plane is established using the high-field magnetization data M(H, T) for T < T N which reveals rapid decrease of T SG with increase in H whereas decrease in T N with increase in H is weaker, typical of AFM systems. For T > T N, the data of χ vs T are fit to the modified Curie–Weiss law, χ = χ 0 + C/(T + θ), with χ 0 = 3.2 × 10−4 emu mol−1 Oe−1 yielding θ = 4 K and C = 2.70 emu K mol−1 Oe−1. This magnitude of C yields effective magnetic moment = 4.65 μ B for Co2+, characteristic of Co2+ ions with some contribution from spin–orbit coupling. Molecular field theory with effective spin S = 3/2 of Co2+ is used to determine the nearest-neighbor exchange constant J 1/k B = 2.39 K AFM and next-nearest-neighbor exchange constant J 2/k B = −0.66 K (ferromagnetic).

The Role of Grain Boundary Diffusion in the Solute Drag Effect.

Molecular dynamics (MD) simulations are applied to study solute drag by curvature-driven grain boundaries (GBs) in Cu–Ag solid solution. Although lattice diffusion is frozen on the MD timescale, the GB significantly accelerates the solute diffusion and alters the state of short-range order in lattice regions swept by its motion. The accelerated diffusion produces a nonuniform redistribution of the solute atoms in the form of GB clusters enhancing the solute drag by the Zener pinning mechanism. This finding points to an important role of lateral GB diffusion in the solute drag effect. A 1.5 at.%Ag alloying reduces the GB free energy by 10–20% while reducing the GB mobility coefficients by more than an order of magnitude. Given the greater impact of alloying on the GB mobility than on the capillary driving force, kinetic stabilization of nanomaterials against grain growth is likely to be more effective than thermodynamic stabilization aiming to reduce the GB free energy.

Strong light-field effects driven by nearly single-cycle 7 fs light-field in correlated organic conductors

We have demonstrated transient charge localization effects with a driving high-frequency field of 7 fs, 1.5-cycle near-infrared light in correlated organic conductors. In a layered organic conductor α-(BEDT-TTF)2I3 (BEDT-TTF: bis[ethylenedithio]-tetrathiafulvalene), a transient short-range charge order (CO) state is induced in a metallic phase. In contrast to such drastic change in the electronic state from the metal to the transient CO in α-(BEDT-TTF)2I3, dynamics of a field-induced reduction of a transfer integral are captured as a red-shift of the plasma-like reflectivity edge in a quasi-one-dimensional organic conductor (TMTTF)2AsF6 (TMTTF: tetramethyltetrathiafulvalene). These studies on the field-induced charge localization have been motivated by the theory of dynamical localization on the basis of tight-binding models with no electron correlation under a strong continuous field. However, the results of pump–probe transient reflectivity measurements using nearly single-cycle 7 fs, 11 MV cm−1 pulses and the theoretical studies which are presented in this review indicate that the pulsed field contributes to the similar phenomenon with the help of a characteristic lattice structure and Coulomb repulsion.

Critical phenomena in amorphous materials

Amorphous material usually exhibit a complex atomic structure including short-range order, long-range disorder and metastable state in thermodynamic, which is one of the existing states of matters. Amorphous alloy, also named metallic glass, is a new metallic material, and has a high strength, a good electromagnetic property, an excellent corrosionresistant and a high elasticity. The system of amorphous alloy can show some critical states and is a complicated system. In recent years, much atttentions have been paid to the researches of the phase transitions and critical phenomena of amorphous material. On a microscale, amorphous alloy can be regarded as a solid composed of many-particle systems. The investigation of the critical phenomena can significantly enhance the understanding of the interactions among these multi-particle systems. The structure of amorphous alloy is randomly and isotropic in macro performance, and ordered and anisotropic on a localized nanometer scale. The characteristics on different scales of amorphous alloy are not isolated. The structure of amorphous alloy determines the performance. The preparation process determines the nature of the microstructure. The microstructure is the internal cause dominating glass transition and deformation. Moreover, the effective cooling rate in preparation process of amorphous alloy affects the short-range rate of the amorphous phase. The nonperiodic short-range order plays a key role in the stability of amorphous phase. Furthermore, the glass transition and deformation of amorphous alloys are the responses to the external energy. The characteristics of the deformation process change with external condition. The external force can lead to the localized shear deformation and transformation between amorphous and liquid in the shear band. High temperature can cause a wide range of transformation from the amorphous solid to the liquid. So it is worth understanding in depth the basic principles of liquid and glass transition in order to prepare amorphous alloy in undercooled liquids. In this review article, we discuss the critical phenomena of amorphous alloys, which include the preparation process, the microstructure, the mechanical property and the electromagnetism. The correlation and the influence of microstructure on the macroscopic properties are analyzed. It will be helpful for understanding the nature of amorphous alloy, improving service reliability and exploring amorphous alloys with application values.

Structural, optical and electrical properties of Ti doped amorphous silicon prepared by co-sputtering

Titanium-doped unhydrogenated amorphous silicon (a-Si: Ti) films were prepared by rf co-sputtering. Structural, optical and electrical properties of a-Si: Ti as a function of Ti content were investigated by RBS(Rutherford Backscattering Spectrometry), XPS(X-ray Photoelectron Spectroscopy), Raman, ESR (Electron Spin Resonance), Spectroscopic Ellipsometry, thermoelectric tests and temperature dependent I–V tests. Introducing Ti impurity in amorphous Si results in n-type material, room temperature conductivity increases more than three orders of magnitude and activation energy decreases obviously. When Ti content is lower than 2 at.%, the degree of short-range order and defect state density of amorphous Si network improve while the optical bandgap changes little. However, with Ti content higher than 2 at.%, the degree of short-range order decreases and the optical gap of a-Si shrinks. The role of Ti atoms in amorphous Si network is discussed. ESR results confirm that introducing low content of Ti can compensate the dangling bonds of amorphous Si, while with Ti content higher than 2%, most Ti atoms compensate the unpaired electrons in the conduction band tail.

On the evolution of long-range order from short-range order in a Ni2(Cr0.5Mo0.5) alloy

Abstract The evolution of long-range order from a disordered state has been studied in a splat-quenched Ni 2 (Cr 0.5 Mo 0.5 ) alloy. The evolution of the order in this alloy follows a continuous ordering mechanism without the need of the superimposition of 〈1 ½ 0〉 type static concentration waves that are believed to be necessary for the evolution of ordered Ni 2 (Cr,Mo) phase (with a Pt 2 Mo type structure) from a short-range order state. The proposed mechanism is based upon the geometrical evidence of the inherent presence of a subunit cell of Pt 2 Mo structure in the N 2 M 2 stacking sequence on {4 2 0} planes in the {1 ½ 0} short range order. Addition of Cr to stoichiometric Ni 2 Mo alloy stabilizes the N 2 M 2 order which in turn stabilizes the Pt 2 Mo state. The stability of the N 2 M 2 stacking sequence and Pt 2 Mo order in the short-range ordered state has been discussed in the light of pair interaction energies.

Investigating the structure of iron–chromium alloys using synchrotron based X-ray microanalysis

The Cr in ferritic steels with high Cr content naturally segregates. Depending on the Cr content, the material can be in a state of short-range order (below approximately 10%Cr) or contain clusters of Cr atoms above this concentration. Using synchrotron based X-ray micro-spectroscopy, SEM imaging and fluorescence mapping, Cr-rich clusters in a high purity Fe–16%Cr alloy are identified and investigated. The Cr local structure is resolved by fitting a simple pure Cr model to extended X-ray absorption fine structure (EXAFS) spectra. The fits indicates that the Debye–Waller factor is higher inside the cluster than in the solid solution, but the nearest neighbor distances are closer to that of the pure Cr model inside the cluster than around it.

Magnetic ordering in the double-layered molecular magnetCu(tetren)[W(CN)8]: Single-crystal study

The results of the single-crystal studies of the quasi-two-dimensional (2D) copper-octacyanotungstenate coordination polymer ${{{({\text{tetrenH}}_{5})}_{0.8}{\text{Cu}}_{4}^{\text{II}}{[{\text{W}}^{\text{V}}{(\text{CN})}_{8}]}_{4}\ensuremath{\cdot}7.2{\text{H}}_{2}\text{O}}}_{n}$ are presented. It was found that the three-dimensional (3D) magnetic ordering at ${T}_{c}\ensuremath{\approx}33\text{ }\text{K}$ gives rise to antiferromagnetic structure which under relatively small magnetic field changes to ferromagnetic. There is strong easy-plane anisotropy confining the magnetic moments to the $ac$ crystallographic plane. Detailed analysis of the scaling behavior of the dc susceptibility above ${T}_{c}$ is performed. For the direction of the external magnetic field parallel to the $ac$ crystallographic plane the ordering process involves one stage only, whereas for the direction parallel to the $b$ crystallographic axis a two-stage process is revealed. The corresponding crossover at about 39 K is from the 2D short-range order state to the 3D long-range one. The well-established short-range order above the transition is consistent with the low value of the entropy amounting to only 15% of the maximal expected value. The scaling behavior of the Berezinski-Kosterlitz-Thouless type has been checked for the temperature dependence of magnetization in the $ac$ plane yielding the value of $\ensuremath{\nu}=0.56$. The spin-flip field is explained by assuming that below the transition temperature there is a domain structure coupled through the dipole-dipole interactions. The ratio of the interbilayer to the intrabilayer exchange interaction was estimated to amount to $1\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}4}$.

Monte Carlo Simulations of Ordering in Ni-Mo–Based Alloys

Monte Carlo simulation of the ordering processes in Ni-Mo–based alloys has been carried out with two distinct sets of pair interaction energies (v1 to v10). In the first case, where the D022 and the N3M structures have nearly equal stability, a state of short-range order showing intensity maxima at {1½0} and equivalent positions results. This is similar to what is observed in binary Ni4Mo alloy. When some of the pair interaction energies are changed to make D022 more stable than N3M, {100}, {110}, and equivalent maxima also appear. This is what is observed when small ternary additions of Al or Mn are made to Ni-Mo.

Specific features of temperature-and strain-induced order-disorder phase transitions

The regularities of temperature-and strain-induced order-disorder phase transitions are determined from analyzing X-ray diffraction data. The results obtained are used to reveal common features in order-disorder transformations induced under energy actions of different types. It is established that, in both cases, the transition from the long-range order state to the short-range order state is accompanied by an increase in the number of defects in the structure of the alloy and the most rapid decrease in the long-range order parameter is associated with the generation of antiphase boundaries and their smearing.

Magnetic instabilities and phase diagram of the double-exchange model in infinite dimensions

Dynamical mean-field theory is used to study the magnetic instabilities and phase diagram of the double-exchange (DE) model with Hund's coupling JH > 0 in infinite dimensions. In addition to ferromagnetic (FM) and antiferromagnetic (AF) phases, the DE model also supports a broad class of short-range ordered (SRO) states with extensive entropy and short-range magnetic order. For any site on the Bethe lattice, the correlation parameter q of an SRO state is given by the average q = ⟨ sin2 (θi/2)⟩, where θi is the angle between any spin and its neighbours. Unlike the FM (q = 0) and AF (q = 1) transitions, the transition temperature of an SRO state with 0 < q < 1 cannot be obtained from the magnetic susceptibility. But a solution of the coupled Green's functions in the weak-coupling limit indicates that an SRO state always has a higher transition temperature than the AF for all fillings p below 1 and even has a higher transition temperature than the FM for 0.26 ⩽ p ⩽ 0.39. For 0.39 < p < 0.73, where both the FM and AF phases are unstable for small JH, an SRO phase has a nonzero transition temperature except close to p = 0.5. As JH increases, the SRO transition temperature eventually vanishes and the FM phase dominates the phase diagram. For small JH, the T = 0 phase diagram of the DE model is greatly simplified by the presence of the SRO phase. An SRO phase is found to have lower energy than either the FM or AF phases for 0.26 ⩽ p < 1. Phase separation (PS) disappears as JH → 0 but appears for any nonzero coupling. For fillings near p = 1, PS occurs between an AF with p = 1 and either an SRO or a FM phase. The stability of an SRO state at T = 0 can be understood by examining the interacting density-of-states, which is gapped for any nonzero JH in an AF but only when JH exceeds a critical value in an SRO state.

Short-range ordered phase of the double-exchange model in infinite dimensions

Using dynamical mean-field theory, we have evaluated the magnetic instabilities and T=0 phase diagram of the double-exchange model on a Bethe lattice in infinite dimensions. In addition to ferromagnetic (FM) and antiferromagnetic (AF) phases, we also study a class of disordered phases with magnetic short-range order (SRO). In the weak-coupling limit, a SRO phase has a higher transition temperature than the AF phase for all fillings p below 1 and can even have a higher transition temperature than the FM phase. At T=0 and for small Hund's coupling J_H, a SRO state has lower energy than either the FM or AF phases for 0.26\le p < 1. Phase separation is absent in the J_H --> 0 limit but appears for any non-zero value of J_H.

Electronic Structure and Catalytic Properties of Transition Metal Nanoparticles: The Effect of Size Reduction

Size reduction of metal particles results in the formation of nanoparticles having short-range order and metastable state.

Ordering and topologically close packed-phase precipitation in a Ni–25at.%Mo–5at.%Al alloy

The quenched in state of short range order in Ni 70Mo 25Al 5 alloy was found to be characterized by intensity maxima at {1 0 0} and equivalent positions in the reciprocal space, in addition to the {1 1 2 0} and equivalent maxima seen in the case of the binary Ni–Mo alloys. Transmission electron microscopy studies of the decomposition of the short range ordered alloy, into two long range ordered phases having the D0 22 and the Pt 2Mo-type structures, at ageing temperatures of 875 and 1000 K have been carried out, in addition to microstructural characterization of the topologically close packed phases, such as the σ-phase, found in both the as solutionised and the aged alloys.

Monte Carlo simulation of ordering transformations in Ni–Mo-based alloys

The quenched in state of short range order (SRO) in binary Ni–Mo alloys is characterized by intensity maxima at {1 1 2 0} and equivalent positions in the reciprocal space. Ternary addition of a small amount of Al to the binary alloy, on the other hand, leads to a state of SRO that gives rise to intensity maxima at {1 0 0} and equivalent, in addition to {1 1 2 0} and equivalent, positions in the selected area electron diffraction patterns. Different geometric patterns of streaks of diffuse intensity, joining the SRO maxima with the superlattice positions of the emerging long range ordered (LRO) structures or in some cases between the superlattice positions of different LRO structures, are observed during the SRO-to-LRO transitions in the Ni–Mo-based and other 1 1 2 0 alloys. Monte Carlo simulations have been carried out here in order to shed some light on the atomic structures of the SRO and the SRO-to-LRO transition states in these alloys.

HRTEM image contrast and atomistic microstructures of long-period ordered Al-rich TiAl alloys.

Long-period superstructures formed in off-stoichiometric L 1(0)-TiAl alloys were investigated by high-resolution transmission electron microscopy (HRTEM). The HRTEM analysis combined with multislice simulation and image processing was carried out to clarify atomistic microstructures of Al5Ti3 and h-Al2Ti ordered states and a short-range ordered (SRO) state in Ti-62.5 at.% Al alloys. Aluminium atoms in the (002) Ti layers form square-, lean rhombus- and fat rhombus-type ordered clusters in the SRO state. The ordered clusters are in contact with each other and form microdomains of various long-period superstructures. The ordered clusters are tiled periodically in a long range to form Al5Ti3 or h-Al2Ti domains and characteristic antiphase boundary structures.

HRTEM Observation of Partially Ordered Long-Period Superstructures in Al-Rich TiAl Alloys

ABSTRACTAtomistic microstructures of long-period superstructures in Al-rich TiAl alloys were investigated by high-resolution transmission electron microscopy. The multi-slice image simulation revealed that Al-atoms in Ti-rich (002) layers of the superstructures are imaged as very bright dots under appropriate observing conditions. By enhancing the contrast of the very bright dots using an image processing technique, partially ordered long-period superstructures were clearly characterized. For the short-range order state, A4B, A2B and A3B type ordered-clusters are generated in the Ti-rich (002) layers. These ordered-clusters are in contact with each other, and form local microdomains of various superstructures, such as Al5Ti3, h-Al2Ti and so on. For the Al5Ti3 and h-Al2Ti ordered states, the ordered-clusters are tiled periodically to form the Al5Ti3 or h-Al2Ti domains and characteristic anti-phase boundary structures.