Using the multiband s-f model we derive the formulae determining the Curie temperature and magnetic moment as functions of concentration x for R^{(1)}_{1-x}R^{(2)}_{x}Al_{2} intermetallic alloys, where R^{1,2} denote magnetic rare earth metals. These formulae, applied to R^{(1)}_{1-x}R^{(2)} alloys (R^{(1)} = La, Lu, Y, Zr and R^{(2)} = Gd) give the linear dependence of the Curie temperature and magnetic moment versus x in full agreement with experimental data.
We studied the ground state properties of the one-dimensional Falicov-Kimball model in the strong coupling limit for special periodic and aperiodic configurations of ions. The ground state phase diagrams of the model for nearest neighbor and next nearest neighbor hopping are discussed.
We study the electronic and magnetic properties of FeRh ordered alloys. The electronic structure is calculated by the tight-binding linear muffin-tin orbital method. We observe the decreasing magnetic moment with increase of the ratio c/a during the structural phase transformation from CsCl to CuAu type structure.
A strongly correlated electronic system is studied within the framework of the t-J model with the help of diagram technique for the Hubbard X-operators. The summing of some series of diagrams is suggested, corresponding to the following three types of approximations: the generalized random phase approximation, the mean field approximation, and the low density approximation. In generalized random phase approximation the dynamic magnetic susceptibility of paramagnetic phase is calculated, and boundaries of stability of this phase are found on the plane of Hamiltonian parameters and electron concentration n. It is shown that at some critical concentration n_{c} the system undergoes a crossover from the itinerant magnetism (n < n_{c}) to the magnetism with localized magnetic moments (n > n_{c}). Simultaneously, the system transfers from the Fermi-liquid behaviour to the regime of strong electron correlations with nonquasiparticle states. In mean field approximation equations for the order parameters were derived and phase transitions temperatures in a ferromagnetic and antiferromagnetic phases were obtained. In low density approximation a state of the saturated ferromagnetism was investigated and a critical concentration n_{s} was calculated when the nonsaturated ferromagnetism follows the saturated ferromagnetism.
The magnetic susceptibility of ferromagnetic GdM alloys (M=Cu_{1-x}Ga_{x}, Mg, Zn) has been investigated under helium gas pressure for temperatures above T_{C}. The evaluated pressure derivatives of the paramagnetic Curie temperature, dlnΘ/dP, appeared to be remarkably different for isovalent GdMg and GdZn compounds (-11.2 and -0.1 Mbar^{-1}, respectively). An analysis of the obtained dlnΘ/dP values for GdCu_{1-x}Ga_{x} alloys and results of ab initio electronic structure calculations have revealed the essential role of 5d electrons as the mediators of exchange coupling in ferromagnetic GdM compounds. The pressure derivatives of T_{C} were calculated by employing the modern mean-field theory, as well as the spin-fluctuation model. As a result, good agreement is found with the experimental values of dlnΘ/dP.
The Hartree-Fock ground-state phase diagram of the one-dimensional Hubbard model is calculated in the μ-U plane, restricted to phases with no charge density modulation. This allows antiferromagnetism, saturated ferromagnetism, spiral spin density waves and a collinear structure with unit cell ↑↑↓↓. The spiral phase is unstable against phase separation near quarter-, half- and three-quarter-filling. For large U this occurs at hole (or electron) doping of (3t/π^{2}U)^{1/3} from half-filling.
Electronic density of states of disordered fcc Pt_{1-x}Mn_{x} and Pt_{1-x}Cr_{x} alloys for 0 < x < 0.35 is calculated by the tight-binding linear muffin-tin orbital (TB LMTO) and the coherent potential approximation (CPA) method. Using the Stoner model it was found that the disordered Pt-Mn and Pt-Cr alloys are paramagnetic.
The effect of frustration in various localized and itinerant vanadium oxide compounds is discussed within next nearest neighbors Heisenberg and spin fluctuation models, respectively. In the localized moment case the S=1/2 J_1-J_2-model on a square lattice exhibits a rich phase diagram with magnetic as well as exotic hidden order phases due to the interplay of frustration and quantum fluctuations. Their signatures in the high field magnetization and in magnetocaloric quantities are surveyed. The possible quantum phase transitions are discussed and applied to layered vanadium oxides of the type AA'VO(PO_4)_2 where A, A' = Pb, Zn, Sr, Ba, Cd. In itinerant electron systems magnetic frustration may emerge as a result of electron correlations on a geometrically frustrated lattice. This mechanism causes enhanced spin fluctuations in a large region of momentum space and therefore can lead to a heavy fermion state at low temperatures as in the 3d spinel compound LiV_2O_4. The evidence from neutron scattering and NMR experiments is discussed within self-consistent renormalization theory based on local density approximation band structure calculations.
The magnetic susceptibility, χ, of the itinerant antiferromagnetic compound UGa_{3} was studied under pressure up to 2 kbar in the temperature range 64-300 K. The measured pressure derivative of the Néel temperature is found to be dT_{N}/dP=-1.1 K/kbar. In order to analyze the experimental magnetovolume effect values, d lnχ/d lnV, the volume dependent electronic structure of UGa_{3} has been calculated ab initio in a paramagnetic phase by employing a relativistic full-potential linear muffin tin orbital method and including an external magnetic field self-consistently. The calculations revealed a predominance of itinerant uranium f-states at the Fermi energy, as well as a large orbital contribution to χ.
We compare the Fermi liquid (FL) and statistical spin liquid (SSL) representations of the electronic properties for strongly correlated systems. In particular, we discuss the entropy, as well as the magnetization as a function of band filling, temperature and magnetic field. These properties are quite different in the two (FL, SSL) states. Antiferromagnetic state disappears for a small number of holes in the doped Mott insulator.
The model of a strongly correlated system in which periodically spaced Anderson-Hubbard centers are introduced into narrow-band metal is considered. Besides the interactions between localized magnetic moments and strong on-site Coulomb interaction, the model takes into account the hybridization of localized and band states. To study the effect of the lattice deformation on the electrical properties of the system, the phonon term and elastic energy have been taken into account. Green functions for band and localized electrons have been found. On this base, the energy spectrum has been investigated as a function of model parameters, temperature and external pressure. The criterion of the metal-insulator transition for an integer value of electron concentration has been derived and the phase diagram of the metal-insulator transition has been built.
The electronic properties of Fe_{3}Al were determined experimentally, with the use of the Mossbauer spectroscopy, and theoretically. The band structure of the compounds was investigated applying the self-consistent tight-binding linear muffin tin orbital method. The calculated Fermi contact term of hyperfine fields and the isomer shifts are in good agreement with the values resulting from analysis of experimental data. The different kinds of electron transfer estimated on the base of the proposed "additive model" are also strongly supported by calculations.
It is demonstrated that a wing of the Hofstadter diagram calculated for the tightly-bound s-electrons in the simple cubic lattice can be reproduced by the dispersion figure of the wave-vector square considered for the electron states of the same lattice, on the condition that the states having equal energies are taken into account. The dispersion splitting increases systematically with the distance of the states from the Brillouin zone center. A similar wing due to dispersion of the electron momentum is calculated also for the s-electron states in the body-centered cubic lattice.
We analyze the influence of hopping interaction on magnetic ordering. Scattering scheme of the Hubbard III approximation with included inter-site kinetic electron-electron correlation is used. The hopping interaction and inter-site correlation lead to two spin dependent effects: the band width correction and the band-shift correction. The band-shift correction factor causes an exchange splitting between the spin-up and spin-down spectrum, and its role is similar to the exchange interaction in the classic Stoner model. The spin dependent band width correction enhanced strongly by the inter-site kinetic correlation lowers the kinetic energy of electrons by decreasing the majority spin band width for some electron occupations with respect to the minority spin band width. The results show that in the case of the symmetrical density of states there is only ferromagnetic enhancement. For the strongly asymmetrical density of states there is a ferromagnetic transition.
The effect of a uniform pressure on the magnetic susceptibility was measured for YbPb_3 compound, wherein a degeneracy point of the energy bands is located just below the Fermi level and responsible for the anomalous diamagnetism. Theoretical analysis of the experimental data has revealed that a pronounced increase of diamagnetism with pressure is governed by closing the degeneracy point towards the Fermi energy
We analyze the possible occurrence of ferromagnetism in the Hubbard model, by means of an exact diagonalization study performed on 3- to 8-site chains with periodic boundary conditions. In the case of one hole in the half-filled configuration, we find that the Nagaoka state is reached only in the 3- and in the 4-site case. Ground states characterized by unsaturated ferromagnetism are found when the case of more than one hole is considered.
We present the influence of local ordering on the electronic and magnetic properties of Heusler-type alloys. The band structure and magnetic moments are calculated by ab initio spin-polarized tight binding linear muffin-tin orbital method. The calculated electronic density of states for Pd_{2}TiAl alloy is similar to ultraviolet photoelectron spectroscopy measurements. The self-consistent band calculations showed that the density of states at the Fermi level in Ni_{2}(Nb_{(1-x)}Ti_{x})Sn and Ni_{2}(Nb_{(1-x)}Ta_{x})Sn alloys decreased with the increase in Ti or Ta concentration. The total and local magnetic moments in ordered Rh_{2}TMSn (TM = Mn, Fe, Co, Ni, Cu) and Rh_{2}MnX (X = Al, Ga, In, Ge, Sb, Pb) Heusler-type alloys are calculated. The difference between theoretical and experimental results can be connected to the partial disorder in the samples.
We discuss the limitations of the local density approximation for high-temperature superconductors and related to them parent antiferromagnetic insulators. The calculated band-gap accuracy is pointed out for two cases: C_{60}-FCC and YBa_{2}Cu_{3}O_{6}. We also compare the results for parent CaCuO_{2} and NiO systems, as well as discuss the role of gradient and self-energy (GW) corrections for the Mott insulators.
We introduce an effective model for correlated e_g electrons which reproduces qualitatively the evolution of magnetic order in monolayer manganites when correlated wave functions are used. Here we address recent optical conductivity measurements suggesting that the splitting between the occupied and empty e_g states is very large in LaSrMnO_4, contrary to what is expected for the effective model. We argue that no contradiction was found but several simple-minded one-atom-based expectations concerning crystal-field splitting and optical conductivity are in error.
The spin-one-half Falicov-Kimball model with spin-dependent on-site interaction between localized (f) and itinerant (d) electrons is studied by small-cluster exact-diagonalization calculations and a well-controlled approximative method in two dimensions. The results obtained are used to categorize the ground-state configurations according to common features (charge and spin ordering) for all f and d electron concentrations (n_f and n_d) on finite square lattices. It is shown that only a few configuration types form the basic structure of the charge phase diagram in the n_f-n_d plane. In particular, the largest regions of stability correspond to the phase segregated configurations and configurations that can be considered as mixtures of chessboard configurations and the full (empty) lattice. Since the magnetic phase diagram is much richer than the charge phase diagram, the magnetic superstructures are examined only at selected values of f and d electron concentrations.
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