Showing papers by "Mikhail A. Zagrebin published in 2020"

Coulomb correlation in noncollinear antiferromagnetic α -Mn

Abstract: We discuss the interplay between magnetic and structural degrees of freedom in elemental Mn. The equilibrium volume is shown to be sensitive to magnetic interactions between the Mn atoms. While the standard generalized gradient approximation underestimates the equilibrium volume, a more accurate treatment of the effects of electronic localization and magnetism is found to solve this longstanding problem. Our calculations also reveal the presence of a magnetic phase in strained $\ensuremath{\alpha}$-Mn that has been reported previously in experiments. This new phase of strained $\ensuremath{\alpha}$-Mn exhibits a noncollinear spin structure with large magnetic moments.

Effects of magnetic and structural phase transitions on the normal and anomalous Hall effects in Ni-Mn-In-B Heusler alloys

Abstract: Magnetization, electrical resistivity, magnetoresistance, and Hall resistivity of Ni50Mn35In14.25B0.75 and Ni50Mn35In14.5B0.5 Heusler alloys were studied in a temperature range T = 80-400 K in magnetic fields up to 20 kOe. Both alloys exhibit a martensitic transformation from a high-temperature ferromagnetic austenite phase to a low-temperature, low-magnetization martensitic phase. The electrical resistivity nearly doubles as a result of the martensitic transformation, reaching 180 and 100 μΩcm in the martensitic states of Ni50Mn35In14.25B0.75 and Ni50Mn35In14.5B0.5, respectively. The temperature dependence of the electrical resistivity does not corresponded with the Mooij correlation. The magnetoresistance is negative with a narrow negative peak at the martensitic transition. Normal and anomalous Hall effect coefficients were determined by fitting the field dependencies of the Hall resistivity using magnetization data. The coefficients of the normal Hall effect for both compositions were found to decrease with temperature from positive values in the austenite to negative values in the martensite phase. None of the known correlations between the anomalous Hall effect coefficient and resistivity were satisfied. Significant changes in the values of the anomalous Hall coefficients during the martensitic transformation are explained by the difference in spin-up and spin-down state

Electronic structure beyond the generalized gradient approximation for Ni 2 MnGa

Abstract: The stability of the nonmodulated martensitic phase, the austenitic Fermi surface, and the phonon dispersion relations for ferromagnetic ${\mathrm{Ni}}_{2}\mathrm{MnGa}$ are studied using density functional theory. Exchange-correlation effects are considered with various degrees of precision, starting from the simplest local spin density approximation (LSDA), then adding corrections within the generalized gradient approximation (GGA), and finally, including the meta-GGA corrections within the strongly constrained and appropriately normed (SCAN) functional. We discuss a simple procedure to reduce a possible overestimation of magnetization and underestimation of nesting vector in SCAN by parametrically decreasing self-interaction corrections.

Prediction of giant magnetocaloric effect in Ni40Co10Mn36Al14 Heusler alloys: An insight from ab initio and Monte Carlo calculations

Abstract: In this work, based on limited experimental magnetocaloric data for Ni–Co–Mn–Al Heusler alloys, we present a theoretical study to predict a composition with higher magnetocaloric properties. By analogy with Ni–Co–Mn–(In, Sn) alloys exhibiting a large magnetization change across the structural transformation, we suppose that the addition of 10 at. % Co in Ni–Mn–Al would yield a similar trend. Our approach is based on the combination of ab initio calculations and Monte Carlo simulations within the framework of the Potts–Blume–Emery–Griffiths model. It follows from ab initio calculations that Co addition modifies the exchange interactions and enhances the ferromagnetism in austenite, while for martensite, the ferromagnetism is substantially suppressed due to the strongest antiferromagnetic Mn–Mn interactions. Thermo-magnetization curves and magnetocaloric properties under magnetic fields of 0.5 and 2 T are simulated by the Monte Carlo method assuming the ab initio exchange-interaction parameters. A large change in magnetization of approximately 100 A m 2 kg − 1, leading to a giant magnetocaloric effect ( Δ T a d ≈ − 7 K) across the martensite–austenite transformation, is predicted.

Exchange-correlation corrections for electronic properties of half-metallic Co2FeSi and nonmagnetic semiconductor CoFeTiAl

Abstract: We consider two cobalt-based full-Heusler compounds, CoFeTiAl and Co 2FeSi, for which Coulomb correlation effects play an important role. Since the standard GGA scheme does not provide a precise description of the electronic properties near the Fermi level, we use a meta-GGA functional capable to improve the description of the electronic properties of CoFeTiAl and Co 2FeSi. In particular, we find a better agreement with the experiment for the magnetic moment and the energy bandgap. Moreover, our calculations show that pressure enhances the insulating properties of Co 2FeSi and CoTiFeAl.

Theoretical Approach to Investigation of the Magnetic and Magnetocaloric Properties of Heusler Ni–Mn–Ga Alloys

Abstract: The magnetic and magnetocaloric properties of Heusler Ni2 + xMn1 – xGa alloys (x = 0.16, 0.18, and 0.3) have been studied using a model based on the Malygin theory of smeared phase transitions, the Bean–Rodbell theory of first-order phase transitions, and the mean-field theory. The temperature dependences of strain, magnetization, and isothermal change in entropy of these alloys have been analyzed. It is shown that the largest change in the magnetic entropy is observed in a Ni2.18Mn0.82Ga alloy, in which the martensitic transition is accompanied by a change in the magnetic ordering. The smallest change in the entropy is observed in a Ni2.3Mn0.7Ga alloy, which exhibits the magnetocaloric effect in a martensitic phase with a change in the magnetic ordering. However, the refrigeration capacity of this alloy is twice as high as that of the other considered compositions.

Exchange-correlation corrections for electronic properties of half-metallic Co$_2$FeSi and nonmagnetic semiconductor CoFeTiAl.

Abstract: We consider two cobalt-based full-Heusler compounds CoFeTiAl and Co$_2$FeSi, for which Coulomb correlation effects play an important role. Since the standard GGA scheme does not provide a precise description of the electronic properties near the Fermi level, we use a meta-GGA functional capable to improve the description of the electronic properties of CoFeTiAl and Co$_2$FeSi. In particular, we find a better agreement with the experiment for the magnetic moment and the energy-band gap. Moreover, our calculations show that pressure enhances the insulating properties of Co$_2$FeSi and CoTiFeAl.

Superconducting and antiferromagnetic properties of dual-phase V3Ga

Abstract: The binary compound V3Ga can exhibit two near-equilibrium phases, the A15 structure that is superconducting and the Heusler D03 structure that is semiconducting and antiferromagnetic. Density functional theory calculations show that these two phases are nearly degenerate, being separated in energy by only ±10 meV/atom. Our magnetization measurements on bulk-grown samples show antiferromagnetism and superconducting behavior below 14 K. These results indicate the possibility of using V3Ga for quantum technology devices exploiting the co-existence of superconductivity and antiferromagnetism in a dual-phase material.

Superconducting and Antiferromagnetic Properties of Dual-Phase V$_3$Ga

Abstract: The binary compound V$_3$Ga can exhibit two near-equilibrium phases, consisting of the A15 structure that is superconducting, and the Heusler D0$_3$ structure that is semiconducting and antiferromagnetic. Density functional theory calculations show that the two phases are closely degenerate, being separated by only ~10 meV/atom. Magnetization measurements on bulk-grown samples show superconducting behavior below 14 K. These results indicate the possibility of using V$_3$Ga for quantum technology devices utilizing both superconductivity and antiferromagnetism at the same temperature.

First principles study of structural and magnetic properties in Fe100−xGex alloys

Abstract: Using density functional theory, the structural, magnetic and electronic properties for Fe100−xGex (x = 12.5–25 at.%) with A2, B2, D03, L12, and D019 structures were studied. Based on the total energy calculation, the phase diagram was plotted as a function of Ge concentration. In agreement with the experimental results, the following sequences of phase transitions were observed: A2→B2→D03 (12.5 ≤ x

Statistical model for the martensitic transformation simulation in Heusler alloys

Abstract: In this work, the one-dimensional statistical model based on the phenomenological approach for the study of the magnetic and magnetocaloric properties in Heusler alloys is considered. The proposed model is approved for Ni-Co-Mn-In exhibiting both the first-order structural transformation and second-order magnetic phase transition. Temperature dependencies of magnetization and magnetic entropy change under applied magnetic field and external pressure are studied. The results of the simulations are in good agreement with the experimental data.

Phase Transformations in Ni(Co)–Mn(Cr,C)–(In,Sn) Alloys: An Ab Initio Study

Abstract: —The phase transformations in the Heusler alloys of Ni(Co)–Mn(Cr,C)–In and Ni(Co)–Mn(Cr,C)–Sn(Al) have been studied in this work using the density-functional theory. The possibility of martensitic phase transitions from the cubic L21-structure into tetragonal L10-state has been predicted, and the transition temperatures have been estimated. Energetically favorable magnetic configurations, lattice parameters, and magnetic moments of austenite and martensite phases have been determined. The Curie temperatures and elastic moduli of cubic phases of alloys have been calculated. In Ni–Mn–Sn alloys with Co addition, the effect of exchange-correlation functional on the ground state has been investigated.

Structural, magnetic and electronic properties of FeRhxPd1-x compounds: Ab initio study

Abstract: In this paper, structural, magnetic and electronic properties of FeRhxPd1-x (x = 0, 0.125, and 0.25) were investigated by first principles calculations. The two types of crystal structures CuAu-type and CsCl-type are considered. The lattice parameters, energy, partial and total magnetic moments, formation energy, total density of electronic states and the projected density of states for t2g and eg orbitals for Fe–Rh–Pd with different magnetic ordering are calculated.

First-Principles Study of the Structure and Properties of Fe 3 Pd and Fe–Pd–Rh Alloys

Abstract: In the present work, the structural, magnetic and electronic properties of Rh-doped Fe–Pd alloys were studied by means of the first-principles approach. The calculations of the lattice parameters, formation energy, magnetic moments and total and partial densities of states were performed for ordered Fe70Pd30, Fe75Pd25, Fe50Pd50, Fe50Pd43.75Rh6.25, Fe50Pd37.5Rh12.5 compositions and disordered Fe75Pd25 one, which were formed by 16-atom supercell approach. It is shown that face-centered tetragonal structure is stable for all considered compositions instead of Fe70Pd30 and Fe75Pd25. For the latter, the disordered base-centered tetragonal structure is found to be favorable. Besides, the Rh addition results in decrease in the tetragonal ratio and to increase in the total magnetic moment.

A ternary map of Ni-Mn-Ga Heusler alloys from ab initio calculations.

Abstract: In the present work, the aspects of magnetic and structural properties of Ni-Mn-Ga alloys are described in the framework of fist-principles approach and mapped into ternary composition diagrams. The stable atomic arrangement and magnetic alignment for compositions with cubic austenite and tetragonal martensite structures across phase diagrams are predicted. It is shown that Ni- and Ga-rich compositions possess the regular Heusler structure in contrast to Mn-rich compositions with inverse Heusler structure as favorable one. Compositions with unstable austenite structure are concentrated in the left and right sides of diagram whereas compositions with unstable martensite structure are located in the low-middle part of diagram. The magnetic phase diagrams showing regions with the ferromagnetic order and the complex ferrimagnetic order for austenitic and martensitic compositions are obtained. The results of calculations are in a good agreement with available experimental data.

Ab Initio Studies of Phase Transformations in Fe 100 – x Si x

Abstract: A concentration phase diagram for Fe100 – xSix (9.375 ≤ x ≤ 25.0 at %) alloys has been built on the base of the structural and magnetic phase transition temperatures estimated theoretically from the first principles. The structural phase transition temperatures for the experimentally observed crystal structures are obtained from the geometric optimization of the crystal structure. The Curie temperatures are estimated in a molecular field approximation using the constants of magnetic exchange interaction calculated ab initio. As temperature increases, the structural transitions from the ordered cubic phase to a completely disordered structure occur via a partially disordered structure over the entire concentration range. As for the magnetic transformations, the ferromagnet–paramagnet transition is observed for all the compositions, but in various crystal phases.

Phase transitions in Fe3Al-based alloys: ab initio study

Abstract: In this work, based on structural and magnetic phase transition temperatures estimated theoretically from the first principles, the concentration phase diagram for Fe100−xAlx (15.625≤x≤31.2...

A Study of the Structure and Magnetic Properties of FeRh 1 – x Ir x ( x = 0.5–1) Alloys by First-Principles Methods

Abstract: The structure and magnetic properties of FeRh1 – xIrx alloys (x = 0.5, 0.625, 0.75, 0.875, and 1) are investigated by first-principles methods using the VASP software package. Two types of structures (CuAu and CsCl) are examined in the study. The equilibrium lattice parameters, total energy, and magnetic moment, as well as the total and partial densities of states for the Fe–Rh–Ir system, are obtained. It is shown that the antiferromagnetic structure with a spin configuration of the AFM-III type is energetically favorable for all the studied alloys.

Coulomb correlation in noncollinear antiferromagnetic α-Mn

Abstract: We discuss the interplay between magnetic and structural degrees of freedom in elemental Mn. The equilibrium volume is shown to be sensitive to magnetic interactions between the Mn atoms. While the standard generalized gradient approximation underestimates the equilibrium volume, a more accurate treatment of the effects of electronic localization and magnetism is found to solve this longstanding problem. Our calculations also reveal the presence of a magnetic phase in strained $\ensuremath{\alpha}$-Mn that has been reported previously in experiments. This new phase of strained $\ensuremath{\alpha}$-Mn exhibits a noncollinear spin structure with large magnetic moments.

Electronic and Magnetic Properties of DyFe 4 Ge 2 Alloys near a Phase Transition

Abstract: The first-principles studies have been performed for the electronic and magnetic properties of DyFe4Ge2 alloys near the P42/mmm–Cmmm phase transition. The calculations are carried out in a local spin density approximation taking into account the Coulomb interaction within the limit of strong localization in a mean field approximation. The electronic and magnetic properties of the tetragonal structure are shown to be weakly changed in the dependence on the Coulomb and exchange interactions and also on the choice of the approximations. In the case of the orthorhombic structure, a change in the parameters of the Coulomb and exchange interactions leads to a change in the magnetic ordering: from the ferromagnetic to ferrimagnetic in the strong localization limit and from the ferromagnetic to paramagnetic in the mean field approximation.