Solid State Phenomena Vol. 215

Abstract: The critical behavior and phase transitions (PT) of the three-dimensional antiferromagnetic Heisenberg model on a layered triangular lattice with nearest-neighbor (J) and next-to-nearest neighbor (J₁) interactions is studied by the replica Monte Carlo method. The first order phase transition has been revealed in the studied model based on the histogram method of processing data of the Monte Carlo for systems with large linear dimensions. Shown that the system exhibits the pseudouniversal critical behavior in the case of small lattices.

Abstract: The magnetic susceptibility of the optimally doped Pr_1-xLaCe_xCuO₄ in the superconducting state is calculated using the t-J model of Cu-O planes, the Mori projection operator technique, and the dispersion of electron bands derived from photoemission experiments. The electron band folding across the antiferromagnetic Brillouin zone border, which is inherent in the crystal, leads to a commensurate low-frequency response. The same band folding causes the appearance of a supplementary spin-excitation branch, which coexists with usual spin excitations. This coexistence can explain two maxima observed in the frequency dependence of the susceptibility. The two nested spin-excitation branches lead to a comb of closely spaced peaks in momentum cuts, which presumably are not resolved in experiment, being seen as a broad commensurate peak up to 100 meV. Reasons for differences in magnetic responses of electron- and hole-doped cuprates are discussed.

Abstract: The critical properties of two-dimensional antiferromagnetic Ising model in square lattice are investigated using the replica Monte-Carlo method with account of interactions of second nearest neighbors. The diagram of critical temperature dependence on an interaction value of second nearest neighbors is plotted. Static critical exponents of the heat capacity α, susceptibility γ, magnetization β, and correlation radius ν are calculated for this model using the finite-size scaling theory.

Abstract: Zero-point spin fluctuations are shown to strongly influence the ground state of ferromagnetic metals and to impose limitations for the fully spin polarized state assumed in half-metallic ferromagnets, which may influence their applications in spintronics. This phenomenon leads to the low-frequency Stoner excitations and cause strong damping and softening of magnons in magnetoresistive manganites observed experimentally.

Abstract: X-ray photoelectron spectra of TiO₂:Fe are measured. Electronic structure and magnetic properties of rutile doped by iron are calculated in frames of the coherent potential approximation. The main experimental spectral features of TiO₂:Fe such as heterovalent cation substitution (Fe³⁺→Ti⁴⁺), decreasing of the band gap value and appearance of additional features at the bottom and top of X-ray photoelectron spectra of valence band in comparison with those for undoped TiO₂ are described.

Abstract: Statistical and multifractal properties of Barkhausen jumps in exchange-coupled ferromagnet/antiferromagnet bilayers are studied on Co/IrMn and NiFe/NiO heterostructures using a magneto-optical indicator film technique. The statistical analysis proves a nonstochastic character of magnetization jumps. In particular, power-law behavior is observed for Co/IrMn samples. Furthermore, the statistics depends on the ferromagnet layer thickness and antiferromagnet layer material structure. The average jump size displays an asymmetry between the forward and backward branches of the hysteresis loop, particularly pronounced for the structure with a thin Co layer. In spite of the observation of such an asymmetry in the activity of the domain nucleation and pinning centers, the statistical distributions of jumps do not show any significant differences for two branches of the hysteresis loop. The conclusion on a nonrandom character of the magnetization process is supported by the multifractal analysis which reveals the presence of correlations in the time arrangement of the Barkhausen jumps.

Abstract: Thermodynamic properties of modulated magnetic ordering nanoparticles are studied by the Monte-Carlo methods. Temperature curves for main thermodynamic parameters are plotted. The availability of free surface is shown to lead to the displacement of phase transition temperatures and the complex dependency of thermodynamic parameters on a temperature. We determine the modulated ordering in nanoparicles is possible at sufficiently larger linear sizes.

Abstract: A method for electronic structure calculations of strongly correlated materials based on the coherent potential approximation is formulated and implemented. The evolution of the electronic structure of the LaMnO perovskite system in dependence on oxygen deficiency is studied.

Abstract: The phase transitions and critical phenomena in two-dimensional 3-state antiferromagnetic Potts model with account of next-nearest neighbors are investigated by Monte-Carlo method. The systems with linear sizes L=20-144 are explored. Following parities of exchange interactions are considered. Moreover, we analyze the character of phase transitions and determine the critical temperatures.