Mechanism Of Exchange Interaction Research Articles

Charge and magnetic states for the Hubbard model on a three-site cluster

The Hubbard model for small size clusters is investigated. Using direct diagonalization of the small cluster Hamiltonian, electronic, magnetic and spin susceptibilities are calculated and their behavior at low temperature is investigated. The temperature behavior of susceptibilities for two- and three-site clusters is investigated for both center and edge of the Brillouin zone. If at the given concentration the ground state is polarized, the susceptibilities diverge following the Curie law and the system is transformed into the magnetically ordered state at T = 0. Compounds with transition- and rare-earth metals (oxides, suldes and others) possess unique physical properties and attract great attention in the elds of practical applications and general-theoretical study. Re- garding the magnetic properties of such compounds, they are ferromagnetic, ferrimagnetic, and antiferromagnetic. The peculiarity of some of them is a possibility of mag- netic transitions which are followed by a change of the magnetic-order type under the external inuence. Ac- cording to the electrical properties, transition- and rare- earth metal compounds may be divided into four groups: insulators, metals, compounds which possess a possibility of a metalinsulator transition (caused by external fac- tors) with a simultaneous change of the magnetic prop- erties and magnetic-disordered compounds in which a metal-insulator transition takes place too. But at the same time, the mechanisms of exchange interactions at the metalinsulator transition in such compounds have been studied insucien tly. This is connected with strong electron correlation. In this respect there is a need for the investigation of physics of such phenomena as ferromag- netism, antiferromagnetism, etc. by using the Hubbard model. The Hubbard model (1 3) was originally proposed for the description of correlations in narrow-band materi- als on a three-dimensional lattice. It takes into account the main system characteristics, namely electron hopping and Coulomb interaction. Its two-dimensional version is often considered as the minimal model for describing the copper oxide planes in high-Tc superconductors (4, 5). The one-dimensional Hubbard model has an exact solu- tion in terms of the Bethe ansatz (6). It displays Luttinger liquid and Mott insulator phases and has received much attention (7). In innite dimensions other exact solutions for the Hubbard model were obtained within Dynamical Mean Field Theory (8). In other dimensions, for which there are no exact so- lutions, a great variety of approximate treatments have been proposed in order to accommodate a suitable theo- retical framework. In this connection, many eorts were exerted to obtain exact ground states of few electrons for the Hubbard model on a nite size cluster (see, Ref. (9) and references therein) from which the ground state ener- gy for the low-dimensional cases can be estimated using a concept of dimensional scaling (10). The latest period has provided new motivations for further investigations into the systems containing a small number of particles conned in a device or unit as, for ex- ample, in the case of quantum dots, quantum well struc- tures, mesoscopic systems, experimental entanglement, etc., for which the knowledge of the ground state does not suce. The consideration of the excited states is a much more complicated problem. It has been studied in detail only for the two-site (11 13) and four-site (14) clus- ters. In Ref. (15) linear chains and rings containing two to six atoms were studied numerically and it was found that with the increase of the cluster size the thermodynamic properties of the model at half-lling approach its one- dimensional limit whereas magnetic susceptibility shows a clear even-odd eect in the very low temperature re- gion.

Preparation and evaluation of nickelmesogen for micropacked gas chromatography

The preparation and mesomorphic properties of a substituted bis(dithiolene)nickel complex derived from 4, 4′-dimethoxybenzil are reported. The phase transition temperatures were based on data obtained by polarized light microscopy and differential scanning calorimetry. The mesogenic phase existed over the temperature range from 77 to 175 °C. A novel micropacked column (1.5 or 3 m×1 mm i.d.) prepared from the slurry of bis[1,2-bis(4- n-undecyloxyphenyl)ethane-1,2-dithiolene] nickel(II) (5%, w/w), coated on Chromosorb W was applied for the separation of dialkyl sulfides. The non-linearity (discontinuity) of Van’t Hoff plots suggests that the liquid crystal property existed even in the coated phase. Factors affecting the retention and the sample selectivity on the prepared column were examined by using a flame photometric detector (FPD). The separation might be based on the mechanism of ligand exchange, shape selectivity and polarity interaction besides the vapor pressure. LOD for the determination of dialkyl sulfides was below 1 ng for most of the analytes.

Ferromagnetism in diluted magnetic semiconductors at low carrier density

AbstractWe present a general approach to the problem of a ferromagnetic phase transition in diluted magnetic semiconductors with free carriers. The Curie temperature of ferromagnetic transition is calculated in the mean field approximation. The approach allows to analyze the effects of magnetic fluctuations and disorder on the Curie temperature. We also propose a new mechanism of exchange interaction between magnetic impurities at vanishing concentration of free carriers.

Exchange interaction and ferromagnetism in III-V semiconductors

The mechanism of indirect exchange interaction leading to ferromagnetism in magnetically doped III-V semiconductors is considered theoretically. The mechanism is based on the interplay of two interactions: (i) hybridization of band states with shallow impurity ones, and (ii) direct exchange coupling between localized spins and the band states. The indirect exchange interaction between two Mn impurities occurs when the wave functions of shallow states associated with the Mn atoms overlap. The mechanism does not rely on degenerate carriers, and therefore can describe the ferromagnetic transition in both degenerate and nondegenerate semiconductors. Ferromagnetic critical temperature has been calculated within the percolation approach, and is in good agreement with available experimental data.

Mechanism of ferromagnetism in diluted magnetic semiconductors at low carrier density

We analyze critical temperature of ferromagnetic ordering in diluted magnetic semiconductors in the mean-field approximation. Using a general field-theoretical approach we formulate a description of magnetic phase transition, which takes into account the interaction of magnetic fluctuations. We also propose a mechanism of exchange interaction between Mn impurity moments in GaMnAs alloys via the Mn-induced acceptor levels. This mechanism may lead to low temperature ferromagnetism at vanishing concentration of free carriers.

Mesoscopic mechanism of exchange interaction in magnetic multilayers

We discuss a mesoscopic mechanism of exchange interaction in ferromagnet-normal metal-ferromagnet multilayers. We show that in the case when the metal's thickness is larger than the electron mean free path, the relative orientation of magnetizations in the ferromagnets is perpendicular. The exchange energy between ferromagnets decays with the metal thickness as a power law.

Chemically Induced Dynamic Electron Polarization Study on the Mechanism of Exchange Interaction in Radical Ion Pairs Generated by Photoinduced Electron Transfer Reactions

Photoinduced electron transfer reactions were studied by using the continuous wave time-resolved electron paramagnetic resonance and Fourier-transformed electron paramagnetic resonance spectroscopy in polar solvents. The chemically induced dynamic electron polarization was investigated in both singlet and triplet precursor intermolecular electron transfer systems. The signs of the exchange interaction, which are defined by the energy differences between the singlet and triplet radical ion pairs, depended on the free energy changes for the charge recombination processes. The results are interpreted in terms of the mechanism that the spin selective stabilization and destabilization are caused by the perturbation through the electronic coupling from the ground state and the locally excited triplet state of the donor -acceptor pair at the equilibrium nuclear coordinate. In the singlet precursor electron transfer systems, the positive exchange interaction resulted from the selective triplet stabilization in the radical ion pair, when the ion pair state crossed with the locally excited triplet state at the normal region. In the triplet precursor electron transfer systems, the negative exchange interaction resulted from the selective singlet stabilization when the ion pair state crossed with the singlet ground state at the normal region. When the free energy change is larger than about 1.8 eV, the positive exchange interaction resulted from the spin-selective destabilization in the singlet ion pair, since the level crossing occurs at the inverted region.

A new type of surface spin wave in magnetoelectric crystals

It is shown for the case of a two-sublattice model of an antiferromagnet with a linear magnetoelectric effect that a new type of surface spin wave can arise at a boundary between a magnetoelectric and a nonmagnetic metal or between a magnetoelectric and a nonmagnetic insulator. This type of surface magnon arises from hybridization of the exchange and electric-dipole spin-spin interaction mechanisms.

Magnetic properties of Cu(II) bischelate complexes with 3-imidazoline nitroxides. 2. Anab initio analysis of exchange interaction mechanisms

The mechanism responsible for the emergence of ferromagnetic exchange interactions in bischelate complexes of Cu2+ with enaminoketone derivatives of 3-imidazoline nitroxide CuL2 is studied by ab initio quantum chemical methods. The parameters Jcu-L and JL-L’ of exchange interactions between the unpaired electrons of the paramagnetic centers (Cu2+ ion and N-O groups of nitroxyl ligands L and L’) of these complexes were calculated in terms of the full 3x3 configuration interaction between the singlet states constructed in a basis set of molecular orbitals of unpaired electrons. It is shown that for variations of the structure of the coordination polyhedron around the Cu2+ ion from square planar to tetrahedral the exchange interactions between the unpaired electrons of the paramagnetic centers is ferromagnetic JCu-L >JL-L’>0, which agrees with the data of magnetic measurements. The principal mechanism of exchange interactions in CuL2 complexes is the delocalization mechanism that is due to a minor transfer of spin density from the 3d-orbitals of Cu2+ to the Σ-orbitals of the N-O groups of L and L’ ligands.

Magnetic ordering in perovskites A1−xMnO3+y (A=La, Bi, rare earth ion) (abstract)

In order to clarify the mechanism of exchange interactions in orthomanganites with perovskite structure a magnetic study of parent compounds as a function of stoichiometry has been made. It was shown that the LaMnO2.99 is a weak ferromagnet with TN=147 K while LaMnO3.13 and La0.9MnO3.05 are ferromagnets with TC=160 and 240 K, respectively. The magnetic data indicate that the mixed magnetic state involving ferro- and antiferromagnetic domains is realized in the intermediate range of oxygen content. The RMnO3+y (R=Nd, Sm, Eu, Gd) are inhomogeneous antiferromagnets. The temperatures of start of magnetic moments freezing inside ferromagnetic clusters depend slightly on stoichiometry and for y∼0 are equal 83, 65, 47, and 22 K, respectively. The coercive field for LaMnO3 is 6 kOe at 4.2 K while that for EuMnO3 is much higher—26 kOe. It is worth noting that the Neel temperatures of RMnO3 (R=Eu, Gd, Tb, Dy) are estimated to be about 40 K independently of the type of rare-earth ion. Magnetic moments of Gd, Tb, and Dy in the rare-earth sublattice are antiferromagnetically ordered below 6–7 K. Applying external fields induces the antiferromagnet–ferromagnet transition in rare-earth sublattice. Hcr at 2 K for RMnO3 (R=Gd, Tb, Dy) are measured to be about 5, 12, and 10 kOe, respectively. In contrast to rare-earth compounds, the BiMnO3 is ferromagnet below 98 K. The deviation from stoichiometry as well as replacement of Bi ions by Sr, Ca, or La ions leads to the suppression of the long-range ferromagnetic order. Magnetic properties are discussed in terms of super exchange interactions between manganese ions via oxygen taking into account the 3d-orbital ordering that occurs in LaMnO3 and TbMnO3 at 400 and 1200°C, respectively.

Molecular Magnets Based on Charge Transfer Complexes

Abstract We investigated two classes of organic charge transfer complexes in the TTF family and related materials, in order to aim at the development of novel magnetic systems. The first one is the complexes with transition metal d-electrons having localized magnetic moments, where the π-electrons in organic donors play an important role in the exchange interaction mechanism between the localized magnetic moments of d-electrons. The second class is obtained on the basis of the localized magnetic moments of π-electrons, where the on-site Coulomb interaction in organic donors overwhelms the transfer integral.

Thermodynamics of intermediate-valence two-band ferromagnets: Magnetic properties

We present the influence of the finite bandwidth of the 4f (5f) band and finite intrasite Coulomb repulsion on the magnetic properties of the generalized s-f model. The calculation of the Curie temperature T C shows that the increase of the 4f (5f) bandwidth suppress the magnetic phase and can completely destroy it. The increase in the Coulomb intrasite repulsion acts in the opposite direction. The mechanism of indirect exchange interaction is discussed in detail to explain the shapes of the T C dependence on different model parameters. The calculated T C dependence on the average occupation number or on the position of the 4f (5f) level shows great similarities to the experimentally measured T C curves versus applied pressure or dopant concentration in many intermediate-valence rare-earth or actinide compounds and alloys.

Ion chromatographic separation of alkylsulphonic acids with conductivity detection

An ion chromatographic (IC) method has been developed for the separation of alkylsulphonic acid. Two different stationary phases, silica-based and polymer-based ion-exchange resins, were studied using pure ion exchange and/or hydrophobic interaction mechanisms. Correlations between analyte hydrophobicity and eluent polarity were made in order to investigate the possibility of changing the dominant separation mechanism by varying the eluent composition. The alkyl chain lengths of the sulphonic acids analysed ranged from C1 to C9. Detection limits in the submicromolar range were obtained by suppressed conductivity detection.

Nonradiative energy transfer Sn 2+ → Mn 2+ in monocrystalline KBr

Energy transfer from Sn 2+ to Mn 2+ ions in single crystals of KBr codoped with tin and manganese ions has been investigated. The spectroscopic data obtained clearly indicate that sensitized luminescence with tin ions as sensitizers and manganese ions as activators takes place in this material even at very low concentrations of the dopants and after the crystals are subjected to severe quenching treatments. This result suggests that the impurity ions are not randomly distributed but rather are forming small Sn-Mn clusters. It is established that an exchange (superexchange) interaction mechanism might be the responsible one for the Sn 2+ → Mn 2+ energy transfer process taking place in KBr.

Magnetic properties of ferromagnetic GdN

A polycrystalline GdN sample was prepared by the direct reaction of gadolinium metal and nitrogen gas at 1600°C and at a pressure of 1300 atm. We measured magnetization and the magnetic field dependence of the susceptibility and specific heat of GdN. Susceptibility and magnetization show ferromagnetic behavior for GdN with the saturation value reached at about 20 kOe. Specific heat measurements revealed a ferromagnetic transition in GdN at T c = 58 K with an additional broad peak around 20 K. Experimental results were explained with an exchange interaction mechanism.

Mechanisms of exchange interactions between carriers and Mn or Eu spins in lead chalcogenides.

Mechanisms of exchange interactions between carriers and Mn or Eu spins in lead chalcogenides.

Exchange Interaction in the Diluted Semimagnetic Semiconductors Hg1−xMnxSe Under Pressure

AbstractGalvanomagnetic properties and Shubnikov‐de Haas oscillations in n‐type Hg1−xMnxSe diluted solutions for Mn concentrations 0.001 ≦ x ≦ 0.23 in magnetic fields up to 7 T at temperatures T = (0.2 to 20) K under hydrostatic pressure up to 1.6 GPa are investigated. New methods based on oscillation measurements are proposed to determine magnetic characteristics (value and sign of exchange interaction energy for magnetic ion clusters and average cluster size) of semimagnetic semiconductor diluted solid solutions. These methods permit to determine the magnetic subsystem characteristics in the range of low temperatures between the spin‐glass transition and the temperature corresponding to the binding energy of magnetic atoms in the cluster. In this temperature range, familiar methods (based on measurements of sample magnetic moments) are not effective. The methods proposed are used to determine the dependence of interaction energy on Mn concentration and pressure. It is found that Mn concentration or pressure increase causes a change in the character of cluster interaction from antiferromagnetic to ferromagnetic. The possible exchange interaction mechanisms and their contributions to the interaction energy are discussed.

Delocalization mechanism of ferromagnetic exchange interactions in complexes of copper(II) with nitroxyl radicals

Delocalization mechanism of ferromagnetic exchange interactions in complexes of copper(II) with nitroxyl radicals

The effect of the spin exchange interaction on SNP and RYDMR spectra of geminate radical pairs

The characteristic features of the SNP and RYDMR spectra of radical pairs (RPs) in micelles and biradicals are analysed in a number of models. It is shown that recently found decrease of the SNP and RYDMR line splittings in these systems is, in general, explained by the fast ST dephasing (STD). Within a simple but reasonable model of spin-space dynamics of RPs, it is demonstrated that STD is most probably induced by the exchange interaction. Recently proposed high reactivity mechanism of the above-mentioned decrease is only a particular and rather artificial way to introduce fast STD which, in addition, is less effective than the exchange interaction mechanism.

On the mechanism of time-dependent spectral diffusion in binary solid solutions: The 1,4-dibromonaphthalene and 1-bromo-4-chloronaphthalene system

The mechanism of the spectral diffusion process has been studied in the presence of both orientational and substitutional disorder with the help of a simple model calculation. The systems of study are different compositions of solid solutions of 1-bromo-4-chloronaphthalene and 1,4-dibromonaphthalene. The isotropic exchange mechanism for different dimensions are considered in detail. The calculation points to a two-dimensional excitation exchange interaction mechanism. Furthermore, the switching of spectral diffusion occurs at a critical effective acceptor concentration.