Solid State Phenomena Vol. 190

Abstract: The dynamic critical behavior of the three-dimensional Heisenberg model with longrangecorrelated disorder was studied by using short-time Monte Carlo simulations at criticality.The static and dynamic critical exponents are determined. The simulation was performed fromordered initial state. The obtained values of the exponents are in a good agreement with resultsof the field-theoretic description of the critical behavior of this model in the two-loopapproximation.

Abstract: In the dynamic spin-fluctuation theory extended by taking into account higher-order terms of the free energy and uniform fluctuations along with the local ones, simple computational formulae for local magnetic moment are obtained. The temperature dependence of local magnetic moment in the Fe_0.65Ni_0.35 Invar is calculated. Good agreement with the experimentally estimated value of the magnetic volume change is achieved.

Abstract: The tunneling of a plane monochromatic acoustic wave through a gap between two ferromagnets under conditions of their relative longitudinal displacement has been considered. It has been shown that, the inclusion of the longitudinal displacement of one of the crystals, in all cases, leads to the violation of resonance conditions and, consequently, to a noticeable decrease in the coefficient of transmission of the acoustic wave through the gap between two ferromagnets; this effect is more pronounced, the higher is the velocity of motion of the crystal. It has been demonstrated that there is a possibility of the wave front reversal occurring with the amplification of the reflected magnetoacoustic wave.

Abstract: Dynamic susceptibilities (Greens function) in a system of two wave fields of different physical nature connected by the zero-mean random coupling parameter are studied. Magnetoelastic waves in zero-mean magnetostrictive media are considered. It is shown that the Greens functions of the spin and elastic waves at the intersection of the initial dispersion laws have the form of broad single-mode peaks with the fine structure on the tops of these peaks: the narrow resonance on the top of and the narrow antiresonance on the top of . For the long-wave inhomogeneities the widths of the broad peaks and narrow peaks are determined by the root-mean-square fluctuation and the correlation wave number, respectively.

Abstract: The problem of discontinuous phase transition in the dynamic spin-fluctuation theory is resolved by taking into account large anharmonic spin fluctuations and nonlocality of the mean Green function. The extended theory is applied to the calculation of magnetic properties of iron.

Abstract: The hybridized sp-states of valence electrons in the carbon atom can be describedby SU(2) matrix considered as the order parameter. The chiral model of graphene based onthis order parameter is suggested in the long-wave approximation, the ideal graphene planebeing determined by the kink-like solution. Corrugation of the graphene surface is describedin the form of ripple and rings. The approximate solution corresponding to an infinite carbonnanotube is found.

Abstract: We introduce and investigate a few models of three-dimensional frustrated magnets withpossible new pseudo-universal behavior appearing upon a phase transition of weak first order.We an-alyze the different broken symmetry scenarios implemented in frustrated XY and Heisenberg magnetsto compare with our results.

Abstract: We consider the magnetic phase diagram of the two-dimensional Hubbard model ona square lattice.We take into account both spiral and collinear incommensurate magnetic states.The possibility of phase separation of spiral magnetic phases is taken into consideration as well.Our study shows that all the listed phases appear to be the ground state at certain parametersof the model. Relation of the obtained results to real materials, e.g. Cu-based high-temperaturesuperconductors, is discussed.

Abstract: The spin-wave spectrum in one-dimensional magnon crystals periodic superlattices (SLs) is investigated for the rectangular and sinusoidal modulation profile of the material parameters. The dependences of band gaps in the spectrum on both the difference of the magnetic parameters in adjacent layers and the ratio of the layer thicknesses are calculated.

Abstract: Detailed study of transverse magnetoresistance (MR) and magnetic susceptibility have been done on the high quality single crystals of antiferromagnet GdB₆ (T_N15.5K) in the wide range of temperatures 2-40K in magnetic fields up to 8T. The data obtained allow to establish the low temperature antiferromagnetic (AFMII) phase below T^*~4.7K with the complex behavior of MR including the field hysteresis of the magnetoresistance. It was shown that MR behavior depends on the cooling-warming prehistory in AFM state of GdB₆. The analysis of experimental data allowed us to deduce three contributions to MR in AFM(I), (T^*) and paramagnetic (PM) phases of GdB6. In addition to the main negative component Δρ˿ρ~ H² interpreted in terms of Yosida model both the linear and nonlinear magnetic contributions were also established. The anomalies of MR found in AFM(I) state seem to be associated with the local spin polarization of 5d-electron states of Gd³⁺ ion.