Long-range Chemical Order Research Articles

Deformation pseudo-twinning of the L21-ordered intermetallic superlattice

Ordered intermetallic materials are promising candidates for applications in extreme environments due to the strong localized bonding schemes that stabilize the structures against degradation. Here, we investigate the propensity for deformation twinning modes in the L21-ordered MnCu2Al intermetallic by inducing a state of severe plastic deformation. Post-mortem microstructural analysis reveals the presence of meso-scale deformation twins accompanied by a high degree of nano-scale crystalline heterogeneity. The orientation relationship between the parent and twin structures is consistent with activation of the 〈111〉{112} deformation twinning mode. Ab initio generalized stacking fault energy curve calculations corresponding to 〈111〉{112} twinning shear suggest that the magnitude of the twinning partial is 14〈111〉{112} and results in the local formation of a metastable orthorhombic phase within the pseudo-twin. This work highlights the discrepancy between the deformation modes of ordered/disordered structures and promotes an increased awareness for the role that long-range chemical order plays in determining deformation modes.

Interaction models and configurational entropies of binary MoTa and the MoNbTaW high entropy alloy

We introduce a simplified method to model the interatomic interactions of high entropy alloys based on a lookup table of cluster energies. These interactions are employed in replica exchange Monte Carlo simulations with histogram analysis to obtain thermodynamic properties across a broad temperature range. Kikuchi's cluster variation method entropy formalism is applied to directly calculate entropy from statistics on short- and long-range chemical order, and we discuss the convergence of the entropy as clusters of differing size are included. A high temperature series expansion aids in our understanding of the convergence. Computer codes implementing these methods, and supporting data, are freely available on the internet.

Structural Changes of Intermetallic Catalysts under Reaction Conditions

Intermetallic Catalysts Intermetallics with long-range chemical ordering have been demonstrated to be excellent catalysts in many reactions. Their surface structures under real reaction conditions are crucial for understanding the catalytic mechanisms. In article number 2100011 by Xiao Li, Jianxiong Zhao and Dong Su, the structural changes of intermetallic catalysts under reaction conditions are systematically discussed. Strategies to improve the activity and durability of catalysts are also presented.

Surfactant-induced chemical ordering of GaAsN:Bi

We have examined the influence of an incorporating surfactant on chemical ordering in GaAsN:Bi alloys. Epitaxy with a (2 × 1) reconstruction leads to the formation of GaAsN alloys, while the introduction of a Bi flux induces long-range chemical ordering of the {111} planes of GaAsN:Bi. We propose a mechanism in which Bi enhances the alignment of dimer rows along the [110] direction, facilitating N incorporation beneath surface dimers and Bi incorporation between dimer rows to form alternating N-rich and Bi-rich {111} planes. These findings suggest a route to tailoring the local atomic environment of N and Bi atoms in a wide range of emerging dilute nitride-bismide alloys.

Atomistic origin of the thermodynamic activation energy for self-diffusion and order-order relaxation in intermetallic compounds I: analytical approach

The general state of knowledge of diffusion and order-order relaxation in a long-range ordered system is summarised. A consistent atomistic description of self-diffusion and the order-order relaxation process is discussed in terms of effective atomic jump frequencies and the current degree of chemical long-range order. It is demonstrated that the thermodynamic activation energies of self-diffusion and the order-order relaxation can be expressed in terms of the activation energies of more elementary processes. The derived expressions differ from each other indicating that although both processes are controlled by the same vacancy-mediated elementary atomic jumps, the values of their thermodynamic activation energies can be different.

Thermal evolution of nanocrystalline co-sputtered Ni–Zr alloy films: Structural, magnetic and MD simulation studies

Monophasic and homogeneous Ni10Zr7 nanocrystalline alloy films were successfully grown at room temperature by co-sputtering in an indigenously developed three-gun DC/RF magnetron sputtering unit. The films could be produced with long-range crystallographic and chemical order in the alloy, thus overcoming the widely acknowledged inherent proclivity of the glass forming Ni–Zr couple towards amorphization. Crystallinity of these alloys is a desirable feature with regard to improved efficacy in applications such as hydrogen storage, catalytic activity and nuclear reactor engineering, to name a few. Thermal stability of this crystalline phase, being vital for transition to viable applications, was investigated through systematic annealing of the alloy films at 473 K, 673 K and 923 K for various durations. While the films were stable at 473 K, the effect of annealing at 673 K was to create segregation into nanocrystalline Ni (superparamagnetic) and amorphous Ni + Zr (non-magnetic) phases. Detailed analyses of the physical and magnetic structures before and after annealing were performed through several techniques effectual in analyzing stratified configurations and the findings were all consistent with each other. Polarized neutron and X-ray reflectometry, grazing incidence x-ray diffraction, time-of-flight secondary ion mass spectroscopy and X-ray photoelectron spectroscopy were used to gauge phase separation at nanometer length scales. SQUID based magnetometry was used to investigate macroscopic magnetic properties. Simulated annealing performed on this system using molecular dynamic calculations corroborated well with the experimental results. This study provides a thorough understanding of the creation and thermal evolution of a crystalline Ni–Zr alloy.

Atomic scale studies of La/Sr ordering in colossal magnetoresistant La(2-2x)Sr(1+2x)Mn2O7 single crystals.

To date, it is unclear whether chemical order (or disorder) is in any way connected to double exchange, electronic phase separation, or charge ordering (CO) in manganites. In this work, we carry out an atomic resolution study of the colossal magnetoresistant manganite La(2-2x)Sr(1+2x)Mn2O7 (LSMO). We combine aberration-corrected electron microscopy and spectroscopy with spectroscopic image simulations, to analyze cation ordering at the atomic scale in real space in a number of LSMO single crystals. We compare three different compositions within the phase diagram: a ferromagnetic metallic material (x=0.36), an insulating, antiferromagnetic charge ordered (AF-CO) compound (x=0.5), which also exhibits orbital ordering, and an additional AF sample (x=0.56). Detailed image simulations are essential to accurately quantify the degree of chemical ordering of these samples. We find a significant degree of long-range chemical ordering in all cases, which increases in the AF-CO range. However, the degree of ordering is never complete nor can it explain the strongly correlated underlying ordering phenomena. Our results show that chemical ordering over distinct crystallographic sites is not needed for electronic ordering phenomena to appear in manganites, and cannot by itself explain the complex electronic behavior of LSMO.

Anomalous Hall effect in epitaxialL10-Mn1.5Ga films with variable chemical ordering

The anomalous Hall effect (AHE) in perpendicularly magnetized $L{1}_{0}$-${\mathrm{Mn}}_{1.5}$Ga epitaxial films is investigated as a function of the degree of long-range chemical ordering at different temperatures. Our results provide conclusive evidence that the only correct AHE scaling is ${\ensuremath{\rho}}_{\mathrm{AH}}\phantom{\rule{0.16em}{0ex}}=\phantom{\rule{0.16em}{0ex}}{a}_{0}{\ensuremath{\rho}}_{xx0}\phantom{\rule{0.16em}{0ex}}+\phantom{\rule{0.16em}{0ex}}b{\ensuremath{\rho}}_{xx}^{2}$. Based on this scaling, we unambiguously identify the pronounced influence of the chemical ordering on both the intrinsic and extrinsic contributions to the AHE, and significant suppression of the extrinsic anomalous Hall conductivity by phonon and magnon scatterings. Our results help clarify the controversy over the AHE scaling and its microscopic mechanisms in this material. The experiments also demonstrate an approach for the study of the AHE by altering the long-range chemical ordering of magnetic materials.

X-ray absorption spectroscopy of Ru-doped relaxor ferroelectrics with a perovskite-type structure

X-ray absorption near-edge structure and extended x-ray absorption fine structure spectroscopy at the Ru $K$ edge of Ru-doped PbSc${}_{0.5}$Ta${}_{0.5}$O${}_{3}$ (PST-Ru), PbSc${}_{0.5}$Nb${}_{0.5}$O${}_{3}$ (PSN-Ru), and 0.9PbZn${}_{1/3}$Nb${}_{2/3}$O${}_{3}$-0.1PbTiO${}_{3}$ (PZN-0.1PT-Ru) as well as at the Ta ${L}_{3}$ edge of PST-Ru and the Nb $K$ edge of PSN-Ru was applied to study the short- and intermediate-range atomic arrangements in perovskite-type (ABO${}_{3}$) relaxor ferroelectrics. The compounds were also analyzed by complementary Raman scattering, visible/near-visible absorption spectroscopy, and synchrotron x-ray single-crystal diffraction. The results show that Ru is octahedrally coordinated in all three relaxor host matrices but the average oxidation state of Ru in PST-Ru and PSN-Ru is \ensuremath{\sim}4.4, whereas it is \ensuremath{\sim}3.8 in PZN-0.1PT-Ru. In PbSc${}_{0.5}$${B}^{\ensuremath{'}\ensuremath{'}}$${}_{0.5}$O${}_{3}$ (${B}^{\ensuremath{'}\ensuremath{'}}$ = Ta, Nb) Ru substitutes for the ${B}^{\ensuremath{'}\ensuremath{'}}$ cations in the form of isolated point defects, while in PZN-0.1PT-Ru Ru replaces adjacent $A$ and $B$ sites, forming a chainlike structural species of face-sharing elongated octahedra. Chemical 1:1 $B$-site order as well as dynamic $B$O${}_{6}$ tilting is observed around both the Ru dopant and the major ${B}^{\ensuremath{'}\ensuremath{'}}$ cation in PST-Ru and PSN-Ru regardless of the fact that according to x-ray diffraction at ambient conditions, the average structure is cubic with weak or no long-range chemical order. Pb cations are off-center displaced from the prototypic cubic $A$ site for all three compounds and in Ru-doped PbSc${}_{0.5}$${B}^{\ensuremath{'}\ensuremath{'}}$${}_{0.5}$O${}_{3}$ the $B$O${}_{6}$ tilt angle correlates with the degree of coherent $B$-Pb distances.

Ab initioinvestigation of the elastic properties of Ni3Fe

Ab initio alloy theory, formulated within the exact muffin-tin orbitals method in combination with the coherent-potential approximation, is used to determine the elastic properties of Ni-Fe alloys with Fe:Ni ratio 1:3. The interplay between magnetic and chemical effects is investigated by computing the lattice parameters and the single-and polycrystal elastic moduli for different partially ordered structures in the ferro-and paramagnetic states. It is found that the influence of long-range chemical order on the bulk properties strongly depends on the magnetic state. The largest magnetic-order-induced changes are obtained for the chemically ordered L1(2) phase. The ferromagnetic L1(2) system possesses similar to 5.4% larger elastic Debye temperature than the paramagnetic L1(2) phase, which in turn has a similar Theta(D) as the chemically disordered face-centered cubic phase in either the ferro-or paramagnetic state. It is concluded that magnetic ordering has a substantially larger impact on the bulk parameters of Ni3Fe than chemical ordering. The calculated trends are explained based on the electronic structure of nonmagnetic, ferromagnetic, and paramagnetic ordered and disordered phases.

Effects of composition and chemical disorder on the magnetocrystalline anisotropy of FexPt1−x alloys

We perform first-principles calculations of the magnetocrystalline anisotropy energy (MAE) of the L10-like FexPt1−x samples studied experimentally by Barmak and co-workers (see J. Appl. Phys., 98 (2005) 033904). The variation of composition and long-range chemical order in the samples was studied in terms of the coherent potential approximation. In accordance with experimental observations, we find that, in the presence of long-range chemical disorder, Fe-rich samples exhibit a larger MAE than stoichiometric FePt. By considering the site-and species-resolved contributions to the MAE, we infer that the MAE is primarily a function of the degree of completeness of the nominal Fe layers in the L10 FePt structure.

Pt3Zr(0001): A substrate for growing well-ordered ultrathin zirconia films by oxidation

We have studied the surface of pure and oxidized Pt${}_{3}$Zr(0001) by scanning tunneling microscopy (STM), Auger electron microscopy, and density functional theory (DFT). The well-annealed alloy surface shows perfect long-range chemical order. Occasional domain boundaries are probably caused by nonstoichiometry. Pt${}_{3}$Zr exhibits ABAC stacking along [0001]; only the A-terminated surfaces are seen by STM, in agreement with DFT results showing a lower surface energy for the A termination. DFT further predicts a stronger inward relaxation of the surface Zr than for Pt, in spite of the larger atomic size of Zr. A closed ZrO${}_{2}$ film is obtained by oxidation in ${10}^{\ensuremath{-}7}$ mbar O${}_{2}$ at 400 ${}^{\ensuremath{\circ}}$C and post-annealing at $\ensuremath{\approx}800{\phantom{\rule{0.16em}{0ex}}}^{\ensuremath{\circ}}$C. The oxide consists of an O-Zr-O trilayer, equivalent to a (111) trilayer of the fluorite structure of cubic ZrO${}_{2}$, but contracted laterally. The oxide forms a $(\sqrt{19}\ifmmode\times\else\texttimes\fi{}\sqrt{19})R{23}^{\ensuremath{\circ}}$ superstructure. The first monolayer of the substrate consists of Pt and contracts, similar to the metastable reconstruction of pure Pt(111). DFT calculations show that the oxide trilayer binds rather weakly to the substrate. In spite of the O-terminated oxide, bonding to the substrate mainly occurs via the Zr atoms in the oxide, which strongly buckle down toward the Pt substrate atoms if near a Pt position. According to DFT, the oxide has a band gap; STM indicates that the conduction band minimum lies $\ensuremath{\approx}2.3$ eV above ${E}_{F}$.

Absence of long-range chemical ordering in equimolar FeCoCrNi

Equimolar FeCoCrNi alloys have been the topic of recent research as “high-entropy alloys,” where the name is derived from the high configurational entropy of mixing for a random solid solution. Despite their name, no systematic study of ordering in this alloy system has been performed to date. Here, we present results from anomalous x-ray scattering and neutron scattering on quenched and annealed samples. An alloy of FeNi3 was prepared in the same manner to act as a control. Evidence of long-range chemical ordering is clearly observed in the annealed FeNi3 sample from both experimental techniques. The FeCoCrNi sample given the same heat treatment lacks long-range chemical order.

Magnetic Anisotropy and Chemical Order of Artificially Synthesized L10-Ordered FeNi Films on Au–Cu–Ni Buffer Layers

L10-FeNi films were grown by alternate monatomic layer deposition on Au–Cu–Ni buffer layers at several substrate temperatures (Ts), and the relation between the uniaxial magnetic anisotropy energy (Ku) and the long-range chemical order parameter (S) was investigated. A large Ku of (7.0 ±0.2) ×106 erg/cm3 and S of 0.48 ±0.05 were obtained. The value of Ku was larger than those reported previously for artificially synthesized FeNi films. It was first found that both Ku and S increased with Ts, and Ku was roughly proportional to S.

Magnetic Anisotropy and Chemical Order of Artificially Synthesized L10-Ordered FeNi Films on Au–Cu–Ni Buffer Layers

L10-FeNi films were grown by alternate monatomic layer deposition on Au–Cu–Ni buffer layers at several substrate temperatures (Ts), and the relation between the uniaxial magnetic anisotropy energy (Ku) and the long-range chemical order parameter (S) was investigated. A large Ku of (7.0 ±0.2) ×106 erg/cm3 and S of 0.48 ±0.05 were obtained. The value of Ku was larger than those reported previously for artificially synthesized FeNi films. It was first found that both Ku and S increased with Ts, and Ku was roughly proportional to S.

Fabrication of (001) Oriented TMR Film With Highly Ordered $L1_{0}$-Fe(Pd,Pt) Alloy Films by Using a Very Thin Fe Underlayer

Magnetic tunnel junctions with L10 -FePd storage layer which has low perpendicular magnetic anisotropy and lower ordering temperature compared with the case of L10-FePt films were fabricated with using a very thin Fe underlayer. The main results are as follows. 1) The long-range chemical order degree and the full width at half maximum of diffraction peak rocking curve for the FePd(002) for the L10-FePd film with a very thin Fe underlayer onto MgO(100) substrate were more than 0.9 and less than 2.0°. 2) The perpendicular coercive force and magnetic anisotropy field for the L10-FePd film with the very thin Fe underlayer were 300 Oe and 15 kOe. 3) The full width at half maximum of diffraction peak rocking curve for the Fe(Pd,Pt)(002) for the FePt/MgO/FePd film with a very thin Fe underlayer onto MgO(100) substrate was less than 2.0°. 4) The out-of-film-plane hysteresis loop of the FePt/MgO/FePd film with the very thin Fe underlayer shows a clear antiparallel state at the magnetic field from 500 to 1000 Oe between FePt reference layer and FePd storage layer. It is found that the very thin Fe underlayer is effective to obtain the FePt/MgO/FePd film with highly (001) orientation and highly L10 ordered structure.

Control of magnetic anisotropy field of (001) oriented L10-Fe(PdxPt1−x) films for MRAM application

L10-Fe(Pd,Pt) films which are expected to have tunable perpendicular magnetic anisotropy and lower ordering temperature compared with the L10-FePt films were fabricated and investigated in order to realize high-performance Magnetic Random Access Memory (MRAM) with spin-transfer magnetization switching method and Magnetic Domain-Wall Racetrack Memory with current-driven domain wall motion. The main results are as follows: (1) The long-range chemical order degree S for the L10-Fe(Pd,Pt) films with the optimized thermal treatment temperature fabricated on MgO(001) substrate was about 0.8. (2) The perpendicular coercive force and magnetic anisotropy field for the L10-FePd film with the thermal treatment temperature of 600 °C were 300 Oe and 14 kOe respectively, whereas the L10-FePt film with the thermal treatment temperature of 800 °C were 2000 Oe and 102 kOe, respectively. (3) The optimized ordering temperature to obtain L10 single phase decreased continuously from 800°C to 600°C with the increasing Pd content for the Fe(Pd,Pt) films. (4) The perpendicular magnetic anisotropy field for the L10-Fe(Pd,Pt) films decreased continuously from 102 kOe to 14 kOe with the increasing Pd content. It is found that the Fe(Pd,Pd) films which have tunable perpendicular magnetic anisotropy field is one of suitable ferromagnetic material for high-performance magnetic recording devices.

Synthesis and characterization of FePt nanoparticles and FePt nanoparticle/SiO2-matrix composite films

Superparamagnetic face-centered cubic (fcc) FePt nanoparticles were synthesized using a polyol process. The effect of reaction temperature and molar ratio of Fe(CO)5 to Pt(acac)2 on the structure, composition and morphology of nanoparticles has been investigated. The optimum processing condition has been obtained for producing well-monodisperse fcc-phase FePt nanoparticles with the 2:1 molar ratio of Fe-Pt at 220 °C. In order to circumvent the problem of FePt particle coalescence during high temperature annealing for the L10 ordering, FePt nanoparticle/SiO2-matrix composite films have been fabricated by sol–gel method. The experimental results confirm that the amorphous SiO2 matrix effectively inhibits the grain growth and particle aggregation during 700 °C annealing for 1 h. Well-monodisperse face-centered tetragonal (fct) FePt particles embedded in the SiO2 matrix can be obtained with the long-range chemical order parameter S of ~0.74, indicating partially ordered L10 phase transition in FePt/SiO2 composite films. The FePt/SiO2 system exhibits a hysteretic behavior with smaller coercive field of 1,450 Oe. The incomplete phase transition from cubic deredat height maxsium (A1-disordered phase to tetragonal L10-ordered phase) might be responsible for it.

Antiphase boundary-like stacking fault in α″-martensite of disordered crystal structure in β-titanium shape memory alloy

The ordering and antiphase boundary (APB)-like fault found in the α″-martensite of β-Ti shape memory alloys are studied. Long-range chemical ordering was not found, but APB-like faults were observed in every martensite plate studied by transmission electron microscopy. These faults have morphology similar to the APBs observed in ordered phases. The superlattice reflections observed in some previous works were a consequence of multiple diffractions. APB-like faults were not observed in the parent phase, leading to the conclusion that the faults were introduced by the martensite transformation. The fault took the form of a wavy tube running perpendicular to the habit plane. The fault was a ‘transformation-induced APB’ with an additional small displacement due to the pre-existing athermal ω phase. The displacement vector was determined to be [–3/50, −23/50, 1/2]. Geometrical aspects of the formation of APB-like faults are also discussed.

Structural modifications during heating of bulk nanocrystalline FeAl produced by high-pressure torsion

The deformation-induced nanostructure developed during high-pressure torsion of B2 long-range ordered FeAl is shown to be unstable upon heating. The structural changes were analyzed using transmission electron microscopy, differential scanning calorimetry and microhardness measurements. Heating up to 220°C leads to the recurrence of the chemical long-range order that is destroyed during deformation. It is shown that the transition to the long-range-ordered phase evolves in the form of small ordered domains homogeneously distributed inside the nanosized grains. At temperatures between 220 and 370°C recovery of dislocations and antiphase boundary faults cause a reduction in the grain size from 77 to 35nm. Grain growth occurs at temperatures above 370°C. The evolution of the strength monitored by microhardness is discussed in the framework of grain-size hardening and hardening by defect recovery.