Exchange Interaction Integrals Research Articles

Low-dimensional Magnetism in Compounds with Different Dimensions of Magnetic Interaction

The results of a comparison of the exchange interaction mechanisms in low dimensional magnetic systems are presented. It has been shown that ZnO crystal may be used as a semiconductor non-magnetic matrix for the formation of quasi-one-dimensional and quasi-zero-dimensional magnetic systems by introducing impurity atoms of Cr, Mn, Fe, Co and Ni. Structural parameters, electronic and magnetic properties were calculated at the atomic level in the framework of quantum mechanical simulation. The exchange interaction integrals were calculated at the microscopic level using the Heisenberg model. The exchange interaction mechanisms were determined on the obtained dependences of the exchange interaction integral on the structural and electronic properties, as well as on the features of the low-dimensional magnetic systems partial density of electronic states. The results of studying the exchange interaction mechanisms in two-dimensional magnetic systems formed in materials of the MAX3 (M= Cr, Fe, A = Ge, Si, X= S, Se, Te) group are summarized. The established mechanisms made it possible to compare the conditions for the formation of a ferromagnetic order in systems with different dimensions of magnetic interaction. The ferromagnetic order in all the structures under study is formed due to the indirect superexchange interaction between orbitals of different symmetry. Strategies aimed at enhancing the superexchange interactions between orbitals of different symmetry or attenuating the contributions of the exchange interaction between orbitals of the same symmetry contribute to the formation of stable hightemperature ferromagnetism.

Electronic properties of quasi two-dimensional transition metals chalcogenides with low-dimensional magnetism

The search for fundamental physical laws which lead to stable high-temperature ferromagnetism is an urgent task. In addition to the already synthesized two-dimensional materials, there remains a wide list of possible structures, the stability of which is predicted theoretically. The article suggests the results of studying the electronic properties of MAX3 (M = Cr, Fe, A = Ge, Si, X = S, Se, Te) transition metals based compounds with nanostructured magnetism. The research was carried out using quantum mechanical simulation in specialized VASP software and calculations within the Heisenberg model. The ground magnetic states of twodimensional MAX3 and the corresponding energy band structures are determined. We found that among the systems under study, CrGeTe3 is a semiconductor nanosized ferromagnet. In addition, one is a semiconductor with a bandgap of 0.35 eV. Other materials are antiferromagnetic. The magnetic moment in MAX3 is localized on the transition metal atoms: in particular, the main one on the d-orbital of the transition metal atom (and only a small part on the p-orbital of the chalcogen). For CrGeTe3, the exchange interaction integral is calculated. The mechanisms of the formation of magnetic order was established. According to the obtained exchange interaction integrals, a strong ferromagnetic order is formed in the semiconductor plane. The distribution of the projection density of electronic states indicates hybridization between the d-orbital of the transition metal atom and the p-orbital of the chalcogen. The study revealed that the exchange interaction by the mechanism of superexchange is more probabilistic.

Magnetic interactions in Cr2Ge2Te6 and Cr2Si2Te6 monolayers: ab initio study

Abstract Magnetic anisotropy in low dimensional semiconductors can lead to long-range ferromagnetic order. This feature is useful for various applications such as spintronic and sensoric. The exchange interaction integrals have been observed through magnetic, structural and electronic properties of Cr2Ge2Te6 and Cr2Si2Te6 first-principles simulation in the VASP software package and Heisenberg model approach. The structural parameters for the bulk layered Cr2Ge2Te6 and Cr2Si2Te6 were determined and in good agreement with the experiment. Calculations have established that the number of layers does not affect the magnetic properties and entire magnetic moment is localized on Cr atoms. Exchange interaction integrals between the first, second and third neighbors have been calculated for monolayers. The dependence of the exchange interaction integral on the Hubbard coefficient has been established. The exchange integral with the first neighbors J1 was -2.34 (Cr2Ge2Te6) and -1.61 (Cr2Si2Te6) meV at the Hubbard coefficient of 3 eV. Thus, strong ferromagnetic order exists in plane of monolayers of Cr2Ge2Te6 , Cr2Si2Te6 and magnetic anisotropy leads to long-range ferromagnetism

GdFe2 Laves phase intermetallic system under pressure: an ab-initio study

Here we perform abinitio study of Curie temperature TC under hydrostatic pressure for intermetallic compound GdFe2. To calculate TC for GdFe2 we applied mean-field solution of the Heisenberg model for several magnetic sublattices with DFT/LDA calculated values of necessary exchange interaction integrals and local magnetic moments. To compare with available experimental data pressure values were taken from zero up to about 70 Kbar. It corresponds to 2% compression of the volume of the unit cell. In agreement with experimental data TC grows under pressure. It was shown that Fe ions magnetic sublattice alone provides only about 75% of the experimental Curie temperature TCexp. Gd sublattice is found to give very weak contribution to the TCexp. Here we show that the missing 25 % of TCexp comes from Fe-Gd exchange pairs.

Monte Carlo simulation of amorphous magnets with random exchange interactions

Using the Monte Carlo method, the computer simulation of magnetic properties of pure amorphous Gd and Re-Gd amorphous alloys was performed. For the model of amorphous Gd the temperature dependencies of magnetization and magnetic susceptibility were calculated at different values of the ratio of exchange interaction integrals wihin the first and the second coordination spheres J1/|J2| . The magnetic phase diagram in coordinates T - J1/|J2| was constructed. For the models of Re-Gd amorphous alloys the dependence of spin-glass transition temperature on concentration of Gd atoms was calculated. For all our models the hysteresis loops were calculated at different temperatures.

Changes in magnetic state of Y2(Fe,Mn)17-H systems: Regularities and potentialities

Abstract A systematic study is made of the magnetic properties of the hydrides of Y2Fe17−xMnxHy intermetallic alloys (0 ⩽ x ⩽ 8 and 0 ⩽ y ⩽ 4.4 at.H/f.u.) and the exchange interaction parameters of these compounds are estimated. It is found that hydrogen atoms enhance the ferromagnetic interactions. The decrease of the Curie temperature observed in both hydrides and parent alloys with substitution of manganese for iron gives an indication of the reinforcement of negative exchange interactions. As a result the magnetic order of the compounds under study is subject to change at high (x > 6) manganese concentration. The transformation of magnetic state in the Y2Fe17−xMnx-H system is explained on the basis of molecular field theory connecting the Curie temperatures with the value and sign of exchange interaction integrals (AFeFe, AMnMn, and AFeMn) for magnetoactive atoms: iron and manganese.

Determination of the intersublattice exchange interactions in GdCo12-xFexB6 (x = 0-3) intermetallic compounds by high field magnetization measurements

High field magnetization measurements up to 60 T on free powder samples from GdCo12-xFexB6 (x = 0-3) compounds are reported. The data were used to evaluate the microscopic exchange interaction integral, JGd-3d, between Gd and 3d (Co,Fe) spins. The systems are ferrimagnets; they order magnetically between TC = 95 K for x = 3 and TC = 165 K for x = 0. The low temperature magnetization curves as well as the temperature dependence of intrinsic magnetic parameters are determined by magnetic measurements in pulsed magnetic field. The average magnetic moment ⟨μCo+Fe⟩ per mean transition metal atom (Co + Fe) is small and increases with increasing Fe concentration from 0.44 μΒ for x = 0 to 0.51 μΒ for x = 3 at T = 4 K. From high field magnetization curves, a value of JGd-3d/kB = −4.65 K is derived for x = 0, whereas mean field approximation yields a much larger 3d-3d exchange integral of JCo-Co/kB = 105 K. The obtained results reveal an increase of −JGd-3d/kB with Fe concentration. For x = 0.5, the intersublattice coefficient nGd-3d is found to keep an almost constant value of 5.87 ± 0.13 T*f.u.*μB−1 whatever the temperature in the 2 to 60 K range.

Magnetic Exchange Interactions and Supertransferred Hyperfine Fields at <sup>119</sup>Sn Probe Atoms in CaCu<sub>3</sub>Mn<sub>4</sub>O<sub>12</sub> Manganite

The hyperfine magnetic interactions of 119Sn probe atoms in the CaCu3Mn3.96Sn0.04O12 double manganite by Mössbauer spectroscopy using magnetic measurements have been investigated. A consistent description of the results obtained in terms of the Weiss molecular field model by taking into account the peculiarities of the local environment of tin atoms has allowed the indirect Cu2+OMn4+ (JCuMn 51 ± 1 K) and Mn4+OMn4+ (JMnMn 0.6 ± 0.6 K) exchange interaction integrals to be estimated. Based on the KanamoriGoodenoughAnderson model, we show that the magnitude and sign of the intrasublattice exchange integral JMnMn correspond to both the electronic configuration of the Mn4+ cations and the geometry of their local crystallographic environment in the compound under study.

Hyperfine magnetic fields at the nuclei of probe 119Sn atoms and exchange interactions in the CaCu3Mn3.96Sn0.04O12 manganite

We have investigated the hyperfine magnetic interactions between the nuclei of probe 119Sn atoms in the CaCu3Mn3.96Sn0.04O12 double manganite by Mossbauer spectroscopy using magnetic measurements. A consistent description of the results obtained in terms of the Weiss molecular field model by taking into account the peculiarities of the local environment of tin atoms has allowed the indirect Cu2+-O-Mn4+ (JCuMn ≈ −51 ± 1 K) and Mn4+-O-Mn4+ (JMnMn ≈ −0.6 ± 0.6 K) exchange interaction integrals to be estimated. Based on the Kanamori-Goodenough-Anderson model, we show that the magnitude and sign of the intrasublattice exchange integral JMnMn correspond to both the electronic configuration of the Mn4+ cations and the geometry of their local crystallographic environment in the compound under study.

Metallic ferromagnetism in a generalized Hubbard model

The possibility of realization of a metallic ferromagnetic state in a generalized Hubbard model with correlated hopping and exchange interaction integrals is investigated. The single-electron energy spectrum recently obtained by means of the mean-field approximation is applied for the description of ground-state and finite-temperature properties of the system. An expression for the Curie temperature is found, and the behavior of the temperature dependences of the magnetization and paramagnetic susceptibility is analyzed. Taking the correlated hopping into account allows one to explain certain peculiarities of the ferromagnetic behavior of transition metals and their compounds.

Quasi-one-dimensional antiferromagnetic spinel compound LiCuVO 4

An antiferromagnetic transition in the quasi-one-dimensional metaloxide compound LiCuVO 4 was found. A broad maximum of the magnetic susceptibility was observed at T M =28 K , while at T N ′=4 K the magnetic susceptibility exhibits a sharp decrease. The Néel temperature T N =3.3 K increases initially with magnetic field, and then decreases. The exchange interaction integral in the chains of edge-sharing CuO 6 octahedra and the interchain interaction integral are estimated to be J 1=22.5 K and J 2=1.3 K , respectively.

High-temperature magnetic susceptibility of Cd1-xFexTe diluted magnetic semiconductors.

The high-temperature magnetic susceptibility of the diluted magnetic semiconductor Cd 1-x Fe x Te (x=3.3%) is investigated in the temperature range 60-300 K. The observed Curie-Weiss behavior is analyzed in terms of the crystal-field model. The sum of the magnetic exchange interaction integrals Σ p≥1 z p J p =-132±16 K, deduced from the Curie-Weiss temperature, is in excellent agreement with the data obtained from our previous analysis of the high-field magnetization (B≤20 T) at low temperature (T=4.2 K)

Magnetic and Mössbauer studies of magnesium‐chromium ferrites

AbstractThe magnetization, Curie temperature, cation distribution, sublattice magnetization, and parameters of Mössbauer spectra are measured in MgFe2‐xCrxO4. The ligand‐cation 3d‐electron transfer parameters and the cation‐cation exchange interaction integrals are found using the superexchange bond model.

Determination of Effective Indirect Exchange Interaction Integrals in Ferrimagnets

AbstractIn terms of a collinear two‐sublattice ferrimagnet a statistical method of estimating exchange interaction integrals is developed. Using the Mössbauer spectroscopy data for the temperature induced variation of the spontaneous magnetization of tetra‐ and octahedral sublattices and the temperature dependence of inverse susceptibility the stable and unambigious estimates of exchange integrals for Y3Fe5O12 and Li0.5Fe2.5O4 are developed.

A study of the valence interaction integrals in effective core potential applications

The valence interactions in two effective core potential (ECP) methods, the frozen orbital ECP and the method of Sakai and Huzinaga, are shown to yield atomic valence orbital Coulomb and exchange interaction integrals closely approximating all-electron calculations. The ECP approximation is studied in some detail with special application to the ScO molecule. The too short bond distance in ScO when the 3s. 3p orbitals are included in the core is shown to stem from a long-range attraction of the ECP.

Triplet excitons in molecular pairs: Inhomogeneous distribution of molecular excitation energies as the origin of inhomogeneously broadened ESR lines

For triplet excitons in AB-pairs of napthalene- h 8, which are embedded in single crystals of napthalene- d 8, it is shown by a model calculation that the inhomogeneous distribution of molecular excitation energies results in an inhomogeneously broadened ESR line. Its width is expressed in terms of the parameters of the model (exchange interaction integral, external magnetic field, fine structure tensors) and by the width of the inhomogeneously broadened optical line of isolated napthalene- h 8 guest molecules.

Raman scattering of light by magnon‐spin flip excitations in antiferromagnetic semiconductors

AbstractEstimates are given for the process by which the s – d exchange interaction may contribute to the Raman scattering due to the magnon–spin flip of the conduction electron processes in antiferromagnetic semiconductors. It is shown that for the typical values of the s – d exchange interaction integral, i.e. 0.01–0.1 eV, the extinction coefficient can be larger by two orders of magnitude than that in ferromagnetic semiconductors. The value of the extinction coefficient in antiferromagnetic semiconductors is found to be 10−9–10−11 cm−1 sr−1. It is also shown that the change of the value of an absorbed energy in an external magnetic field allows us to determine the gyromagnetic factor for the conduction electrons.

The influence of exciton motion on spin-resonance absorption

We use a stochastic model for the exciton motion which comprises both the coherent and the incoherent motion. The incoherent part is taken care of by a stochastic process which allows the local excitation energy and the transition matrix element to fluctuate by means of a Markovian process. The interaction between the spins and their surroundings is described by the usual spin-Hamiltonian which is, however, simplified to a spin 1/2 particle (instead of the triplet state). In the present paper we solve exactly the two limiting cases of completely coherent and incoherent motion (for two molecules). In the incoherent case the influence of the exchange interaction integral is taken into account by perturbation theory. We find expressions which are immediately comparable with ESR-experimental data. This comparison and additional information derived from optical absorption measurements allow us to determine all free parameters of our model uniquely. In particular, the fluctuations of the exchange interaction integral (with strength γ1) play an important role. From these parameters we may furthermore calculate the correlation time of the proton spin resonance in agreement with experimental data. The results show clearly that at room temperature in anthracene crystals the exciton undergoes a hopping process.

The coupled coherent and incoherent motion of excitons and its influence on the line shape of optical absorption

We treat the coupled coherent and incoherent motion of Frenkel excitons by a model calculation. The model contains the four parametersa (distance of neighbouring atoms),J (exchange interaction integral), γo (describing the strength of the local energy fluctuations) and γ1 (a measure of the fluctuations of the exchange interaction integral, i.e. nonlocal fluctuations). Calculation of the optical absorption of systems with two differently oriented molecules/unit cells results in the Davydov-splitting given by Δ=8J and the linewidth given by Γ=γo+γ1. From the equation of motion of the density matrix we derive a diffusion equation. The diffusion constant is given by $$D = \frac{{a^2 }}{\hbar }\left( {2\gamma _1 + \frac{{J^2 }}{{\gamma _1 + \Gamma }}} \right)$$ . Comparison with experiment yields γo=70cm−1, γ1=0.1 cm−1 at room temperature and Γ<1 cm−1 at 4.2 °K. Using these numerical values and the criterium of Haken and Strobl we derive that at room temperature the exciton motion is incoherent and may be described by a hopping process whereas at low temperature it is coherent.

Structures et proprietes magnetiques de quelques composes de formule M Fe F 3 (M  Na, K, Rb, Cs, NH 4, T1)

The relations between structures and magnetic properties of some compounds with formula M Fe F 3 are discussed. The exchange interaction integrals are calculated by the Bethe-Peierls-Weiss methods.