Solid State Phenomena Vols. 152-153

Abstract: Basing on the study of the transport in disordered carbon nanomaterials we argue that the correct description of the magnetoresistance can be obtained in a spin polarization theory, which processes separately the spin and orbital degrees of freedom in hoping conductivity in magnetic field. It is found that the polarization of the spin part of the electron wave function controls the probabilities of the hops between the single and double occupied localized states and, moreover, the spin polarization process can be treated as in the paramagnet with spin S=1/2. The experimental checking of the universal magnetoresistance scaling expected in the spin polarization model is provided.

Abstract: A model for iron/vanadium superlattices was proposed to study the phase transition in multilayers. The temperature dependence of magnetization, susceptibility, and specific heat were obtained. Two specific heat peaks are revealed at small ratios of interlayer and intralayer exchange.

Abstract: Transition from high-spin state to low-spin state is considered in this work. The self - energy in one-loop approximation was calculated. The three – orbital periodic Anderson-Hubbard model is used to obtain an initial segment of an optical branch in view of degeneration. The possibility of experimental verification of the received result by a method of non-elastic dispersion of slow neutrons is mentioned.

Abstract: The spin-spiral (SS) type of magnetization is studied with the Hubbard model. Consideration of noncollinearity of the magnetic moments results in a phase diagram which consists of regions of the SS and paramagnetic states depending on the number of electrons nel and the parameter U/t (U is the Hubbard repulsion, and t is an overlap integral). A possibility of stabilization of the SS state with three nonzero components of magnetic moment is considered.

Abstract: Thermodynamics of Heisenberg ferromagnets is described by means of a fluctuating exchange field operating on the atomic magnetic moments. A self-consistent method for calculating field characteristics is developed at arbitrary temperatures. It is shown that transitions in a paramagnetic state may pass both continuously and stepwise on temperature subject to magnitude of single-site susceptibility.

Abstract: The model of exchange coupling between ferromagnetic metal layers across a non-degenerate semiconductor spacer with point-like defects is considered. Calculation of an asymptotic behavior of the exchange integral is carried out. The exchange integral is found to alternate its sign depending on the position and filling of the deep impurity energy levels into the spacer. The results provide a basis for interpretation of recent experiments in iron/silicon multilayers.

Abstract: The effect of quenched nonmagnetic impurities on phase transitions in the three-dimensional Potts model with the number of spin states q=3 is studied using the Wolff single-cluster algorithm of the Monte Carlo method. By the method of fourth-order Binder cumulants, it is demonstrated that the second-order phase transition occurs in the model under study at spin concentrations p=0.9, 0.8, 0.7, and 0.65, while the first-order phase transition is observed in the pure model (p=1.0). The static critical exponents (CEs) α (heat capacity), γ (susceptibility), β (magnetization), and ν (correlation length) are calculated based on the finite-size scaling theory.

Abstract: The results for 3D anisotropic Ising model with competing interactions (ANNNI) investigated by the Monte Carlo methods are presented. The temperature dependence of thermal parameters is calculated. The character of all possible phase transitions in the model is analyzed.

Abstract: Temperature dependence of magnetic characteristics of Fe and Fe-Ni invar is considered in the dynamic nonlocal approximation of the spin-fluctuation theory. Numerical calculations show that the magnetic characteristics can have a discontinuous jump at high temperatures, well below the Curie temperature. Using methods of the catastrophe theory, we investigate the effect of small changes in the initial data on the results of the calculation. It is demonstrated that the discontinuous jump can only be smoothed but cannot be eliminated entirely without a significant change in the system of equations of the spin-fluctuation theory. Possible variants of such changes are discussed.

Abstract: Electron coherent and incoherent pairings and other types of correlations are studied exactly for the ensemble of small clusters with different geometries under variation of interaction strength, electron doping and temperature. Charge and spin collective excitations yield intriguing insights into level crossing degeneracies, phase separation and condensation. Criteria for spin-charge separation and recombination driven by interaction strength and temperature are found. Phase diagrams resemble a number of inhomogeneous, nanoscale phases and pseudogap regions seen recently in high TC cuprates, manganites, multiferroics and CMR nanomaterials.