Exchange Interaction Constants Research Articles

Theoretical study of room temperature ferromagnetism and band gap energy of pure and ion doped In2O3 nanoparticles

Using the s-d microscopic model including the electron-phonon interaction and the Green’s function theory we have considered the origin of room temperature ferromagnetism (RTFM) in pure and ion doped In2O3 nanoparticles (NPs). The magnetization M increases with decreasing particle size. M of Fe, Tb and Mn doped In2O3 NPs is investigated, which increases, decreases and has a maximum, respectively, with increasing doping concentration. The RTFM is due to surface oxygen vacancies and different ionic radius of the dopants compared to that of the host ions. This differences lead to different strains which changes the exchange interaction constants. We have calculated the dependence of the band gap energy on the particle size in In2O3 NPs and the Fe concentration of Fe doped In2O3 NPs. The results are in good qualitative agreement with the experimental data.

Акцепторный центр Mn-=SUB=-Ga-=/SUB=- в GaAs ( О б з о р )

AbstractThe model of the Mn_Ga acceptor in GaAs in which initial ground state of a coupled hole Γ_8 is changed due to the antiferromagnetic exchange interaction with five 3 d electrons of the Mn core is described. The acceptor energy spectrum and the wave functions of its states and also their changes under action of deformations, electric and magnetic fields are considered. Expressions are presented for the description of various properties of isolated Mn_Ga acceptors in GaAs, and the data of some experiments (changes in the $${\text{Mn}}_{{{\text{Ga}}}}^{{\text{0}}}$$ luminescence and absorption spectra and polarization under uniaxial pressures and in magnetic field, EPR spectra, temperature dependence of the magnetic susceptibility, circular polarization of photoluminescence during excitation by polarized light in magnetic field, etc.) are analyzed. It is demonstrated that, in some cases, it is necessary to take into account the existence of random electric and strain fields splitting the acceptor states in the crystal. The analysis results show that this model agrees well with most of the experimental results. The exchange interaction constant is in the range of 3–5 meV.

Энергетическая структура одиночного акцептора Mn в GaAs : Mn

AbstractThe energy structure of the Mn acceptor, which is a complex of Mn^2+ ion plus valence band hole, is investigated in the external magnetic field and under presence of an uniaxial stress has been studied. The spin-flip Raman spectra are studied under resonant excitation of exciton bound to the Mn acceptor. The gfactors of the ground F = 1 and the first excited F = 2 states are determined and selection rules for the optical transitions between the acceptor states are described. The value of the random field (stress or electric field) acting on manganese acceptor and the deformation potential for the exchange interaction constant of the Mn^2+ + hole complex are obtained. A theoretical model is developed that takes into account the influence of random internal and uniaxial external stress and magnetic field. The proposed model describes well the lines of spin-flip Raman scattering of Mn acceptor.

Magnetic properties of Fe100−xGax: Ab initio and Monte Carlo study

This work presents a theoretical study of magnetic properties for Fe100-xGax (x = 25, 28.125%) alloys in the framework of first-principles calculations and Monte Carlo simulations. In particular, the zero-temperature ground state properties, magnetocrystalline energy, and magnetic exchange coupling constants for both D03 and L12 structures are calculated. It is shown that for both structures, the magnetocrystalline anisotropy energy change the sign from positive to negative during tetragonal distortion from c/a<1 to c/a>1, respectively. Using the exchange interaction constants within Heisenberg model, the thermomagnetization curves and Curie temperatures for compositions studied are obtained. Calculated data are in a good agreement with experimental ones.

Critical and compensation behavior of a mixed spin-5/2 and spin-3/2 Ising antiferromagnetic system in a core/shell nanowire

In this work, Monte Carlo simulations based on metropolis algorithm were performed to study the critical and compensation temperatures of a core–shell nanowire with spins and , respectively, considering an Ising antiferromagnetic system. The influence of nearest neighbors exchange interactions and crystal field anisotropy on the critical and compensation behaviors of the system has been analyzed. The effects of the nanowire height in the critical and compensation temperatures were evaluated. The results show that, for a system with given values of exchange interaction constants and crystal field anisotropy, a compensation point only appears if two requirements are satisfied. First, the weight of the core magnetization in the total magnetization must be greater than the weight of the shell magnetization at zero temperature. And second, the exchange constant of shell ions must be greater than a certain value. This value is, at the same time, greater than the exchange constant of core ions. The critical and compensation temperatures are very sensitive to variations in the exchange constant of the shell ions and core ions, respectively, while the crystal field anisotropy affects both temperatures.

Size effect on the hysteresis characteristics of a system of interacting core/shell nanoparticles

In this work, a model of interacting two-phase core/shell nanoparticles is presented. An analysis of the effect of exchange coupling, size and magnetic interaction between Fe/Fe3O4 nanoparticles on the coercive field Hc, the remanent saturation magnetisation Mrs and the saturation magnetisation Ms has been carried out. We show that the hysteresis characteristics grow together with the size of nanoparticles. Changing the exchange interaction constant leads to a nonmonotonic change of Hc and Mrs. Magnetic dipole-dipole interaction between core/shell nanoparticles results in a decrease of Hc and Mrs, due to the chaotisation of magnetic moments of the interacting nanoparticles.

First-principles calculations on magnetism and exchange interactions in GaMnAs and GaMnAsP

Based on first principles calculations using a full potential linearized augmented plane wave (FP-LAPW) method in the local spin density approximation (LSDA) we calculated the electronic and magnetic properties of Ga1−xMnxAs and Ga1−xMnxAs1−yPy. The calculated electronic band structure and density of states of both diluted magnetic semiconductors (DMSs) are discussed. These results show a reduction in energy band gap in Ga1−xMnxAs doped with a small content of P atoms (6.25% atomic percent) compared to the undoped alloy Ga1−xMnxAs. Low exchange interaction constants, N0β, and JMn−Mn, and low exchange splitting, Δxpd, are observed in Ga1−xMnxAs1−yPy. The total magnetic moment calculated for Ga1−xMnxAs1−yPy is mainly attributed to the Mn atom, whereas the contributions from Ga, As, and P are minimal. The difference between the magnetic properties of Ga1−xMnxAs and Ga1−xMnxAs1−yPy is also discussed.

Temperature magnetism of disordered iron–aluminum alloys in a model of two intra-atomic interactions

A system of band electrons with the two constants of intra-atomic exchange interaction simulates the different local environment of magnetic atoms in alloys. Temperature behavior of the magnetic characteristics of disordered Fe-Al alloys is investigated on the basis of an improved theory of dynamic fluctuations in electron spin density. The nature of the alloys’ different properties and the unusual temperature dependence of the magnetization and magnetic susceptibility is discussed.

Critical and compensation behavior of a mixed spin-3/2 and spin-5/2 Ising ferrimagnetic system in a graphene layer

We have studied the magnetic properties of the mixed spin σ=±32,±12 and spin S=±52,±32,±12 Ising ferrimagnetic system in a graphene layer by means of Monte Carlo simulations. The effects of next-nearest neighbors exchange interactions and crystal field anisotropy on the critical and compensation behavior of the system have been investigated. The results show that, for a system with given values of the crystal field anisotropy and exchange interaction constants, a compensation point only exists if the values of the spins in the ground state are such that |S|>|σ| and Jσ is higher than a certain value Jσmin. It was shown that the relationship between Jσmin and JS is linear for a given value of the crystal field constant. The compensation and the critical temperature are very sensitive to the change of JS and Jσ, respectively, while the crystal field anisotropy affects both temperatures to a large extent.

Calculation of the electronic structure, lattice dynamics, and optical and magnetic properties of europium tetraborate EuB4O7

The electronic band structure, lattice vibration frequencies, and optical and magnetic properties of EuB4O7 crystal with α-SrB4O7-type structure are calculated within the density functional method. It is found that this compound is a dielectric with a band gap of the order of 4 eV. It is found that the ground state of this crystal is the state with ferromagnetic ordering of Eu2+ spins. Exchange interaction constants are calculated; the ferromagnetic ordering temperature is estimated within the molecular field approximation (T c ≈ 1 K). The EuB4O7 compound is a magnetic pyroelectric and, hence, can exhibit magnetoelectric properties. The calculated polarization change during ferromagnetic ordering of this crystal is 3973 μC/m2.

Structure, lattice dynamics, and exchange interaction in Lu2V2O7, Y2V2O7: an ab initio approach

Ab initio calculations of the crystal structure and fundamental vibrations of vanadium pyrochlores Lu2V2O7 and Y2V2O7 are performed. The calculations are performed in the framework of the density functional theory (DFT) with the use of hybrid functionals. The ions involved in the vibrations are determined by the isotope substitution method. Values of the isotropic exchange interaction constant were calculated. Theoretical results for the crystal structure parameters, the vibrational frequencies, and the isotropic exchange interaction parameter are compared with the experimental data.

Magnetospectroscopy of excited states in charge-tunable GaAs/AlGaAs [111] quantum dots

We present a combined experimental and theoretical study of highly charged and excited electron-hole complexes in strain-free (111) GaAs/AlGaAs quantum dots grown by droplet epitaxy. We address the complexes with one of the charge carriers residing in the excited state, namely, the ``hot'' trions X$^{-*}$ and X$^{+*}$, and the doubly negatively charged exciton X$^{2-}$. Our magneto-photoluminescence experiments performed on single quantum dots in the Faraday geometry uncover characteristic emission patterns for each excited electron-hole complex, which are very different from the photoluminescence spectra observed in (001)-grown quantum dots. We present a detailed theory of the fine structure and magneto-photoluminescence spectra of X$^{-*}$, X$^{+*}$ and X$^{2-}$ complexes, governed by the interplay between the electron-hole Coulomb exchange interaction and the heavy-hole mixing, characteristic for these quantum dots with a trigonal symmetry. Comparison between experiment and theory of the magneto-photoluminescence allows for precise charge state identification, as well as extraction of electron-hole exchange interaction constants and $g$-factors for the charge carriers occupying excited states.

Magnetostructural Correlation for High-Nuclearity Iron(III)/Oxo Complexes and Application to Fe5, Fe6, and Fe8 Clusters.

The synthesis and characterization are reported of two new polynuclear Fe(III) complexes containing the anion of 8-hydroxyquinoline (hqnH), an N,O-chelating ligand. The complexes are [Fe8O4(O2CPh)10(hqn)4(OMe)2] (1) and [Fe6O2(OH)2(O2CPh)10(hqn)2] (2) and were obtained from reactions in MeOH (1) or H2O (2) using either low-nuclearity preformed clusters or simple metal salts as starting materials. Variable-temperature, solid-state dc and ac magnetic susceptibility studies were carried out and indicate S = 0 and S = 5 ground states for 1 and 2, respectively. In order to rationalize the ground states of these and other higher-nuclearity Fe(III)/O clusters, a magnetostructural correlation (MSC) has been developed specifically for polynuclear Fe(III)/O systems that predicts the exchange interaction constant (Jij) between two Fe(III) atoms based on the Fe-O distances and Fe-O-Fe angles at monoatomically bridging ligands. This correlation was refined using selected tri- and tetranuclear complexes in the literature for which both crystal structures and reliable experimentally determined Jij values were available. The predictive capability of the MSC was evaluated by rationalizing the ground-state spins of 1, 2, and other Fe5-Fe8 clusters, simulating the dc magnetic susceptibility data of polynuclear Fe(III) complexes, and fitting experimental dc magnetic susceptibility vs T data. The latter fits were evaluated to identify and eliminate systematic errors, and this allowed a protocol to be developed for application of this MSC to other polynuclear Fe(III)/oxo clusters.

Dependence of the spin-wave resonance spectra in ferrite–garnet films on the microwave field frequency and damping parameter

The factors affecting the slope of the dispersion curve of the spin-wave resonance spectrum in multilayer films are determined. It is shown that an increase in the slope of the curve for the transverse orientation of the constant magnetic field relative to the film upon an increase in frequency is due to enhancement of dynamic as well as dissipative mechanisms of spin pinning. It is found that an increase in the damping parameter increases the degree of spin pinning in the case when the pinning layer is a reactive medium for spin oscillations and can decrease the degree of pinning when it is a dispersive medium. The conditions ensuring a higher degree of accuracy in determining the exchange interaction constant from the spin-wave resonance spectrum in multilayer films are determined.

Phase transitions in a two-dimensional antiferromagnetic Potts model on a triangular lattice with next-nearest neighbor interactions

Phase transitions (PTs) and frustrations in two-dimensional structures described by a three-vertex antiferromagnetic Potts model on a triangular lattice are investigated by the Monte Carlo method with regard to nearest and next-nearest neighbors with interaction constants J1 and J2, respectively. PTs in these models are analyzed for the ratio r = J2/J1 of next-nearest to nearest exchange interaction constants in the interval |r| = 0–1.0. On the basis of the analysis of the low-temperature entropy, the density of states function of the system, and the fourth-order Binder cumulants, it is shown that a Potts model with interaction constants J1 0 and J2 < 0, frustrations arise in the range 0.5 < |r| < 1.0, while, in the interval 0 ⩽ |r| ⩽ 1/3, the model exhibits a second-order PT.

The effect of the frequency of an alternating magnetic field on the dispersion characteristics of the spin-wave resonance spectra in multilayer films

It is demonstrated that the misalignment of dispersion curves for the spin-wave resonance (SWR) spectra under perpendicular and parallel (with respect to the film plane) orientations of a constant magnetic field is reduced when the frequency of an SHF field is increased. It is found that the main factor driving the increase in the curve slope under a perpendicular orientation is related to the transition of the fixation layer from the state of a reactive medium to the state of a dispersive one. It is shown that a dispersion curve plotted for an orientation under which the fixation layer becomes a dispersive medium should be used in order to determine correctly the exchange interaction constant and the exchange rigidity based on the SWR spectrum.

Spin Fluctuations in Disordered Metallic Ferrimagnetic Alloys

We propose a theory for the quantitative description of the thermodynamic, magnetic and electronic properties of disordered alloys of transition metals and normal, for example, the system Fe-Al, as obtained from melting, and in the form of metastable states produced by mechanical grinding. System of itinerantd-electrons with two constants of the intra-atomic exchange interaction is investigated by the fluctuating field approach. Fundamentally new feature of the system is the presence of several ferrimagnetic solutions, transitions between which give variety to the temperature dependence of the magnetic characteristics of disordered alloys. Results of calculation of the temperature dependences of the magnetic characteristics of iron aluminum alloys are considered as an application of the theory.

Axial quadrupole phase of a uniaxial spin-1 magnet

The axial quadrupole phase of uniaxial spin-1 magnet in an external magnetic field has been investigated. The case of magnetic system with the most general form of single‐ion anisotropy and anisotropic biquadratic exchange interaction is considered. It is shown that the relative magnetization in the molecular field approximation does not depend on temperature and linearly increases with external magnetic field. Two branches of the spin excitation spectrum are determined. The boundary between the axial quadrupole and angular phases is defined by the condition for softening of the spectrum. The critical temperature of the corresponding phase transition considerably depends on the anisotropy constants of the biquadratic exchange interaction.

Intergrain exchange interaction estimation from the remanence magnetization analysis

Abstract Analysis of δ m ( H ) = [ M d ( H ) − M r ( ∞ ) + 2 M r ( H ) ] / M r ( ∞ ) curves constructed from dc demagnetization and isothermal remanent magnetization ( M d ( H ) and M r ( H ) respectively) is important for characterization of the interactions in ferromagnets. Up to now, it has been mainly used for qualitative deductions about them. In this work, the novel functional relation between the maximum of the δ m ( H ) plot and the microscopic parameters of the weakly coupled Stoner–Wohlfarth ensemble with the isotropic distribution of easy magnetization axes was established using computer modeling. It allows quantitative analysis in the frame of the model to be performed. Finally, a new method of estimating the intergrain exchange interaction constant for nanostructured high anisotropy magnets could be formulated taking into account the results of the modeling.

Magnetic relaxation of 1D coordination polymers (X)₂[Mn(acacen)Fe(CN)₆], X = Ph₄P⁺, Et₄N⁺.

Substitution of the organic cation X in the 1D polymer, (X)2[Mn(acacen)Fe(CN)6], leads to an essential change in magnetic behavior. Due to the presence of more voluminous Ph4P(+) cations, the polyanion has a more geometrically distorted chain skeleton and, as a consequence, enhanced single chain magnet (SCM) characteristics compared to those for Et4N(+). The Arrhenius relaxation energy barriers, the exchange interaction constant and the zero-field splitting anisotropy of Mn(III) are determined from the analysis of magnetic measurements. The discussion is supported with ligand field calculations for [Fe(CN)6](3-) that unveils the significant anisotropy of Fe magnetic moments.