Exchange Interaction Constants Research Articles

Large Magnetic Anisotropy in van der Waals Ferromagnet Fe3GaTe2 above Room Temperature.

Discoveries of above-room-temperature intrinsic ferromagnetism in two-dimensional (2D) van der Waals (vdW) materials offer a platform for studying fundamental 2D magnetism and spintronic devices, especially the recently discovered above-room-temperature 2D vdW Fe3GaTe2 (FGaT). However, the magnetic mechanism in FGaT remains elusive. Here, a detailed investigation using magnetic force microscopy on the thickness-dependent magnetic behavior of FGaT single crystals is reported. The Heisenberg exchange interaction constant (J) at room temperature is determined to be 1.32836 × 10-12 J/m. Our study combining angle-resolved photoemission spectroscopy and density functional theory suggests that the high Curie temperature in FGaT is attributed to the shift of the localized Fe d band toward the Fermi level as well as the enhanced magnetic exchange effect due to the strong itinerant ability of Fe. This work sheds light on the understanding of magnetism in FGaT and provides a promising platform to investigate the mechanisms of 2D magnetic materials.

Theoretical Study of Co-Doping Effects with Different Ions on the Multiferroic Properties of BiFeO3 Nanoparticles.

Using a microscopic model and the Green's function theory, the size and co-doping effects on the multiferroic and optical (band gap) properties of BiFeO3 (BFO) nanoparticles are investigated. The magnetization increases, whereas the band gap energy decreases with decreasing nanoparticle size. The substitution with Co/Mn, Nd/Sm, Ce/Ni, and Cd/Ni is discussed and explained on a microscopic level. By the ion co-doping appear different strains due to the difference between the doping and host ionic radii, which leads to changes in the exchange interaction constants for tuning all properties. It is observed that by co-doping with Nd/Sm at the Bi site or with Co/Mn at the Fe site, the multiferroic properties are larger than those by doping with one ion. Moreover, by doping with Ni, the multiferroic properties are reduced. But by adding another ion (for example Ce or Cd), an increase in these properties is obtained. This shows the advantages of the co-doping, its flexibility, and its greater possibility of tuning the multiferroic properties compared to single ion substitution. The band gap energy decreases for all co-dopants. The polarization increases with increasing magnetic field. This is evidence of magnetoelectric coupling, which is enhanced by co-doping with Co/Mn. The observed theoretical results are in good qualitative agreement with the existing experimental data.

Theoretical Study of the Multiferroic Properties of Ion-Doped CaBaCo4O7

Using a microscopic model and Green’s function theory, we investigated the magnetization, specific heat, and polarization properties of CaBaCo4O7 (CBCO), scrutinizing their variations with temperature, magnetic field strength, and doping effects. Our analysis revealed a conspicuous kink in the specific heat curve near the critical temperature (TC), indicative of a phase transition. Additionally, the observed increase in polarization, P with escalating magnetic field strength serves as compelling evidence for the multiferroic nature of CBCO. Substituting Co ions with Fe ions resulted in an augmentation of the CBCO magnetization, M, while doping with Zn, Mn, or Ni ions led to a decline. Similarly, doping CBCO with Y or Sr ions at the Ca site exhibited divergent effects on magnetization, M, with an increase in the former and a decrease in the latter case. This modulation of the magnetization, M, can be attributed to the varying strains induced by the doping ions, thereby altering the exchange interaction constants within the system. The polarization, P, increases by Ni, Mn, or Zn substitution on the kagome layer Co sites. It can be concluded that Ni, Mn, or Zn doping enhances the magnetoelectric effect of CBCO. Notably, our findings align qualitatively well with experimental observations, reinforcing the validity of our theoretical framework.

Theoretical Study of the Magnetic and Optical Properties of Ion-Doped LiMPO4 (M = Fe, Ni, Co, Mn).

Using a microscopic model and Green's function theory, we calculated the magnetization and band-gap energy in ion-doped LiMPO4 (LMPO), where M = Fe, Ni, Co, Mn. Ion doping, such as with Nb, Ti, or Al ions at the Li site, induces weak ferromagnetism in LiFePO4. Substituting Li with ions of a smaller radius, such as Nb, Ti, or Al, creates compressive strain, resulting in increased exchange interaction constants and a decreased band-gap energy, Eg, in the doped material. Notably, Nb ion doping at the Fe site leads to a more pronounced decrease in Eg compared to doping at the Li site, potentially enhancing conductivity. Similar trends in Eg reduction are observed across other LMPO4 compounds. Conversely, substituting ions with a larger ionic radius than Fe, such as Zn and Cd, causes an increase in Eg.

Doping Effects on the Multiferroic Properties of KNbO3 Nanoparticles

The magnetization, polarization, and band-gap energy in pure and ion-doped KNbO3 (KNO) bulk and nanoparticles (NPs) are investigated theoretically using a microscopic model and Green’s function theory. It is shown that KNO NPs are multiferroic. The size dependence of M and P is studied. The magnetization M increases with decreasing NP size, whereas the polarization P decreases slightly. The properties of KNO can be tuned by ion doping, for example, through the substitution of transition metal ions at the Nb site or Na ions at the K site. By ion doping, depending on the relation between the doping and host ion radii, different strains appear. They lead to changes in the exchange interaction constants, which are inversely proportional to the lattice parameters. So, we studied the macroscopic properties on a microscopic level. By doping with transition metal ions (Co, Mn, Cr) at the Nb site, M increases, whereas P decreases. Doped KNO NPs exhibit the same behavior as doped bulk KNO, but the values of the magnetization and polarization in KNO NPs are somewhat enhanced or reduced due to the size effects compared to the doped bulk KNO. In order to increase P, we substituted the K ions with Na ions. The polarization increases with increasing magnetic field, which is evidence of the multiferroic behavior of doped KNO bulk and NPs. The behavior of the band-gap energy Eg also depends on the dopants. Eg decreases with increasing Co, Mn, and Cr ion doping, whereas it increases with Zn doping. The results are compared with existing experimental data, showing good qualitative agreement.

Multiferroic Properties of Co, Ru, and La Ion Doped KBiFe2O5.

The magnetic, electric, dielectric, and optical (band gap) properties of ion doped multiferroic KBiFe2O5 (KBFO) have been systematically investigated utilizing a microscopic model and the Green's function theory. Doping with Co at the Fe site and Ru at the Bi site induces changes in magnetization, coercive field, and band gap energy. Specifically, an increase in magnetization is observed, while the coercive field and band gap energy decrease. This behavior is attributed to the distinct ionic radii of the doped and host ions, leading to alterations in the exchange interaction constants. The temperature dependence of the polarization P reveals a distinctive kink at the Neel temperature TN, which shifts to higher temperatures with an increase in the applied magnetic field h. Furthermore, doping with Ru and La leads to an increase in polarization. The temperature dependence of the dielectric constant exhibits two peaks at the Neel temperature TN and the Curie temperature TC. Notably, these peaks diminish with increasing frequency. Additionally, the dielectric constant demonstrates a decrease with the rise in the applied magnetic field h. This study sheds light on the intricate interplay between ion doping, structural modifications, and multifunctional properties in KBFO, offering valuable insights into the underlying mechanisms governing its behavior across various physical domains.

Advancing understanding of structural, electronic, and magnetic properties in 3d-transition-metal TM-doped α-Ga2O3 (TM = V, Cr, Mn, and Fe): A first-principles and Monte Carlo study

In this paper, we investigated the properties of transition metal (TM)-doped α-Ga2O3 using first-principles calculations and Monte Carlo simulations. α-Ga2O3 is a wide-bandgap semiconductor material with enhanced performance and lower fabrication costs on sapphire substrates compared to β-Ga2O3. Doping with TMs can modify electrical transport, optical absorption, and magnetic properties, yet theoretical studies on this are scarce. Our study focused on V, Cr, Mn, and Fe impurities. We introduced a newly proposed scheme for efficiently determining the ground-state defect configuration during structural relaxation. We adopt a recent, novel image charge correction method to accurately calculate formation enthalpy and thermodynamic transition levels for spin-polarized transition metal ion doping, without employing the empirical dielectric constant. Results showed Cr ions tend to neutral substitutional Ga, while V, Mn, and Fe impurity ions tend to carry a negative charge in common n-type α-Ga2O3. Magnetic moments and spin-splitting impurity levels primarily arise from transition metal impurities and their d orbitals. We used the generalized four-state method to calculate exchange interaction constants between substitution lattice sites and identified (anti) ferromagnetic couplings at specific distances in a 120-atom supercell, which are negligible in total energy calculations. Monte Carlo simulations indicated a Curie temperature of 360 K in n-type α-Ga2O3: Mn system with 12.5% doping, suggesting intrinsic ferromagnetic ordering based on the Heisenberg model. Our study contributes to understanding TM-doped α-Ga2O3 electronic structure and magnetic properties through improved methodologies. The approach can be applied in research involving other TM-doped oxides or wide-bandgap semiconductors.

Regarding certain stages of the development of quantum chemistry in Russia: Experience from the Ya.K. Syrkin Department of Physical Chemistry of the M.V. Lomonosov Institute of Fine Chemical Technologies

Objectives. To analyze the history of the development of quantum chemistry and software for quantum chemical calculations in Russia at the Ya.K. Syrkin Department of Physical Chemistry of the M.V. Lomonosov Institute of Fine Chemical Technologies of RTU MIREA.Results. This work presents a historical overview of the development of quantum chemistry at the Ya.K. Syrkin Department of Physical Chemistry from Academician Ya.K. Syrkin to Professor V.R. Flid. It provides a summary of the work with the participation of the author in 1980s–1990s. Quantum-chemical models used to describe some of the intercalation reactions in a bond are considered in comparison with the well-known Woodward–Hoffman and Fukui approaches. The work outlines fundamentals of studies on the design of bifunctional compounds.Conclusions. The physical significance of the exchange interaction constant is given a visual meaning: it establishes the change in spin density on the metals forming complexes of the type in question when passing from isolated cations in the composition of the complexes. The work provides recommendations to synthetic chemists regarding the selection of components in the synthesis of magnetic sublattices of bifunctional materials. It also examines the high level of scientific research carried out at the Ya.K. Syrkin Department of Physical Chemistry and its relevance to the world science level.

Regarding certain stages of the development of quantum chemistry in Russia: Experience from the Ya.K. Syrkin Department of Physical Chemistry of the M.V. Lomonosov Institute of Fine Chemical Technologies

Objectives. To analyze the history of the development of quantum chemistry and software for quantum chemical calculations in Russia at the Ya.K. Syrkin Department of Physical Chemistry of the M.V. Lomonosov Institute of Fine Chemical Technologies of RTU MIREA.Results. This work presents a historical overview of the development of quantum chemistry at the Ya.K. Syrkin Department of Physical Chemistry from Academician Ya.K. Syrkin to Professor V.R. Flid. It provides a summary of the work with the participation of the author in 1980s–1990s. Quantum-chemical models used to describe some of the intercalation reactions in a bond are considered in comparison with the well-known Woodward–Hoffman and Fukui approaches. The work outlines fundamentals of studies on the design of bifunctional compounds.Conclusions. The physical significance of the exchange interaction constant is given a visual meaning: it establishes the change in spin density on the metals forming complexes of the type in question when passing from isolated cations in the composition of the complexes. The work provides recommendations to synthetic chemists regarding the selection of components in the synthesis of magnetic sublattices of bifunctional materials. It also examines the high level of scientific research carried out at the Ya.K. Syrkin Department of Physical Chemistry and its relevance to the world science level.

The Study of Transport Properties of (III−Mn) V Diluted Magnetic Semiconductors

We investigated the transport properties of diluted magnetic semiconductors (DMSs) theoretically by using the Heisenberg and Ruderman–Kittel–Kasuya–Yosida (RKKY) exchange interaction models by considering both spin and charge disorder. The formalism is applied to the specific case of Ga 1 − x Mn x As . Using the Heisenberg model and the Green function formalism the total thermal excitation of the magnon is calculated. The magnetization and Curie temperature of Mn -doped GaAs is calculated. The theoretical calculation of T C of Ga 1 − x Mn x As at x = 0.08 has a good agreement with the experimental calculation at x = 0.08 (i.e., 162 k). The exchange interaction constant and spin-dependent relaxation time is calculated by using RKKY interaction. The electrical conductivity and hole mobility are calculated by using the Boltzmann transport equation and the spin-dependent relaxation time. The electrical conductivity of Mn -doped III–V DMS is exponentially increased with temperature and magnetic impurity concentration. Hole mobility of Mn -doped III–V diluted magnetic semiconductor is increased with the magnetic impurity concentration.

Comparison of the Multiferroic Properties of Ion‐Doped Hexagonal LuFeO3 and LaFeO3

Using a microscopic model, the magnetic and electric properties of pure and ion‐doped (on the Lu or Fe sites) hexagonal LuFeO3 (LuFO) are studied and compared with the properties of ion‐doped LaFeO3 (LaFO). The magnetization increases by Sr and Sc doping which is caused by the strain and the changes of the exchange interaction constants at the doped states. It is observed that by Sm doping of LuFO the polarization decreases, whereas by Sm doping of LaFO, the polarization increases with increasing Sm concentration. By Ir or Co ion doping on the Fe site, the magnetization and the phonon energy increase whereas the bandgap energy decreases. By doping on the Fe sites with the same ion, for example, Sc, there are no differences between LuFO and LaFO. A good agreement with the experimental data is observed. The doping effects can be used for different applications.

Exchange interaction for Mn acceptors in GaAs: Revealing its strong deformation dependence

In this paper we calculate exchange interaction constant between manganese ion inner electronic $d$-shell and GaAs valence band bounded hole using their microscopic multiparticle wave functions. We reveal its parametric dependence on crystal lattice deformations and find out that it could be about and even more than dozens percent when the strain tensor reaches values of $10^{-3} \div 10^{-2}$. This fact is in accordance with the previous hypothesis of deformation dependence of Mn acceptors in GaAs fine energy structure obtained from Raman spectroscopy, and we show that this dependence has the same magnitude. Also, we resolve here the problem of a substantial high temperature mismatch between well-developed theory and experimental data for the static magnetic susceptibility of Mn ions in GaAs. We show by numerical estimates and calculations that quite a strong parametric dependence of the exchange coupling value on GaAs lattice expansion determines the high temperature (above $50~$K) magnetic susceptibility reduction as well.

Magnetic, Optical and Phonon Properties of Ion-Doped MgO Nanoparticles. Application for Magnetic Hyperthermia

The influence of size and doping effects on the magnetization M, phonon ω and band gap energy Eg of MgO nanoparticles is studied using a microscopic model. The room-temperature ferromagnetism is due to surface or/and doping effects in MgO nanoparticles (NPs). The influence of the spin-phonon interaction is discussed. M increases with decreasing NP size. M and Eg can increase or decrease by different ion doping (Co, Al, La, Fe) due to the different strain that appears. It changes the lattice parameters and the exchange interaction constants. We found that MgO NP with size of 20 nm and Fe- or Co-doping concentration x = 0.1 and x = 0.2, respectively, have a Curie temperature TC = 315 K, i.e., they are appropriate for application in magnetic hyperthermia, they satisfy the conditions for that. The energy of the phonon mode ω = 448 cm-1 increases with decreasing NP size. It increases with increasing Co and Fe, or decreases with Sr ion doping.

Magnetization, Band Gap and Specific Heat of Pure and Ion Doped MnFe2O4 Nanoparticles

We have studied the magnetic properties of ion doped MnFe2O4 nanoparticles with the help of a modified Heisenberg model and Green’s function theory taking into account all correlation functions. The magnetization Ms and the Curie temperature TC increase with decreasing particle size. This is the opposite behavior than that observed in CoFe2O4 and CoCr2O4 nanoparticles. By Co, Mg or Ni doping, Ms and TC increase with enhancing the dopant concentration, whereas, by La or Gd doping, the opposite effect is obtained due to the different doping and host ionic radii which change the exchange interaction constants. The band gap energy Eg is calculated from the s–d model. It can decrease or increase by different ion doping. The peak observed in the temperature dependence of the specific heat at TC is field dependent.

The Effect of the Interface Width on the Exchange Interaction Constant between Ferro- and Antiferromagnets

Within the model of two-phase (core/shell) ellipsoidal nanoparticles, the effects of the interfacial exchange interaction, size, elongation, and orientation of the major axes of the core and shell on the meta-stability of magnetic states have been simulated. Using the example of Fe2.44Ti0.56O4 nanoparticles,Fe3O4 it is shown that the metastability of magnetic states is realized in a limited range of the constant of interfacial exchange interaction between the core and shell, Ain , and geometric parameters. With an increase in the absolute value of the interfacial exchange interaction constant, , from Ain = 0 to some finite value, theAin metastability of magnetic states decreases monotonically from maximum to zero, regardless of the angle between the major axes of the core and the shell.

The Effect of the Interface Width on the Exchange Interaction Constant between Ferro- and Antiferromagnets

In the approximation of the average spin method, a system of equations for determining the average magnetic moments of atoms at the interface between a ferromagnet and an antiferromagnet is formulated. It is possible to simulate the dependences of interfacial exchange interaction constant Ain on the temperature, antiferromagnetic layer thickness, and interface width by solving the system of equations for an ultrathin Ni/NiO film. It is found that the interfacial exchange interaction constant in a film with a fixed interface width at low temperatures increases with an increase in the thickness of the antiferromagnetic layer. With anincrease in the interface width, the Ain value decreases by a factor of 1.3 and reaches its minimum level.

Ab-initio study and atomistic spin model simulations of Cr2O3 thin films

Magnetoelectric random access memory (MERAM) is the next generation technology of data storage with high-density and low power. Magnetoelectric material can be controlled by voltage such as an electric field to transform spin domain directions for storing data. One of the most promising materials that possess such properties near room temperature is Cr2O3. Although, the bulk has perfect properties for MERAM, it is necessary to fabricate thin films for applicable spintronic devices, i.e., reducing the required external control voltage; however, the film magnetic and physical properties might be different from bulk. In this work, we propose that the film properties might be induced by surface effects and are confirmed with Ab-initio calculation and atomistic spin simulation. The main simulation processes are the density functional theory (DFT) calculating, mapped with the classical Heisenberg model for exchange interaction constants up to the fifth nearest neighbour (J 1-J 5). The atomistic spin model was then used to approximate the Néel temperature (T N), comparing two methods i.e., Monte Carlo simulation and the Landau–Lifshitz–Gilbert (LLG) equation. The Néel temperatures of the film were found to be slightly lower than the bulk, which is in good agreement with previous experiments. Furthermore, the spin dynamic model gives more correct result comparing to the Monte Carlo method.

Band Gap Tuning in Transition Metal and Rare-Earth-Ion-Doped TiO2, CeO2, and SnO2 Nanoparticles

The energy gap Eg between the valence and conduction bands is a key characteristic of semiconductors. Semiconductors, such as TiO2, SnO2, and CeO2 have a relatively wide band gap Eg that only allows the material to absorb UV light. Using the s-d microscopic model and the Green's function method, we have shown two possibilities to reduce the band-gap energy Eg-reducing the NP size and/or ion doping with transition metals (Co, Fe, Mn, and Cu) or rare earth (Sm, Tb, and Er) ions. Different strains appear that lead to changes in the exchange-interaction constants, and thus to a decrease in Eg. Moreover, the importance of the s-d interaction, which causes room-temperature ferromagnetism and band-gap energy tuning in dilute magnetic semiconductors, is shown. We tried to clarify some discrepancies in the experimental data.

Magnetic, Phonon and Optical Properties of Transition Metal and Rare Earth Ion Doped ZnS Nanoparticles.

The surface, size and ion doping effects on the magnetic, phonon and optical properties of ZnS nanoparticles are studied based on the s-d model including spin-phonon and Coulomb interaction, and using a Green's function theory. The changes of the properties are explained on a microscopic level, due to the different radii between the doping and host ions, which cause different strains-compressive or tensile, and change the exchange interaction constants in our model. The magnetization increases with increasing small transition metal (TM) and rare earth (RE) doping concentration. For larger TM dopants the magnetization decreases. The phonon energies increase with increasing TM, whereas they decrease by RE ions. The phonon damping increases for all doping ions. The changes of the band gap energy with different ion doping concentration is also studied. Band gap changes in doped semiconductors could be due as a result of exchange, s-d, Coulomb and electron-phonon interactions. We have tried to clarify the discrepancies which are reported in the literature in the magnetization and the band gap energy.

Dynamics of magnetic system formed by two semi-skyrmions in circular nano-pillars with a perpendicular anisotropy

Magnetic skyrmion has the advantages of stable topology and small volume. Many researchers choose different materials or build double free layers for using skyrmions in spin torque nano-oscillators capable of producing GHz frequencies. In this paper, the dynamics of the two semi-skyrmions in a circular nano-pillar with perpendicular magnetic anisotropy free layer and a spin polarizer are studied using micromagnetic simulation. The oscillation frequency of two semi-skyrmions is more than two times higher that of the single semi-skyrmion. In addition, we also explore the influences of different parameters (current density, damping coefficient, anisotropy constant, and temperature) on the motion of two semi-skyrmions. The results show that damping coefficient and exchange interaction constant have the most pronounced influence on the oscillation frequency of the system.