Evidence for a large Rashba splitting in PtPb[subscript 4] from angle-resolved photoemission spectro
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Lee, Kyungchan↗
Engineering topics
Nguyen, Manh Cuong
Publications and source records attributed to Nguyen, Manh Cuong.
Spatial decomposition of magnetic anisotropy in magnets: Application to doped Fe 16 N 2
We propose a scheme of decomposition of the total relativistic energy in solids to intra- and interatomic contributions. The method is based on a site variation of such fundamental constant as the speed of light. As a practical illustration of the method, we tested such decomposition in the case of a spin-orbit interaction variation for the decomposition of the magnetic anisotropy energy (MAE) in CoPt. We further studied the α" - Fe 16 N 2 magnet doped by Bi, Sb, Co, and Pt atoms. It was found that the addition of Pt atoms can enhance the MAE by as much as five times while Bi and Sb substitutions double the total MAE. Using the proposed technique, we demonstrate the spatial distribution of these enhancements. Our studies also suggest that Sb, Pt, and Co substitutions could be synthesized by experiments.
75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
Handbook of Materials Modeling: Chapter title - Adaptive Genetic Algorithm for Structure Prediction and Application to Magnetic Materials
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Zhao, Xin↗
Concentration-tuned tetragonal strain in alloys: Application to magnetic anisotropy of $\mathrm{FeNo}_{1-x}\mathrm{Co}_x$
In this paper, we explore an opportunity to induce and control tetragonal distortion in materials. The idea involves formation of a binary alloy from parent compounds having body-centered and face-centered symmetries. The concept is illustrated in the case of $\mathrm{FeNo}_{1-x}\mathrm{Co}_x$ magnetic alloy formed by substitutional doping of the $L1_0$ FeNi magnet with Co. Using electronic structure calculations we demonstrate that the tetragonal strain in this system can be controlled by concentration and it reaches maximum for $x = 0.5$. This finding is then applied to create a large magnetocrystalline anisotropy (MAE) in $\mathrm{FeNo}_{1-x}\mathrm{Co}_x$ system by considering an interplay of the tetragonal distortion with electronic concentration and chemical anisotropy. In particular, we identify an ordered $\mathrm{FeNi_{0.5}Co_{0.5}}$ system with MAE larger by a factor 4.5 from the $L1_0$ FeNi magnet.
36 MATERIALS SCIENCE↗
Quantum phase transition and ferromagnetism in C o 1 + x Sn
The onset of ferromagnetism in cobalt-tin alloys is investigated experimentally and theoretically. The Co 1+x Sn alloys were prepared by rapid quenching from the melt and form a modified hexagonal NiAs-type crystal structure for 0.45 ≤ x ≤ 1. The magnetic behavior is described analytically and by density-functional theory using supercells and the coherent-potential approximation. The excess of Co concentration x, which enters the interstitial 2d sites in the hypothetical NiAs-ordered parent alloy CoSn, yields a Griffiths-like phase and, above a quantum critical point (x c ≈ 0.65), a quantum phase transition to ferromagnetic order. Quantum critical exponents are determined on the paramagnetic and ferromagnetic sides of the transition and related to the nature of the magnetism in itinerant systems with different types of chemical disorder.
75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
First-principles study, fabrication and characterization of (Zr 0.25 Nb 0.25 Ti 0.25 V 0.25 )C high-entropy ceramics
The formation possibility of a new (Zr 0.25 Nb 0.25 Ti 0.25 V 0.25 )C high-entropy ceramics (ZHC-1) was first analyzed by the first-principles calculations and thermodynamical analysis and then it was successfully fabricated by hot pressing sintering technique. The first-principles calculation results showed that the mixing enthalpy of ZHC-1 was 5.526 kJ/mol and the mixing entropy of ZHC-1 was in the range of 0.693R–1.040R. The thermodynamical analysis results showed that ZHC-1 was thermodynamically stable above 959 K owing to its negative mixing Gibbs free energy. The experimental results showed that the as-prepared ZHC-1 (95.1% relative density) possessed a single rock-salt crystal structure, some interesting nano-plate-like structures, and high compositional uniformity from nanoscale to microscale. By taking advantage of these unique features, compared with the initial metal carbides (ZrC, NbC, TiC and VC), it showed a relatively low thermal conductivity of 15.3 ± 0.3 W/(m·K) at room temperature, which was due to the presence of solid solution effects, nanoplates and porosity. Meanwhile, it exhibited the relatively high nanohardness of 30.3 ± 0.7 GPa and elastic modulus of 460.4 ± 19.2 GPa and the higher fracture toughness of 4.7 ± 0.5 MPam 1/2 , which were attributed to the solid solution strengthening mechanism and nanoplate pullout and microcrack deflection toughening mechanism.
36 MATERIALS SCIENCE↗
Single-Crystal Permanent Magnets: Extraordinary Magnetic Behavior in the Ta-,Cu-, and Fe-Substituted CeCo 5 Systems
To reduce material and processing costs of commercial permanent magnets and to attempt to fill the empty niche of energy products, 10–20 MGOe, between low-flux (ferrites, alnico) and high-flux (Nd 2 Fe 14 B- and SmCo 5 -type) magnets, we report the synthesis, structure, magnetic properties, and modeling of Ta-, Cu-, and Fe-substituted CeCo 5 . Using a self-flux technique, we grow single crystals of Ce 15.1 Ta 1.0 Co 74.4 Cu 9.5 , Ce 16.3 Ta 0.6 Co 68.9 Cu 14.2 , Ce 15.7 Ta 0.6 Co 67.8 Cu 15.9 , Ce 16.3 Ta 0.3 Co 61.7 Cu 21.7 , and Ce 14.3 Ta 1.0 Co 62.0 Fe 12.3 Cu 10.4 . X-ray-diffraction analysis shows that these materials retain a CaCu 5 substructure and incorporate small amounts of Ta in the form of “dumbbells,” filling the 2e crystallographic sites within the one-dimensional hexagonal channel with the 1a Ce site, whereas Co, Cu, and Fe are statistically distributed among the 2c and 3g crystallographic sites. Scanning-electron-microscopy, energy-dispersive-x-ray-spectroscopy, and scanning-transmission-electron-microscopy examinations provide strong evidence of the single-phase nature of the as-grown crystals, even though they readily exhibit significant magnetic coercivities of approximately 1.6 kOe to approximately 1.8 kOe caused by Co-enriched, nanosized structural defects and faults that can serve as pinning sites. Heat treatments at 1040°C for 10 h and hardening at 400°C for 4 h lead to the formation of a so-called composite crystal with a bimodal microstructure that consists of a Ta-poor matrix and Ta-rich laminal precipitates. Formation of the composite crystal during the heat treatment creates a three-dimensional array of extended defects within a primarily single-grain single crystal, which greatly improves its magnetic characteristics. Possible causes for the formation of the composite crystal may be associated with Ta atoms leaving matrix interstices at lower temperatures and/or matrix degradation induced by decreased miscibility at lower temperatures. Furthermore Fe strongly increases both the Curie temperature and magnetization of the system resulting, in ( BH ) max ≈ 13 MGOe at room temperature.
36 MATERIALS SCIENCE↗
Pressure-induced multiple phase transformations of the BaBi 3 superconductor
Measurements of temperature-dependent resistance and magnetization under hydrostatic pressures up to 2.13 GPa are reported for single-crystalline, superconducting BaBi 3 . A temperature-pressure phase diagram is determined and the results suggest three different superconducting phases α, β, and γ in the studied pressure range. We further show that the occurrence of the three superconducting phases is intuitively linked to phase transitions at higher temperature, which are likely first order and structural in nature. With the α phase being the ambient-pressure tetragonal structure ( P 4/mmm), our first-principles calculations suggest the β phase has a cubic structure ( P m–3m) and the γ phase has a distorted tetragonal structure where the Bi atoms are moved out of the face-centered position. Finally, an analysis of the evolution of the superconducting upper critical field with pressure further confirms these transitions in the superconducting state and suggests a possible change of band structure or a Lifshitz transition near 1.54 GPa in γ phase. As a result, given the large atomic numbers of both Ba and Bi, our results establish BaBi 3 as a good candidate for the study of the interplay of structure with superconductivity in the presence of strong spin-orbit coupling.
75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
Is it possible to stabilize the 1144-phase pnictides with tri-valence cations?
A lately discovered 1144 phase has generated significant interest for its high superconducting temperatures, disorder-free doping, and various chemical substitutions. However, it has only been found in iron arsenides ( AB Fe 4 As 4 ), and cations are limited to +1 or +2 valence states (e.g., alkali metals, alkaline earth elements, and Eu). Whether more 1144 phases could be stabilized and whether intriguing properties exist are questions of general interest. In this work, we investigate 1144 iron and cobalt arsenides with tri-valence cations (La, Y, In, Tl, Sm, Gd). We study phase stability among other competing phases: 122 solution phase and phase decomposition. With La as the cation, we predict room-temperature stable 1144 structures: La A Fe 4 As 4 ( A =K, Rb, and Cs). Other La-contained 1144 structures tend to form solution phase. The solubility of La is estimated and compared with the experiment. By contrast, we do not find stable 1144 structures with Y as the cation. For In and Tl as cations, two 122-phase compounds are remarkably stable: InCo 2 As 2 and TlCo 2 As 2 , which adds to our knowledge about the In(Tl)-Co-As phase diagram. Stable 1144 phases are found in InKCo4As4 and InRbCo 4 As 4 . With Sm and Gd as cations, 1144- or 122-phase iron arsenides are generally unstable. Among structures investigated, we recognize two critical factors for 1144-phase stability: size effect and charge balance, which yields a merging picture with the rule found in previous 1144 systems. Furthermore, La A Fe 4 As 4 ( A =K, Rb, and Cs), InCo 2 As 2 , and TlCo 2 As 2 are exhibiting semimetal features and a two-dimensional Fermi surface, similar to iron superconductors.
36 MATERIALS SCIENCE↗
Structures and magnetic properties of iron silicide from adaptive genetic algorithm and first-principles calculations
We performed a systematic search for low-energy structures of binary iron silicide over a wide range of compositions using the crystal structure prediction method based on adaptive genetic algorithm. 36 structures with formation energies within 50 meV/atom (11 of them are within 20 meV) above the convex hull formed by experimentally known stable structures are predicted. Magnetic properties of these low-energy structures are investigated. Some of these structures can be promising candidates for rare-earth-free permanent magnet.
36 MATERIALS SCIENCE↗
Magnetocrystalline anisotropy in cobalt based magnets: a choice of correlation parameters and the relativistic effects
The dependence of the magnetocrystalline anisotropy energy (MAE) in MCo 5 (M = Y, La, Ce, Gd) and CoPt on the Coulomb correlations and strength of spin orbit (SO) interaction within the GGA + U scheme is investigated. A range of parameters suitable for the satisfactory description of key magnetic properties is determined. We show that for a large variation of SO interaction the MAE in these materials can be well described by the traditional second order perturbation theory. We also show that in these materials the MAE can be both proportional and negatively proportional to the orbital moment anisotropy (OMA) of Co atoms. Dependence of relativistic effects on Coulomb correlations, applicability of the second order perturbation theory for the description of MAE, and effective screening of the SO interaction in these systems are discussed using a generalized virial theorem. Finally, such determined sets of parameters of Coulomb correlations can be used in much needed large scale atomistic simulations.
75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
Using first-principles calculations to screen for fragile magnetism: Case study of LaCrGe 3 and LaCrSb 3
In this study, we present a coupled experimental/theoretical investigation of pressure effect on the ferromagnetism of LaCrGe 3 and LaCrSb 3 compounds. The magnetic, electronic, elastic, and mechanical properties of LaCrGe 3 and LaCrSb 3 at ambient condition are studied by first-principles density-functional theory calculations. The pressure dependences of the magnetic properties of LaCrGe 3 and LaCrSb 3 are also investigated. The ferromagnetism in LaCrGe 3 is rather fragile, with a ferro- to paramagnetic transition at a relatively small pressure (around 7 GPa from our calculations, and 2 GPa in experiments). The key parameter controlling the magnetic properties of LaCrGe 3 is found to be the proximity of the peak of Cr density of states to the Fermi level, a proximity that is strongly correlated with the distance between Cr atoms along the c axis, suggesting that there would be a simple way to suppress magnetism in systems with one-dimensional arrangement of magnetic atoms. By contrast, the ferromagnetism in LaCrSb 3 is not fragile. In conclusion, our theoretical results are consistent with our experimental results and demonstrate the feasibility of using first-principles calculations to aid experimental explorations in screening for materials with fragile magnetism.
36 MATERIALS SCIENCE↗
Fe–Si networks and charge/discharge-induced phase transitions in Li 2 FeSiO 4 cathode materials
Here, structural phase transitions of electrode materials are responsible for poor reversibility during charge/discharge cycling in Li-ion batteries.
25 ENERGY STORAGE↗
Stability of the 1144 phase in iron pnictides
A series of iron arsenides (e.g., CaRbFe 4 As 4 , SrCsFe 4 As 4 ) have been discovered recently, and have provoked a rise in superconductor searches in a different phase, known as the 1144 phase. For the presence of various chemical substitutions, it is believed that more 1144 compounds remain to be discovered. Here in this work, we perform general model analysis as well as scenario calculation on a basis of density functional theory to investigate phase stability in a variety of compounds. We predict that the 1144-type phase could be stabilized in EuKFe 4 As 4 , EuRbFe 4 As 4 , EuCsFe 4 As 4 , CaCsFe 4 P 4 , SrCsFe 4 P 4 , BaCsFe 4 P 4 , InCaFe 4 As 4 , InSrFe 4 As 4 , etc. Remarkably, it involves rare earths, trivalence elements (e.g., indium) and iron phosphides, which greatly expands the range of its existence and suggests a promising prospect for experimental synthesis. In addition, we find that the formation of many random doping compounds (e.g., Ba 0.5 Cs 0.5 Fe 2 As 2 , Ba 0.5 Rb 0.5 Fe 2 As 2 ) is driven by entropy and could be annealed to a 1144-type phase. Eventually, we plot a phase diagram about two structural factors Δa and Δc, giving a bird's-eye view of stability of various 1144 compounds.
75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
New structures of Fe3S for rare-earth-free permanent magnets
We applied adaptive genetic algorithm (AGA) to search for low-energy crystal structures of Fe 3 S. A number of structures with energies lower than that of the experimentally reported Pnma and I-4 structures have been obtained from our AGA searches. These low-energy structures can be classified as layer-motif and column-motif structures. In the column-motif structures, Fe atoms self-assemble into rods with bcc type of underlying lattice, which are separated by the holes terminated by S atoms. In the layer-motif structures, the bulk Fe is broken into slabs of several layers passivated by S atoms. Magnetic properties calculations showed that the column-motif structures exhibit reasonably high uniaxial magnetic anisotropy. In addition, we examined the effect of Co doping to Fe 3 S and found magnetic anisotropy can be enhanced through Co doping.
36 MATERIALS SCIENCE↗
Elastic and electronic tuning of magnetoresistance in MoTe 2
Quasi–two-dimensional transition metal dichalcogenides exhibit dramatic properties that may transform electronic and photonic devices. We report on how the anomalously large magnetoresistance (MR) observed under high magnetic field in MoTe 2 , a type II Weyl semimetal, can be reversibly controlled under tensile strain. The MR is enhanced by as much as ~30% at low temperatures and high magnetic fields when uniaxial strain is applied along the a crystallographic direction and reduced by about the same amount when strain is applied along the b direction. We show that the large in-plane electric anisotropy is coupled with the structural transition from the 1T' monoclinic to the T d orthorhombic Weyl phase. A shift of the T d -1T' phase boundary is achieved by minimal tensile strain. The sensitivity of the MR to tensile strain suggests the possibility of a nontrivial spin-orbital texture of the electron and hole pockets in the vicinity of Weyl points. Our ab initio calculations show a significant orbital mixing on the Fermi surface, which is modified by the tensile strains.
36 MATERIALS SCIENCE↗
Cluster-Expansion Model for Complex Quinary Alloys: Application to Alnico Permanent Magnets
An accurate and transferable cluster-expansion model for complex quinary alloys is developed. Lattice Monte Carlo simulation enabled by this cluster-expansion model is used to investigate temperature-dependent atomic structure of alnico alloys, which are considered as promising high-performance non-rare-earth permanent-magnet materials for high-temperature applications. The results of the Monte Carlo simulations are consistent with available experimental data and provide useful insights into phase decomposition, selection, and chemical ordering in alnico. The simulations also reveal a previously unrecognized D 0 3 alloy phase. This phase is very rich in Ni and exhibits very weak magnetization. In conclusion, manipulating the size and location of this phase provides a possible route to improve the magnetic properties of alnico, especially coercivity.
36 MATERIALS SCIENCE↗
Prediction of novel stable Fe-V-Si ternary phase
Genetic algorithm searches based on a cluster expansion model are performed to search for stable phases of Fe-V-Si ternary. Here, we identify a new thermodynamically, dynamically and mechanically stable ternary phase of Fe 5 V 2 Si with 2 formula units in a tetragonal unit cell. The formation energy of this new ternary phase is -36.9 meV/atom below the current ternary convex hull. The magnetic moment of Fe in the new structure varies from -0.30-2.52 μ B depending strongly on the number of Fe nearest neighbors. The total magnetic moment is 10.44 μ B /unit cell for new Fe 5 V 2 Si structure and the system is ordinarily metallic.
36 MATERIALS SCIENCE↗