Korringa Kohn Rostoker Research Articles

The importance of long range Fe–Fe interactions in magnetically ordered 2D Fe0.2Ni0.8/Ni(001) and Fe0.3Ni0.7/Ni(001) alloy systems

We develop a theoretical approach to study the magnonic properties of the 2D ordered surface Fe–Ni alloys with 20% and 30% Fe on a face-centered cubic Ni(001) surface substrate. The surface alloy nanostructures in stable equilibrium are determined by computing the lowest energy configurations of constitutive supercells of the ordered alloys. The magnetic exchange constants of the ground state of the stable systems are calculated to define their Heisenberg Hamiltonians using the DFT Spin-Polarized Relativistic Korringa–Kohn–Rostoker (SPRKKR) method and considering Fe–Fe interactions up to third nearest neighbors. To determine the propagating exchange spin-wave modes, evanescent modes, and the consequent magnonic properties, the spin dynamics of the magnetic surface alloy nanostructures are solved using the phase field matching theory (PFMT). Our results indicate that the Fe–Fe exchange interactions up to third nearest neighbors are vital for accurately describing their magnonics. In particular, the exchange interactions influence the form of dispersion branches for the high-energy spin-wave modes of the Fe–Ni surface alloys considered. Appropriate Green’s functions are also derived from the PFMT method to compute the local densities of states (LDOS) for the atomic sites at the surface boundary. For the case of 20% Fe surface alloy, the Fe–Fe exchange interactions shift the Fe LDOS peaks to lower frequency intervals. We also distinguish the emergence of remarkable LDOS peaks for the he case of the 30% Fe surface alloy at specific Ni sites. These peaks change positions under second and third nearest neighbor Fe–Fe interactions, heralding new accessible spin-wave channels. Our results contribute to establishing a systematic understanding of the magnonics of the magnetic surface alloys of transition metal (TM) materials. This new knowledge will enable the study of the quantum transport of spin waves at interfacial alloy nanojunctions composed of such materials. The present DFT-PFMT theoretical approach is general and can be applied to other surface TM alloy magnetic nanostructures.

Understanding thermal transport in magnesium solid solutions through first-principles approaches and machine learning feature screening

The solid solution phases have a large influence on the thermal conductivity of alloys, but the physics behind this effect is complicated. Existing works mainly utilize experimental methods to study the thermal conductivity of alloys. However, due to the coexistence of multi-phase and insufficient understanding of the microscopic property variations in different alloy systems, it remains unclear how different solute atoms can affect the thermal transport of solid solutions. In this work, we employ first-principles simulations based on the Korringa–Kohn–Rostoker coherent potential approximation method and the Boltzmann transport equation to calculate the thermal conductivity of 38 binary magnesium solid solutions with dilute solute atom concentration. Solute atoms can greatly reduce the thermal conductivity of magnesium. Phonon contribution is found to be less than 10 %. The machine learning feature screening methods are further applied to identify the key properties that influence their thermal conductivity the most. Different from the commonly used Linde's law, which states that the resistivity is proportional to the square of valence difference, the machine learning screening indicates that thermal conductivity is predominantly influenced by six features in total. Among them, the variance of the density of states of s + p, d + f orbitals at Fermi level is highly correlated to the electronic thermal conductivity of magnesium alloys. These correlations underscore the role of virtual bond states formed in magnesium alloys due to the presence of solute atoms. The developed method and uncovered physical mechanism can potentially be applied to other solid solution systems, which is an important step to achieve predictive design of high thermal conductivity metallic alloy.

Theoretical Study of the Magnetic Properties of the SmFe12−xMox (x = 1, 2) and SmFe10Mo2H Compounds

We present theoretical investigations examining the electronic and magnetic properties of the SmFe12−xMox (x = 1, 2) and SmFe10Mo2H compounds, including magneto-crystalline anisotropy, magnetic moments, exchange-coupling parameters, and Curie temperatures. The spin-polarized fully relativistic Korringa–Kohn–Rostoker (SPR-KKR) band structure method has been employed, using the coherent potential approximation (CPA) to deal with substitutional disorder. Hubbard-U correction was applied to the local spin density approximation (LSDA+U) in order to account for the significant correlation effects arising from the 4f electronic states of Sm. According to our calculations, the total magnetic moments increases with H addition, in agreement with experimental data. Adding one H atom in the near-neighbor environment of the Fe 8j site reduces the magnetic moments of Fe 8j and enhances the magnetic moment of Fe 8f. For every investigated alloy, the site-resolved spin magnetic moments of Fe on the 8i, 8j, and 8f sites exhibit the same magnitude sequence, with msFe (8i) > msFe (8j) > msFe (8f). While the addition of H has a positive impact on magneto-crystalline anisotropy energy (MAE), the increase in Mo concentration is detrimental to MAE. The computed exchange-coupling parameters reveal the highest values between the closest Fe 8i spins, followed by Fe 8i and Fe 8j spins, for all investigated alloys. The Curie temperature of the alloys under investigation is increased by decreasing the Mo concentration or by H addition, which is qualitatively consistent with experimental findings.

Magnetic Anisotropy Tailoring by 5d-Doping in (Fe,Co)5SiB2 Alloys

Band-structure calculations were performed using the spin-polarized relativistic Korringa–Kohn–Rostoker (SPR-KKR) band-structure method, determining intrinsic magnetic properties, such as magnetic moments, magnetocrystalline anisotropy energy (MAE), and Curie temperatures, of Fe5−x−yCoxMySiB2 (M = Re, W) alloys. The general gradient approximation (GGA) for the exchange–correlation potential and the atomic sphere approximation (ASA) were used in the calculations. Previous studies have shown that for Fe5SiB2, the easy magnetization direction is in-plane, but it turns axial for Co-doping in the range 1 < x ≤ 2.5 (y = 0). Furthermore, studies have shown that 5d-doping enhances the MAE by enabling the strong spin–orbit coupling of Fe–3d and M–5d states. The aim of the present theoretical calculations was to find the dependence of the anisotropy constant K1 for combined Co- and M-doping, building a two-dimensional (2D) map of K1 for 0 ≤ x ≤ 2 and 0 ≤ y ≤ 1. Similar theoretical 2D maps for magnetization and Curie temperature vs. Co and M content (M = W and Re) were built, allowing for the selection of alloy compositions with enhanced values of uniaxial anisotropy, magnetization, and Curie temperature. The magnetic properties of the Fe4.1W0.9SiB2 alloy that meet the selection criteria for axial anisotropy K1 > 0.2 meV/f.u., Curie temperature Tc > 800 K determined by the mean-field approach, and magnetization µ0Ms > 1 T are discussed.

Theoretical study on the origin of anomalous temperature-dependent electric resistivity of ferromagnetic semiconductor

Employing Korringa–Kohn–Rostoker Green’s function methodology, our investigation elucidates the previously obscure origins of the anomalous temperature-dependent electrical resistivity behavior of (Ga,Mn)As ferromagnetic semiconductors. Phonon and magnon excitations induced by temperature effects are addressed via the coherent potential approximation, while the Kubo–Greenwood formula is employed to compute transport properties. Consequently, the anomalous temperature-dependent electrical resistivity arising from the ferromagnetic–paramagnetic transition is successfully replicated. Our examination of electronic structures and magnetic interactions reveals pivotal roles played by antisite defects and interstitial Mn atoms in governing this behavior. As this approach enables both the estimation of temperature-dependent transport properties and the assessment of underlying mechanisms from a microscopic standpoint, it holds significant potential as a versatile tool across diverse fields.

Soft X-ray Fermi surface tomography of palladium and rhodium via momentum microscopy

Fermi surfaces of transition metals, which describe all thermodynamical and transport quantities of solids, often fail to be modeled by one-electron mean-field theory due to strong correlations among the valence electrons. In addition, relativistic spin–orbit coupling pronounced in heavier elements lifts the degeneracy of the energy bands and further modifies the Fermi surface. Palladium and rhodium, two 4d metals attributed to show significant spin–orbit coupling and electron correlations, are ideal for a systematic and fundamental study of the two fundamental physical phenomena and their interplay in the electronic structure. In this study, we explored the Fermi surface of the 4d noble metals palladium and rhodium obtained via high-resolution constant initial state momentum microscopy. The complete 3D-Fermi surfaces of palladium and rhodium were tomographically mapped using soft X-ray photon energies from 34 eV up to 660 eV. To fully capture the orbital angular momentum of states across the Fermi surface, the Fermi surface tomography was performed using p- and s- polarized light. Applicability and limitations of the nearly-free electron final state model in photoemission are discussed using a complex band structure model supported by experimental evidence. The significance of spin–orbit coupling and electron correlations across the Fermi surfaces will be discussed within the context of the photoemission results. State-of-the-art fully relativistic Korringa–Kohn–Rostoker (KKR) calculations within the one-step model of photoemission are used to support the experimental results.

First-principles calculation of magnetocrystalline anisotropy of Y(Co,Fe,Ni,Cu)[formula omitted] based on full-potential KKR Green’s function method

The performance of permanent magnets YCo5 can be improved by replacing cobalt with other elements, such as iron, copper, and nickel. In order to determine its optimum composition, it is necessary to perform systematic theoretical calculations in a consistent framework. In this study, we calculated the magnetocrystalline anisotropy constant Ku of Y(Co1−x−yFexCuy)3(Co1−zNiz)2 on the basis of the full-potential Korringa–Kohn–Rostoker Green’s function method in conjunction with the coherent potential approximation. The calculated Ku of YCo5 was smaller than the experimental value because of a missing enhancement due to orbital polarization. Although the value of Ku of Y(Co1−x−yFexCuy)3(Co1−zNiz)2 was systematically underestimated compared to their experimental counterparts, the doping effect can be analyzed within a consistent framework. The results have shown that YFe3Co2 has much higher Ku=5.00 MJ/m3 than pristine YCo5 (Ku=1.82 MJ/m3), and that nickel as a stabilization element decreases Ku and magnetization in YFe3(Co1−zNiz)2. However, the anisotropy field of z∼0.5 can compete with the value of YCo5.

Effect of atomic anti-site disorder on the anisotropic magnetoresistance in Fe50Co50 alloys

In order to understand the anti-site disorder effect on the anisotropic magnetoresistance (AMR) of alloys, alloys were studied in this work using the fully relativistic spin-polarized Korringa–Kohn–Rostoker method. The anti-site disorder was modeled by interchanging Fe and Co atoms and treated by the coherent potential approximation. It is found that the anti-site disorder broadens the spectral function and decreases the conductivity. Our work emphasizes that the absolute variations of resistivity under magnetic moment rotation are less affected by atomic disorder. The annealing procedure improves the AMR by reduction of the total resistivity. At the same time, we also find that the fourth-order term in the angular dependent resistivity becomes weaker when the disorder increases, resulting from increased scattering of the states around the band-crossing.

Conservation of the robust ferromagnetic half-conductivity in 110-surface half-Heusler NaCrAs alloy by strains and defects

We have studied the structural, electronic and magnetic properties of non-polar 110-surface of half-Heusler NaCrAs alloy (110-SHH NaCrAs) using the density functional theory and Korringa–Kohn–Rostoker Green’s function method. Our results show that ferromagnetic half-conductivity (FHC) is exhibited by both the bulk and 110-SHH NaCrAs with a large spin-down energy gap of 2.61 eV and 1.70 eV respectively. The bulk high Curie temperature of 1223 K originates from the direct ferromagnetic exchange between nearest-neighbor (NN) Cr and As atoms as well as the Ruderman–Kittel–Kasuya–Yosida-type interaction between NN Cr atoms. Magnetic moment of 3.92 μB per formula unit was obtained for both the bulk and 110-SHH NaCrAs structures. We used biaxial strain to model the lattice mismatch between deposited thin film and the substrate. Our results reveal that FHC is retained in 110-SHH NaCrAs structure with −3% to 10% strain. Our phase diagram suggests that the 110-SHH NaCrAs composed of NaNaAs, NaCrNa, NaCrCr, CrCrAs, AsCrAs and NaAsAs can be synthesized under appropriate conditions. Additionally, electronic structure calculations show that the surface FHC is preserved only in NaNaAs, VcCrAs, CrCrAs, NaCrCr, NaVcAs and AsCrAs terminated surfaces. Overall, our results demonstrate that the half-Heusler NaCrAs alloy is a potential viable spin filter and spin source in spintronics devices.

Theoretical spectral function of CH3NH3PbI3 hybrid perovskite around the Fermi level

Methylammonium lead iodide CH3NH3PbI3 (MAPI) is one of the hybrid organic–inorganic perovskites (HOIPs) widely considered for photovoltaic devices. Since photoemission is possibly the best technique for the experimental determination of the bands, we have calculated photoemission spectra at the main photon energies available at conventional laboratories (He I - 21.2 eV, He II - 40.8 eV) by performing fully relativistic Spin-Polarized Relativistic Korringa–Kohn–Rostoker (SPRKKR) calculations. Similarly, we have studied how s- and p-polarization affect to the calculated spectra. These studies could help to reach a better understanding of photoemission measurements on MAPI.

Equations of state for B2 and bcc Fe1-xSix

Composition, chemical disorder, and magnetism significantly affect the volume and bulk modulus of iron‑silicon (FeSi) alloys at ambient pressure. Here we computed the equations of state for bcc-like (ordered B2 and disordered bcc) Fe-Si alloys up to the inner-core pressure using the first-principles Korringa–Kohn–Rostoker method. Ferromagnetic (FM) and nonmagnetic (NM) states were investigated over a wide compositional range from Fe to FeSi. The present static calculations revealed that magnetism and chemical disorder increased the volume and decreased the bulk modulus also at high pressures. In order to obtain their density and bulk sound velocity under high temperatures relevant to the inner core, we calculated the Helmholtz energy by using the quasi-harmonic approximation with the assumption that the Poisson's ratio of Fe-Si alloys is equivalent to that observed in the inner core. This assumption is valid when the inner core consists of bcc-like Fe-Si. Nevertheless, the obtained density and bulk sound velocity do not match seismological observations unless a temperature gradient inside the inner core is unrealistically large, indicating that the inner core is not a bcc-like Fe-Si alloy.

Effect of Mg Substitution on Structural Stabilization of NaxMnO2 Cathode for Na‐Ion Batteries

As the Li‐ion batteries market encounters numerous obstacles, such as not sufficient production to the demands, there is a need to develop post‐lithium technologies, such as Na‐ion batteries. Na0.67MnO2 cathode material provides high capacity; however, it suffers from phase transitions during the (de)intercalation process. Herein, it is presented that magnesium substitution in Na0.67Mg0.2Mn0.8O2 stabilizes the crystal structure during cycling, which was confirmed by operando X‐ray diffraction measurements as well as the long‐term cycling experiment. It also enhances the electronic conductivity from 9 × 10−6 to 7 × 10−5 S cm−1 and the ionic one from 3 × 10−6 to 4 × 10−5 S cm−1. X‐ray absorption spectroscopy O K‐edge spectra are in line with the enormous exceeding of the theoretical capacity of Na/Na+/NaxMg0.2Mn0.8O2 cell (nonreversible discharge capacity of 310 with 155 mAh g−1 of theoretical capacity). Electronic structure calculations performed by the Korringa–Kohn–Rostoker method in the framework of local spin‐density approximation + U in ordered Na2/3Mg1/3Mn2/3O2 approximant support its semiconducting properties as well as confirm that substitution of Mn with Mg markedly enhances the role of oxygen in constituting electronic states around the Fermi level and its role in the electrochemical process leading to the improved capacity.

Experimental and simulation study of microstructure and magnetic properties of (AlFeMnNi)1−x Nd x

ABSTRACT The high-entropy alloy (Al25Fe30Mn25Ni20)1−x Nd x (all in at.%) (AlFeMnNi)1−x Nd x ) was prepared by vacuum arc melting (AM) and casting method (CM). X-Ray Diffraction (XRD) and Scanning Electron Microscopy, transmission electron microscope and Integrated Energy-Dispersive X-ray spectroscopy were used to study the structure of our system. The (Al25Fe30Mn25Ni20)1−x Nd x crystallises in a Face-centred cube and centred cubic (BCC), depending on the x (x = 0.00, x = 0.01 at.% Nd). The increase of Neodymium (0.01 at.% Nd) content results in the formation of BCC structure. The chemical composition analysis of FCC and BCC crystal structures, their lattice constants demonstrate that the formation of a single BCC solid solution between 760°C and 1480°C. The Korringa–Kohn–Rostoker Method and Green's function combined with coherent potential approximation were used to study the magnetoelectronic properties of (Al25Fe30Mn25Ni20)1−x Nd x . We have obtained the value optimised of lattice parameter 2.9 Å and it is near to that obtained by experimental value using High Score programme and results XRD experimental.

Topological Phase Transitions Driven by Sn Doping in (Mn1−xSnx)Bi2Te4

The antiferromagnetic ordering that MnBi2Te4 shows makes it invariant with respect to the combination of the time-reversal and primitive-lattice translation symmetries, giving rise to its topologically nontrivial nature and a number of fundamental phenomena. At the same time, the possibility to control the electronic and magnetic properties of this system can provide new effective ways for its application in devices. One of the approaches to manipulate MnBi2Te4 properties is the partial substitution of magnetic atoms in the compound with atoms of non-magnetic elements, which inevitably affect the interplay of magnetism and band topology in the system. In this work, we have carried out theoretical modelling of changes in the electronic structure that occur as a result of increasing the concentration of Sn atoms at Mn positions in the (Mn1−xSnx)Bi2Te4 compound both using Korringa–Kohn–Rostoker (KKR) Green’s function method as well as the widespread approach of using supercells with impurity in DFT methods. The calculated band structures were also compared with those experimentally measured by angle-resolved photoelectron spectroscopy (ARPES) for samples with x values of 0, 0.19, 0.36, 0.52 and 0.86. We assume that the complex hybridization of Te-pz and Bi-pz orbitals with Sn and Mn ones leads to a non-linear dependence of band gap on Sn content in Mn positions, which is characterized by a plateau with a zero energy gap at some concentration values, suggesting possible topological phase transitions in the system.

Ab-initio study of tuning the electronic and magnetic properties of [formula omitted] Heusler alloy by Co and Mn compound doping

We report the effects of Mn and Co doping on the electronic properties, magnetic exchange interaction, and Curie temperature of intermetallic Ni2MnGa by Green’s function based Korringa–Kohn–Rostoker method with coherent potential approximation(KKR-CPA). The effect of single and compound doping of Mn and Co at different crystallographic positions on these properties are evaluated by computing the electronic and magnetic structures of Ni2MnGa. The study revealed the possibility of tuning magnetic exchange interaction(Jij) and Curie temperature(TC) upon doping. Moreover, it is noted that doping can stabilize the Jahn–Teller distortion. It is also worth noting that TC responds in a different way with concentration as well as the site of the dopant. This study helps in understanding and realizing the cause for magnetic properties in Ni2MnGa, and experimental peers can also use it for further research on doped Ni2MnGa.

Color optimization of a core–shell nanoparticles layer using machine learning techniques

Neural networks were recently introduced in the field of nanophotonics as an alternative and powerful way to obtain the non-linear mapping between the geometry and composition of arbitrary nanophotonic structures on one hand, and their associated properties and functions on the other. Taking into account the recent advances in the application of the machine learning concept to the design of nanophotonic devices, we employ this tool for the optimization of photonic materials with specific color properties. We train a deep neural network (DNN) to solve the inverse problem, i.e., to obtain the geometrical parameters of the structure that best produce a desired reflected color. The analyzed system is a single layer of core–shell spheres composed of melanin and silica embedded in air, arranged in a hexagonal matrix. The network is trained using a dataset of the three CIE 1976 (L*a*b*) color coordinates obtained from the simulated reflectance spectra of a large set of structures. The direct problem is solved using the Korringa–Kohn–Rostoker method (KKR), widely applied to calculate the optical properties of sphere composites. The color optimization approach used in this work opens up new alternatives for the design of artificial photonic structures with tunable color effects.

Magnetocaloric Effect, Structural, Magnetic and Electronic Properties of High Entropy Alloys AlCoxCr1−xFeNi: First-Principle Calculations and Monte Carlo Simulations

The magnetocaloric effect, electronic structure and magnetic properties of High Entropy Alloys AlCoxCr[Formula: see text]FeNi ([Formula: see text]) were calculated using the Monte Carlo simulation, the Korringa–Kohn–Rostoker method combined with the coherent potential approximation (KKR-CPA), method of linear augmented plane wave (FPLAPW) within generalized gradient approximation (GGA) and modified Becke–Johnson potential (TB-mBJ). Density of states of AlCoxCr[Formula: see text]FeNi are studied and analyzed. The total magnetic moments of magnetic atoms were calculated and compared with theoretical and experimental results. On the other side, the magnetocaloric effect in AlCoxCr[Formula: see text]FeNi was studied by first-principle calculations (FPCs) and Monte Carlo simulations. The magnetic phase transition temperature is estimated. The Curie temperature and exchange integrals were found to decrease with Co substitution. The magnetic entropy changes and relative cooling power are found.

Contrasting role of bismuth doping on the thermoelectric performance of VFeSb half-Heusler

Isoelectronic heavy elemental substitutions are generally preferred for enhancing the phonon scattering in a material without deteriorating its electrical transport. In this work, we demonstrate the efficacy and contrasting role of bismuth, an isoelectronic substitute of antimony, for enhancing the thermoelectric transport in structurally ordered cubic VFeSb half-Heusler. Despite its limited solubility, Bi-substitution at Sb-site, was found to be effective in enhancing the electrical power factor near room temperature. Alongside, synergistic lowering of lattice thermal conductivity was observed due to point defect scattering of phonons by Bi-induced mass and strain fluctuations. The electronic transport properties of nominal compositions and effects of Bi-doping induced disorder were evaluated using the Korringa–Kohn–Rostoker method with a coherent potential approximation (KKR-CPA). A highest thermoelectric figure-of-merit ( zT ) ~ 0.22 at 478 K was attained with improved weighted mobility and quality factor for optimally doped VFeSb 0.98 Bi 0.02 half-Heusler alloy making them prospective compositions for near room temperature thermoelectric applications. • Contrasting role of bismuth as prospective substitute for isovalent antimony in Sb-based half-Heusler. • High-power factor > 3.0 × 10 −3 Wm −1 K −2 at room temperature. • Lattice thermal conductivity reduction by point defect scattering in Bi-substituted VFeSb alloys. • Enhanced weighted mobility, thermoelectric quality factor and zT in Bi-substituted VFeSb alloys.

Ab initio description of disorder effects in layered cathode active materials by the coherent potential approximation

Disorder effects in alloys are usually modeled by averaging various supercell calculations considering different positions of the alloy atoms. This approach, however, is only possible as long as the portion of the individual components of the alloy is sufficiently large. Herein, we present an ab initio study considering the lithium insertion material Li1−x [Ni0.33Co0.33Mn0.33]O2 as model system to demonstrate the power of the coherent potential approximation within the Korringa–Kohn–Rostoker Green’s function method. This approach enables the description of disorder effects within alloy systems of any composition. It is applied in this study to describe the (de-)intercalation of arbitrary amounts of lithium from the cathode active material. Moreover, we highlight that using either fully optimized structures or experimental lattice parameters and atomic positions both lead to comparable results. Our findings suggest that this approach is also suitable for modeling the electronic structure of state-of-the-art materials such as high-nickel alloys.

Structural and magnetic properties of Fe–Al alloys: Ab initio studies

The structural and magnetic properties of Fe100−xAlx (5≤x≤25 at.%) alloys with different structural order are investigated within the framework of density functional theory. Using the Korringa–Kohn–Rostoker Green’s function method with a coherent potential approximation, the equilibrium lattice parameters, ground state energy and shear moduli for D03, B2 and A2 structures are calculated. For all structures, the optimized lattice constant increases and the shear modulus shows a decrease with increasing Al content. The tetragonal magnetostriction constants are estimated by the torque method. The A2 and B2 structures provide a positive contribution to the tetragonal magnetostriction. Using Monte Carlo simulations of the three-dimensional Heisenberg model, Curie temperatures are obtained over a wide concentration range.