Coherent Potential Approximation Research Articles

Physical properties of MnFeAl-based alloys affected by Mn content and annealing

The present study is devoted to the Mn2.4Fe0.8Al0.8 and Mn2FeAl alloys prepared by induction melting and studied in their original and subsequently annealed states. The annealing was carried out at 773 K/100 h and 1073 K/100 h in the argon atmosphere. The microstructure, phase composition, magnetic properties, and atom arrangement are followed with regard to Mn content and annealing conditions.The scanning electron microscopy completed by the energy dispersive X-ray spectroscopy and neutron activation analysis has detected single-phase alloys with compositions close to the nominal ones. Their structure, analyzed by X-ray diffraction, was found to be primitive cubic β-Mn with the lattice parameters of 0.6359(2) nm for Mn2.4Fe0.8Al0.8 and 0.6339(1) nm for Mn2FeAl. The coherent potential approximation calculations and positron annihilation spectroscopy have allowed obtaining an overview to the arrangement of Mn, Fe, and Al atoms in the β-Mn structure and formation of the open volume defects. It is shown that Mn atoms occupy predominantly 8c Wyckoff sites and remaining Mn, Fe and Al atoms occupy 12d sites in equal 1/3 proportion. The open volume defects, vacancies and vacancy clusters, occur in both alloys and both annealed states in a very low concentration. From the magnetic viewpoint, both alloys in the as-prepared state and after annealing at 773 K/100 h are paramagnetic at room temperature with transition to antiferromagnetic state at about 30-40 K. The ac susceptibility measurements have indicated spin glass nature of the Mn2FeAl alloys. The annealing at a higher temperature, 1073 K/100 h, has affected mainly Mn2.4Fe0.8Al0.8 alloy manifesting a weak ferro-/ferrimagnetic contribution at room temperature contributing to a strong magnetic ordering below 42 K.

Phase stability and elastic properties of NbMoTaWMx (M = Al, V, Zr, Tc, Re and Ir) RHEAs: A first-principles assessment

Using the exact muffin-tin orbital (EMTO) method in combination with the coherent potential approximation (CPA), we studied the influence of alloying elements Al, V, Zr, Tc, Re and Ir on the phase stability, lattice parameters, densities, and elastic properties of NbMoTaWMx (M = Al, V, Zr, Tc, Re, Ir; 0 ≤ x ≤ 1.0) refractory high entropy alloys. The predicted lattice parameter and elastic parameters agree with the available experimental and theoretical data. We calculated the atomic size difference (δ), the valence electron concentration (VEC) and the total energy of face-centered cubic (fcc), body-centered cubic (bcc) and hexagonal closed-pack (hcp) structures. We found that all studied NbMoTaWMx (M = Al, V, Zr, Tc, Re, Ir; 0 ≤ x ≤ 1.0) systems have the bcc structure. We showed that Al and Ir decrease the stability of the bcc structure and Zr significantly enlarges the lattice parameter of the host. Vanadium increases the ductility of NbMoTaW system. On the other hand, Al and Ir have strongly non-linear effect on the ductility, with clear minimum at x = 0.55. The present results provide consistent theoretical data and call for experimental investigation for these alloys.

Coherent potential approximation study of impurity effect on monolayer hexagonal boron phosphide

Impurity doping is a necessary technology for the application of semiconductor materials in microelectronic devices. The quantification of doping effects is crucial for controlling the transport properties of semiconductors. Here, taking two-dimensional (2D) hexagonal boron phosphide semiconductor as an example, we employ coherent potential approximation method to investigate the electronic properties of 2D semiconductor materials at low doping concentrations, which cannot be exploited with conventional density function theory. The results demonstrate that the positive or negative impurity potential in 2D semiconductors determines whether it is p-type or n-type doping, while the impurity potential strength decides whether it is shallow-level or deep-level doping. Impurity concentration has important impacts on not only the intensity but also the broadening of impurity peak in band gap. Importantly, we provide the operating temperature range of hexagonal boron phosphide as a semiconductor device under different impurity concentrations and impurity potentials. The methodology of this study can be applied to other 2D semiconductors, which is of great significance for quantitative research on the application of 2D semiconductors for electronic devices.

The effects of V doping on the intrinsic properties of SmFe10Co2 alloys: A theoretical investigation

The present study focuses on the intrinsic properties of the SmFe10Co2-xVx (x = 0–2) alloys, which includes the SmFe10Co2 alloy, one of the most promising permanent magnets with the ThMn12 type of structure due to its large saturation magnetization (µ0Ms = 1.78 T), high Curie temperature (Tc = 859 K), and anisotropy field (µ0Ha = 12 T) experimentally obtained. Unfortunately, its low coercivity (<0.4 T) hinders its use in permanent magnet applications. The effect of V-doping on magnetization, magnetocrystalline anisotropy energy, and Curie temperature is investigated by electronic band structure calculations. The spin-polarized fully relativistic Korringa-Kohn-Rostoker (SPR-KKR) band structure method, which employs the coherent potential approximation (CPA) to deal with substitutional disorder, has been used. The Hubbard-U correction to local spin density approximation (LSDA + U) was used to account for the large correlation effects due to the 4f electronic states of Sm. The computed magnetic moments and magnetocrystalline anisotropy energies were compared with existing experimental data to validate the theoretical approach’s reliability. The exchange-coupling parameters from the Heisenberg model were used for obtaining the mean-field estimated Curie temperature. The magnetic anisotropy energy was separated into contributions from transition metals and Sm, and its relationships with the local environment, interatomic distances, and valence electron delocalization were analyzed. The suitability of the hypothetical SmFe10CoV alloy for permanent magnet manufacture was assessed using the calculated anisotropy field, magnetic hardness, and intrinsic magnetic properties.

Temperature dependence on magnetic moment formation at a Ce impurity in Gd, Tb, and Dy: An intermediate valence model

Based on the influence of the Ce intermediate valence on magnetic properties in metallic systems, we theoretically study the local magnetic moment formation at a Ce impurity diluted in Gd, Tb, and Dy rare earth metals at finite temperatures. To investigate this point, our model considers the functional integral formalism, coherent potential approximation, and extracting relative charge from the Ce 4f-resonance and transferring it to the 5d Ce band at the temperature range from 0K to Curie critical temperature. As a result, our self-consistent calculation, focusing on temperature dependence, shows that the intermediate valence model explains the regimes of the local magnetic moments and magnetic hyperfine fields. Moreover, our results for the temperature dependence on magnetic hyperfine fields at the Ce site in Gd and Tb agree with experimental reports.

Spin transport study in a disordered-metal/ferromagnetic-insulator heterostructure based on full counting statistics within the coherent potential approximation

Spin transport study in a disordered-metal/ferromagnetic-insulator heterostructure based on full counting statistics within the coherent potential approximation

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.

Impurity Resistivity of the Earth's Inner Core

AbstractSeismic observations suggest that the Earth's inner core has a complex structure (e.g., the isotropic layer at the top, innermost inner core, and hemispherical dichotomy). These characteristics are believed to reflect the history of dynamics and temperature profile of the inner core. One critical physical property is the inner core's thermal conductivity. The thermal conductivity of metals can be estimated from their electrical resistivity using the Wiedemann‐Franz law. Recent high‐pressure and temperature experiments revealed that the temperature dependence of electrical resistivity is small for Fe‐Si alloys. The small temperature coefficient means that it is essential to determine the impurity resistivity of Fe alloys to constrain the inner core's thermal conductivity. Therefore, this study systematically calculated the impurity resistivities of 4‐ and 6‐component alloys at inner core pressure by combining the Korringa‐Kohn‐Rostoker method with the coherent potential approximation. As a result, we obtained the thermal conductivity of the inner core to be 150–263 W/m/K. The inner core cannot maintain thermal convection with such a high thermal conductivity, resulting in a flat temperature profile. In materials science, it is widely known that polycrystals soften suddenly at high temperatures a few percent below their melting temperature. If such a pre‐melting occurs in the inner core, the flat temperature profile due to high thermal conductivity causes variations in the attenuation within the inner core. This may explain the observation that the upper inner core is more strongly attenuated than the innermost inner core.

Electronic structure of SrFe1−x(Mn,Co)xO3-δ: A CPA case study

A new methodology is suggested for determining and using the Coulomb interaction parameter U(t2g), taking into account the different energy localizations of t2g and eg states in disordered transition metal perovskite-like oxides, based on the local spin density approximation with Coulomb interaction correction method and the coherent potential approximation. The adjustment of U(t2g)’s relies on the best coincidence of the experimental and calculated maxima in X-ray absorption/reflection spectra at simultaneous demand for the calculated magnetic moments to closely correspond to their experimental values in cubic oxides SrFeO3, SrMnO3, and SrCoO3. Then, the obtained U’s are utilized in order to construct the Green functions for calculating the electronic properties of strongly correlated perovskite-like solid solutions within the coherent potential method. The high efficiency of the proposed methodology has been demonstrated by a close coincidence of the experimentally observed and calculated X-ray emission/absorption spectra, magnetic moments, density of states at the Fermi level, and electronic conductivity in the disordered non-stoichiometric SrFeO3-δ, SrFe0.75Mn0.25O3-δ, and SrFe0.85Co0.15O3-δ phases. The composition SrFe0.85Co0.15O3-δ is proposed as a promising electrode material with enhanced characteristics of electric transport.

Superconductivity in high-entropy alloy system containing Th

Th-containing superconducting high entropy system with the nominal composition (NbTa)_{0.67}(MoWTh)_{0.33} was synthesized. Its structural and physical properties were investigated by X-ray diffraction, scanning electron microscopy, energy dispersive X-ray spectroscopy, specific heat, resistivity and magnetic measurements. Two main phases of alloy were observed: major bcc structure and minor fcc. The experimental results were supported by numerical simulation by the DFT Korringa-Kohn-Rostoker method with the coherent potential approximation (KKR-CPA).

Temperature-dependent models for wave velocity of oil sand

Knowledge of how temperature affects the oil–sand acoustic response is useful to exploit these reservoir rocks with seismic methods. We propose three models: double-porosity coherent potential approximation (CPA), lower-bound Hashin–Shtrikmann (HS-), and contact cement (CC), based on different spatial distributions of heavy oil and temperature and frequency-dependent empirical equations. The shear modulus and S-wave velocity are affected by temperature in all the cases. Moreover, the properties of oil sands with heavy oil as a continuous matrix and higher viscosity are more sensitive to temperature. The models can provide theoretical support for the establishment of rock physical models in heavy oil reservoirs so as to quantitatively characterize the seismic response changes caused by thermal mining.

First-principles study of the phase stability and elastic properties of TiVNbMoM (M = Al, Sc, Ni and Cu) high entropy alloys

The ab initio exact muffin-tin orbitals (EMTO) method in combination with the coherent potential approximation (CPA) were used to study the influence of alloying elements M = Al, Sc, Ni and Cu on the phase stability, lattice constants, elastic constants, polycrystalline elastic moduli and electronic structure of equiatomic and non-equiatomic TiVNbMoM refractory high entropy alloys. The agreement between our results and the available experimental and theoretical data is quite good. It was found that the equiatomic systems are stable in the body-centered cubic (bcc) structure. Alloying elements decrease the stability of the bcc against the face-centered cubic (fcc) and the hexagonal close-packed (hcp) structures. Scandium enlarges the lattice constants of equiatomic and non-equiatomic systems significantly. According to the calculated bulk modulus to shear modulus, Poisson’s ratio and Vickers hardness, all studied equiatomic and non-equiaomic systems are found to be ductile. However, alloying elements Al, Ni and Cu reduce the ductility and improve the hardness of equiatomic and non-equiatomic TiVNbMoM systems, while the ductility (hardness) of non-equiatomic systems enhances (reduces) by substitution with Sc element. The present theoretical results provide insight for the design and improvement of high entropy alloys and complete information on the alloying effects.

Enhanced Superconducting Critical Parameters in a New High-Entropy Alloy Nb0.34Ti0.33Zr0.14Ta0.11Hf0.08.

The structural and physical properties of the new titanium- and niobium-rich type-A high-entropy alloy (HEA) superconductor Nb0.34Ti0.33Zr0.14Ta0.11Hf0.08 (in at.%) were studied by X-ray powder diffraction, energy dispersive X-ray spectroscopy, magnetization, electrical resistivity, and specific heat measurements. In addition, electronic structure calculations were performed using two complementary methods: the Korringa-Kohn-Rostoker Coherent Potential Approximation (KKR-CPA) and the Projector Augmented Wave (PAW) within Density Functional Theory (DFT). The results obtained indicate that the alloy exhibits type II superconductivity with a critical temperature close to 7.5 K, an intermediate electron-phonon coupling, and an upper critical field of 12.2(1) T. This finding indicates that Nb0.34Ti0.33Zr0.14Ta0.11Hf0.08 has one of the highest upper critical fields among all known HEA superconductors.

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 3d transition metal impurities doping on electronic and magnetic properties of CeO2

The effect of transition metal (TM) impurities, namely Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Cu and Zn, on the electronic and magnetic properties of Ceria dioxide is studied using the Korringa-Kohn-Rostoker method combined with the coherent potential approximation (KKR-CPA). Calculations indicate that pure CeO2 is a nonmagnetic indirect semiconductor. Doping this compound with transition metals results in three main groups based on their electromagnetic behavior. The first group involves CeO2 doped with Sc, Ti and Zn which show a lack of magnetism at low concentrations. The second group of TMs, including V, Mn, Fe and Ni, induces half-metallic character and ferromagnetic behavior. Finally, when Cr, Co and Cu are substituted in CeO2, this doping creates magnetic semiconductors with a direct band gap. The origin of the magnetic states is further discussed and the Curie temperatures are determined. The present work shows that controlling impurity type and concentration renders Cerium dioxide a potential candidate for spintronic applications.

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.

First-principles study on the effects of atomic configuration on the magnetic anisotropy energy of (Fe,Co)16(N,C)2 alloys

Herein, the first-principles technique combined with a coherent potential approximation is utilized to investigate the magnetic anisotropy energy (MAE) of (Fe1-xCox)16(N,C)2 alloys. The alloys contain an ordered α″-phase and partially ordered α′-phase, where α-Fe is distorted to form a bct structure with c/a = 1.1 owing to the random intrusion of (N, C) atoms. For the α″-phase, the MAE is sensitive to Co substitution sites and reaches approximately 3 MJ/m3 at an x of 0.25 when Co atoms occupy the 4d sites. For the α′-phase, the lattice distortion c/a = 1.1 alone cannot provide a positive MAE when x = 0 (FeN0.125 and FeC0.125). Evidently, both Co substitution and further increasing the c/a over 1.1 must be performed to realize a positive MAE in the α'-phase. Finally, the importance of local distortion of the (Fe, Co) lattice in further increasing the c/a beyond 1.1 is discussed as it increases the effective c/a even when the global c/a is fixed at 1.1.

Predicting elastic properties of refractory high-entropy alloys via machine-learning approach

Refractory high entropy alloy (RHEA) is an important family of alloy with excellent mechanical properties and broad industrial applications. Different chemical and stoichiometric compositions make RHEA a highly tunable material with a huge space for optimization. However, the virtual screening of RHEA is extremely nontrivial, considering the high cost of conventional first-principles calculations. In this work, we compute the ground state and the elastic properties of 2487 RHEAs using exact muffin-tin orbital method combined with coherent potential approximation (EMTO-CPA). Using these data, we train an accurate and robust random forest model that can predict the elastic properties of RHEA systems much more efficiently. Furthermore, we also examine the correlation between seven independent input features and the elastic properties of the alloy. Through this analysis, we identify the most important features that control the elastic properties of RHEA, paving the road to a rational design of the RHEA materials.

Properties of electronic and magnetic states of ternary (MgA)O diluted magnetic insulators (A = Cr, Mn and Co)

Diluted magnetic states of the ternary compounds (Mg1-xAx)O are calculated by the method of Korringa-Kohn-Rostoker (KKR) Green's function, where A and x denote the Cr, Mn, and Co atoms with fractional concentration. The fractional impurities are controlled with the coherent potential approximation (CPA). The total energy of the ferromagnetic (FM) and local moment disordered (LMD) states are calculated for the compounds and found them stable in FM structure. The difference of energy between the FM and the LMD states for each unit cell is utilized to estimate the Curie temperature (TC) within framework of the mean-field approximation (MFA). The values of TC for (Mg1-xMnx)O is higher than that of the room temperature (RT) for x = 3%–15% of Mn dopant and follow a decreasing trend with dopant concentrations. Moreover, the low TC is reported for Cr, and Co doped compounds. Furthermore, the induced local moments and the half-metallic conditions of the compounds are obtained, where the magnetic atoms act as the sole contributor for the carrier of magnetic moments.

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.