arXiv subjects

L. M. Sandratskii

Publications and source records attributed to L. M. Sandratskii.

17 recordsLinked to original sources

arXiv2026

Spin-orbit coupling effects in altermagnets: Interplay of weak spin and orbital ferromagnetism with relativistic splitting of electron states

The aim of the paper is to contribute to reaching a deeper understanding of the formation of relativistic effects in altermagnets. The focus of the paper is on the phenomena of weak ferromagnetism (WFM) and relativistic DFT calculations combined with the symmetry analysis on the basis of spin space groups. The consideration is performed on two different levels. On the first level, the atomistic magnetic structure of weak ferromagnetic state is calculated. Both spin and orbital atomic moments are taken into account. We study the dependence of the WFM moment on the strength of the SOC and obtain a peculiar nonmonotonous type of dependence. An interesting result is obtained in quasisymmetry (QS) calculation where only the component of the SOC collinear to the Néel vector is taken into account. In QS calculation the spin WFM is absent while the orbital WFM is present. This reveals a principal difference in the formation of the spin and orbital magnetic moments. On the second level, the study is focused on the properties of individual electron states. We introduce the notion of the magnetic structure of the electron state (MSES). It is shown how the collinear spin-MSESs of both metal and ligand atoms and compensated orbital-MSES of the ligand obtained in the nonrelativiatic calculation transform into complex noncollinear 3D MSESs of both spin and orbital nature. An important role in the formation of MSESs is played by the relativistic splitting of the accidental spin degeneracies at general {\bf k} points filling the volume of the Brillouin zone. The formation of the regions of avoided crossings in the relativistic band structure is related to the nonmonotonous behavior of the WFM moment. The importance of the metal-ligand hybridization in the formation of the AM properties is discussed. Most of the calculations are performed for MnTe.

cond-mat.mtrl-sci↗

arXiv2025

Direct ab initio calculation of magnons in altermagnets: method, spin-space symmetry aspects, and application to MnTe

We suggest the method for direct ab initio calculation of magnons in complex collinear magnets. The method is based on the density-functional-theory calculation under two different constraints: one constraint governs the change of the magnetization with respect to the ground state, and the other is the symmetry constraint responsible for the value of the magnon wave vector. The performance of the method is demonstrated by the application to an altermagnet MnTe. An important role in both the formulation and the application of the method play the aspects of generalized symmetry described by the spin-space groups. The symmetry analysis connects in one coherent picture the following three parts of the consideration: (i) the generalized translational symmetry of the magnons as a crucial condition for their efficient ab-initio calculation, (ii) altermagnetic spin-splitting of the electron states in the ground magnetic state, and (iii) chirality splitting of the magnon excitations. It is demonstrated that both the spin splitting of the electron states and the chirality splitting of the magnons have identical patterns in the corresponding wave vector spaces. Since the altermagnetism of MnTe is the consequence of the presence of the Te atoms, an adequate attention is devoted to the symmetry analysis and calculation results for the Te moments induced in the magnon states. The knowledge of the symmetry properties of the Te moments allows to accelerate the numerical convergence of the magnon states and serves as a test for the accuracy of the calculations. To expose the connection between electron band structures of the magnon states of the system and the chirality properties of these states we investigate the transformation of the electron structure in the transition from the collinear ground state to a noncollinear magnon state.

cond-mat.mtrl-sci↗

arXiv2024

Multiple localized-itinerant dualities in magnetism of 5f electron systems. The case of UPt$_2$Si$_2$

The paper deals with the U based compound UPt$_2$Si$_2$ (UPS). The material was first treated as a localized 5f-electron system. Later, an opposite opinion of a predominantly itinerant nature of the system was put forward. The most recent publications treat UPS as a dual material. We suggest a material specific theoretical model based on the density functional theory plus Hubbard $U$ (DFT+$U$) calculations that describes the set of fundamental ground-state properties and high magnetic field experiment. The ground state properties include antiferromagnetic magnetic structure, magnetic easy axis, and the value of the U atomic moment. The in-field experiment shows the presence of a strong metamagnetic transition for the field parallel to the easy axis in contrast to the hard field direction where such a feature is absent. On the other hand, comparable induced magnetization values are obtained for both easy and hard field directions. Within the framework of the suggested model we show that the compound possesses well-formed atomic moments built by electrons treated as delocalized. To understand the experimental high-field properties we estimate exchange energy, magnetic anisotropy energy, and Zeeman energy. All three energies are shown to have comparable values what is crucial for the interpretation of the experiment. At all steps of the study we devote special attention to revealing and emphasizing the dual itinerant-localized properties of the material. The obtained forms of the duality are different: well defined atomic moments formed by the itinerant electrons, interplay of the single-site and two-site anisotropies, strong localization of two of the 5f electrons in contrast to the itinerant nature of the 5f electrons contributing to the states around the Fermi level.

cond-mat.str-el↗

arXiv2021

Interplay of spin magnetism, orbital magnetism, and atomic structure in layered van der Waals ferromagnet VI$_3$

Recently discovered ferromagnetism of the layered van der Waals material VI$_3$ attracts much research attention. Despite substantial progress,in the following important aspects no consensus has been reached: (i) a possible deviation of the easy axis from the normal to the VI$_3$ layers, (ii) a possible inequivalence of the V atoms, (iii) the value of the V magnetic moments. The theoretical works differ in the conclusions on the conduction nature of the system,the value and the role of the V orbital moments. To the best of our knowledge there is no theoretical works addressing issues (i) and (ii) and only one work dealing with the reduced value of the V moment. By combining the symmetry arguments with density functional theory (DFT) and DFT+$U$ calculations we have shown that the antidimerization distortion of the crystal structure reported in Phys. Rev. B {\bf 99}, 041402(R) (2019) must lead to the deviation of the easy axis from the normal to the VI$_3$ layers in close correlation with the experimental results. The antidimerization accompanied by the breaking the inversion symmetry leads to the inequivalence of the V atoms. Our DFT+U calculations result in large value 0.8μ_B$ of the V orbital moments of the V atoms leading to reduced total V moment in agreement with a number of experimental results and with the physical picture suggested in Phys. Rev. B bf 101, 100402(R) (2020). We obtained large intraatomic noncollinearity of the V spin and orbital moments revealing strong competition between effects coursed by the on-site electron correlation, spin-orbit coupling, and interatomic hybridization since pure intraatomic effects lead to collinear spin and orbital moments. Our calculations confirm the experimental results of strong magnetoelastic coupling revealing itself in the strong dependence of the magnetic properties on the distortion of the atomic structure.

cond-mat.mtrl-sci↗

arXiv2007

Role of the conduction electrons in mediating exchange interactions in Heusler alloys

Because of large spatial separation of the Mn atoms in Heusler alloys the Mn 3d states belonging to different atoms do not overlap considerably. Therefore an indirect exchange interaction between Mn atoms should play a crucial role in the ferromagnetism of the systems. To study the nature of the ferromagnetism of various Mn-based semi- and full-Heusler alloys we perform a systematic first-principles calculation of the exchange interactions in these materials. The calculation of the exchange parameters is based on the frozen-magnon approach. The calculations show that the magnetism of the Mn-based Heusler alloys depends strongly on the number of conduction electrons, their spin polarization and the position of the unoccupied Mn 3d states with respect to the Fermi level. Various magnetic phases are obtained depending on the combination of these characteristics. The Anderson's s-d model is used to perform a qualitative analysis of the obtained results. The conditions leading to diverse magnetic behavior are identified. If the spin polarization of the conduction electrons at the Fermi energy is large and the unoccupied Mn 3d states lie well above the Fermi level, an RKKY-type ferromagnetic interaction is dominating. On the other hand, the contribution of the antiferromagnetic superexchange becomes important if unoccupied Mn 3d states lie close to the Fermi energy. The resulting magnetic behavior depends on the competition of these two exchange mechanisms. The calculational results are in good correlation with the conclusions made on the basis of the Anderson s-d model which provides useful framework for the analysis of the results of first-principles calculations and helps to formulate the conditions for high Curie temperature.

cond-mat.mtrl-sci↗

arXiv2006

Magnetic phase diagram of the semi-Heusler alloys from first-principles

The magnetic phase diagram of the Mn-based semi-Heusler alloys is determined at T=0 using first-principles calculations in conjunction with the frozen-magnon approximation. We show that the magnetism in these systems strongly depends on the number of conduction electrons, their spin polarization and the position of the unoccupied Mn 3d states with respect to Fermi energy. Various magnetic phases are obtained depending on these characteristics. The conditions leading to diverse magnetic behavior are identified. We find that in the case of a large conduction electron spin polarization and the unoccupied Mn 3d states lying far above the Fermi level, an RKKY-like ferromagnetic interaction is dominating. On the other hand, the antiferromagnetic superexchange becomes important in the presence of large peaks of the unoccupied Mn 3d states lying close to the Fermi energy. The overall magnetic behavior depends on the competition of these two exchange mechanisms. The obtained results are in very good agreement with the available experimental data.

cond-mat.mtrl-sci↗

arXiv2005

Exchange interactions and temperature dependence of the magnetization in half--metallic Heusler alloys

We study the exchange interactions in half-metallic Heusler alloys using first-principles calculations in conjunction with the frozen-magnon approximation. The Curie temperature is estimated within both mean-field (MF) and random-phase-approximation (RPA) approaches. For the half-Heusler alloys NiMnSb and CoMnSb the dominant interaction is between the nearest Mn atoms. In this case the MF and RPA estimations differ strongly. The RPA approach provides better agreement with experiment. The exchange interactions are more complex in the case of full-Heusler alloys Co$_2$MnSi and Co$_2$CrAl where the dominant effects are the inter-sublattice interactions between the Mn(Cr) and Co atoms and between Co atoms at different sublattices. For these compounds we find that both MF and RPA give very close values of the Curie temperature slightly underestimating experimental quantities. We study the influence of the lattice compression on the magnetic properties. The temperature dependence of the magnetization is calculated using the RPA method within both quantum mechanical and classical approaches.

cond-mat.mtrl-sci↗

arXiv2005

Above-room-temperature ferromagnetism in half-metallic Heusler compounds NiCrP, NiCrSe, NiCrTe and NiVAs: A first-principles study

We study the interatomic exchange interactions and Curie temperatures in half-metallic semi Heusler compounds NiCrZ (Z=P, Se, Te) and NiVAs. The study is performed within the framework of density functional theory. The calculation of exchange parameters is based on the frozen-magnon approach. It is shown that the exchange interactions in NiCrZ vary strongly depending on the Z constituent. The Curie temperature, Tc, is calculated within the mean field and random phase approximations. The difference between two estimations is related to the properties of the exchange interactions. The predicted Curie temperatures of all four systems are considerably higher than room temperature. The relation between the half-metallicity and the value of the Curie temperature is discussed. The combination of a high spin-polarization of charge carriers and a high Curie temperature makes these Heusler alloys interesting candidates for spintronics applications.

cond-mat.mtrl-sci↗

arXiv2005

First-principles study of exchange interactions and Curie temperatures of half-metallic ferrimagnetic full Heusler alloys Mn2VZ (Z=Al, Ge)

We report the parameter-free, density functional theory calculations of interatomic exchange interactions and Curie temperatures of half-metallic ferrimagnetic full Heusler alloys Mn2VZ (Z=Al, Ge). To calculate the interatomic exchange interactions we employ the frozen-magnon approach. The Curie temperatures are calculated within the mean-field approximation to the classical Heisenberg Hamiltonian by solving a matrix equation for a multi-sublattice system. Our calculations show that, although a large magnetic moment is carried by Mn atoms, competing ferromagnetic (inter sublattice) and antiferromagnetic (intra sublattice) Mn-Mn interactions in Mn2VAl almost cancel each other in the mean-field experienced by the Mn atoms. In Mn2VGe the leading Mn-Mn exchange interaction is antiferromagnetic. In both compounds the ferromagnetism of the Mn subsystem is favored by strong antiferromagnetic Mn-V interactions. The obtained value of the Curie temperature of Mn2VAl is in good agrement with experiment. For Mn2VGe there is no experimental information available and our calculation is a prediction.

cond-mat.mtrl-sci↗

arXiv2005

Magnetic exchange coupling and Curie temperature of Ni(1+x)MnSb (x=0, 0.25, 0.5, 0.75, 1) from first principles

We study the dependence of magnetic interactions and Curie temperature in Ni(1+x)MnSb system on the Ni concentration within the framework of the density-functional theory. The calculation of the exchange parameters is based on the super-cell and frozen-magnon approaches. The Curie temperatures, Tc, are calculated within the random-phase approximation. In agreement with experiment we obtain decrease of the Curie temperature with increasing Ni content.

cond-mat.mtrl-sci↗

arXiv2005

Pressure dependence of the Curie temperature in Ni2MnSn Heusler alloy: A first-principles study

The pressure dependence of electronic structure, exchange interactions and Curie temperature in ferromagnetic Heusler alloy Ni2MnSn has been studied theoretically within the framework of the density-functional theory. The calculation of the exchange parameters is based on the frozen--magnon approach. The Curie temperature, Tc, is calculated within the mean-field approximation by solving the matrix equation for a multi-sublattice system. In agrement with experiment the Curie temperature increased from 362K at ambient pressure to 396 at 12 GPa. Extending the variation of the lattice parameter beyond the range studied experimentally we obtained non-monotonous pressure dependence of the Curie temperature and metamagnetic transition. We relate the theoretical dependence of Tc on the lattice constant to the corresponding dependence predicted by the empirical interaction curve. The Mn-Ni atomic interchange observed experimentally is simulated to study its influence on the Curie temperature.

cond-mat.mtrl-sci↗

arXiv2005

Stability of ferromagnetism in the half-metallic pnictides and similar compounds: A first-principles study

Based on first-principles electron structure calculations and employing the frozen-magnon approximation we study the exchange interactions in a series of transition-metal binary alloys crystallizing in the zinc-blende structure and calculate the Curie temperature within both the mean-field approximation (MFA) and random-phase approximation (RPA). We study two Cr compounds, CrAs and CrSe, and four Mn compounds: MnSi, MnGe, MnAs and MnC. MnC, MnSi and MnGe are isovalent to CrAs and MnAs is isoelectronic with CrSe. Ferromagnetism is particular stable for CrAs, MnSi and MnGe: All three compounds show Curie temperatures around 1000 K. On the other hand, CrSe and MnAs show a tendency to antiferromagnetism when compressing the lattice. In MnC the half-metallic gap is located in the majority-spin channel contrary to the other five compounds. The large half-metallic gaps, very high Curie temperatures, the stability of the ferromagnetism with respect to the variation of the lattice parameter and a coherent growth on semiconductors make MnSi and CrAs most promising candidates for the use in spintronics devises.

cond-mat.mtrl-sci↗

arXiv2004

On-site Coulomb interaction and the magnetism of (GaMn)N and (GaMn)As

We use the local density approximation (LDA) and LDA+U schemes to study the magnetism of (GaMn)As and (GaMn)N for a number of Mn concentrations and varying number of holes. We show that for both systems and both calculational schemes the presence of holes is crucial for establishing ferromagnetism. For both systems, the introduction of $U$ increases delocalization of the holes and, simultaneously, decreases the p-d interaction. Since these two trends exert opposite influences on the Mn-Mn exchange interaction the character of the variation of the Curie temperature (T$_C$) cannot be predicted without direct calculation. We show that the variation of T$_C$ is different for two systems. For low Mn concentrations we obtain the tendency to increasing T$_C$ in the case of (GaMn)N whereas an opposite tendency to decreasing T$_C$ is obtained for (GaMn)As. We reveal the origin of this difference by inspecting the properties of the densities of states and holes for both systems. The main body of calculations is performed within a supercell approach. The Curie temperatures calculated within the coherent potential approximation to atomic disorder are reported for comparison. Both approaches give similar qualitative behavior. The results of calculations are related to the experimental data.

cond-mat.mtrl-sci↗

arXiv2004

First-principles calculation of the intersublattice exchange interactions and Curie temperatures of full Heusler alloys Ni2MnX (X=Ga, In, Sn, Sb)

The interatomic exchange interactions and Curie temperatures in Ni-based full Heusler alloys Ni2MnX with X=Ga, In, Sn and Sb are studied within the framework of the density-functional theory. The calculation of the exchange parameters is based on the frozen-magnon approach. Despite closeness of the experimental Curie temperatures for all four systems their magnetism appeared to differ strongly. This difference involves both the Mn-Mn and Mn-Ni exchange interactions. The Curie temperatures, Tc, are calculated within the mean-field approximation by solving a matrix equation for a multi-sublattice system. Good agreement with experiment for all four systems is obtained. The role of different exchange interactions in the formation of Tc of the systems is discussed.

cond-mat.mtrl-sci↗

arXiv2003

Exchange interactions in (ZnMn)Se

One of the remarkable properties of the II-VI diluted magnetic semiconductor (ZnMn)Se is the giant spin splitting of the valence band states under application of the magnetic field (giant Zeeman splitting). This splitting reveals strong exchange interaction between Mn moments and semiconductor states. On the other hand, no magnetic phase transition has been observed for systems with small Mn content up to very low temperatures. The latter property shows weakness of the exchange interaction between Mn moments. In this paper, the local density approximation (LDA) and the LDA+U techniques are employed to study exchange interactions in (ZnMn)Se. Supercell and frozen-magnon approaches applied earlier to III-V diluted magnetic semiconductors are used. It is found that both LDA and LDA+U describe successfully the combination of the strong Zeeman splitting and weak interatomic exchange. However, the physical pictures provided by two techniques differ strongly. A detailed analysis shows that the LDA+U method provides the description of the system which is much closer to the experimental data.

cond-mat.str-el↗

arXiv2003

Electronic structure, exchange interactions and Curie temperature in diluted III-V magnetic semiconductors: (GaCr)As, (GaMn)As, (GaFe)As

We complete our earlier (Phys. Rev. B, {\bf 66}, 134435 (2002)) study of the electronic structure, exchange interactions and Curie temperature in (GaMn)As and extend the study to two other diluted magnetic semiconductors (GaCr)As and (GaFe)As. Four concentrations of the 3d impurities are studied: 25%, 12.5%, 6.25%, 3.125%. (GaCr)As and (GaMn)As are found to possess a number of similar features. Both are semi-metallic and ferromagnetic, with similar properties of the interatomic exchange interactions and the same scale of the Curie temperature. In both systems the presence of the charge carriers is crucial for establishing the ferromagnetic order. An important difference between two systems is in the character of the dependence on the variation of the number of carriers. The ferromagnetism in (GaMn)As is found to be very sensitive to the presence of the donor defects, like As$_{\rm Ga}$ antisites. On the other hand, the Curie temperature of (GaCr)As depends rather weakly on the presence of this type of defects but decreases strongly with decreasing number of electrons. We find the exchange interactions between 3d atoms that make a major contribution into the ferromagnetism of (GaCr)As and (GaMn)As and propose an exchange path responsible for these interactions. The properties of (GaFe)As are found to differ crucially from the properties of (GaCr)As and (GaMn)As. (GaFe)As does not show a trend to ferromagnetism and is not half-metallic that makes this system unsuitable for the use in spintronic semiconductor devices.

cond-mat.mtrl-sci↗

arXiv2002

Exchange interactions and Curie temperature in (GaMn)As

We use supercell and frozen-magnon approaches to study the dependence of the magnetic interactions in (Ga,Mn)As on the Mn concentration. We report the parameters of the exchange interaction between Mn spins and the estimates of the Curie temperature within the mean-field and random-phase approximations. In agreement with experiment we obtain a nonmonotonous dependence of the Curie temperature on the Mn concentration. We estimate the dependence of the Curie temperature on the concentration of the carries in the system and show that the decrease of the number of holes in the valence band leads to fast decrease of the Curie temperature. We show that the hole states of the valence band are more efficient in mediating the exchange interaction between Mn spins than the electron states of the conduction band.

cond-mat.mtrl-sci↗