Abstract: Thermodynamic properties of cubic Heisenberg ferromagnets with competing exchange interactions are considered near the frustration point where the coefficient D in the spin-wave spectrum Ek ~ Dk2 vanishes. Within the Dyson-Maleev formalism it is found that at low temperatures thermal fluctuations stabilize ferromagnetism by increasing the value of D. For not too strong frustration this leads to an unusual "concave" shape of the temperature dependence of magnetization, which is in agreement with experimental data on the europium chalcogenides. Anomalous temperature behavior of magnetization is confirmed by Monte Carlo simulation. Strong field dependence of magnetization (paraprocess) at finite temperature is found near the frustration point.
Abstract: In the past few years, systems with slow dynamics have attracted considerable interest. Coarsening effects are exhibited in a wide range of systems. Non-equilibrium critical behavior of 2D XY-model demonstrates slow dynamics in a wide temperature range. The coarsening in pure and diluted 2D XY-model are investigated for various defects concentration. The period of logarithmic grows of cluster size was found.
Abstract: The matrix (5x5) Green's function of magnetoelastic waves in an isotropic magnetoelastic medium taking into account the dipole-dipole interaction has been derived. The imaginary parts of diagonal and off-diagonal elements of the Green's function in the crossing resonance of spin and longitudinal elastic waves has been investigated. It is shown that under changing the angle of wave propagation θ the amplitudes of Green functions change drastically due redistribution of energy between the magnetic and elastic oscillation. Approximate expressions for the dependence of the widths of the gaps in the spectrum on the angle θ have been also derived.
Abstract: We consider the generalization of the scalar chiral model of graphene, the 8-spinor field being included for the description of spin and quasi-spin excitations. The invariant Lagrangian density quadratic in derivatives is constructed using the principle of energy positivity and that of correspondence with the scalar model. The electromagnetic interaction is included through the extension of derivatives, the Pauli direct interaction term being added. We consider in detail an example of interaction with the external uniform static magnetic field.
Abstract: A simple low-temperature dynamic spin-fluctuation theory of ferromagnetic metals is developed. The theory is based on the functional integral formalism for the multiband Hubbard Hamiltonian and takes into account both single-site and nonlocal spin fluctuations. We show that our approach correctly reproduces the T3/2 law at low temperatures. The calculated results of magnetic properties for Fe and Fe0.65Ni0.35 Invar demonstrate that the approach works on a much wider temperature interval than the spin-wave approximation.
Abstract: We propose a theory for the quantitative description of the thermodynamic, magnetic and electronic properties of disordered alloys of transition metals and normal, for example, the system Fe-Al, as obtained from melting, and in the form of metastable states produced by mechanical grinding. System of itinerant d-electrons with two constants of the intra-atomic exchange interaction is investigated by the fluctuating field approach. Fundamentally new feature of the system is the presence of several ferrimagnetic solutions, transitions between which give variety to the temperature dependence of the magnetic characteristics of disordered alloys. Results of calculation of the temperature dependences of the magnetic characteristics of iron aluminum alloys are considered as an application of the theory.
Abstract: The anisotropic Ising model with competing interactions in the region of transition from a modulated phase into paramagnetic state is investigated by the Monte-Carlo methods. By means of histogram analysis and the finite-size scaling method, the modulated – paramagnetic phase transition is shown to be a second order phase transition. Critical parameters and temperatures of phase transitions in this region are calculated.
Abstract: Theoretical description of ultrathin ferromagnetic Fe and Ni alloy film adsorption on a nonmagnetic metal substrate is considered by the spin-density functional method. The conditions for the formation of ultrathin ferromagnetic films stable with respect to island adsorption with a change in the coverage parameter θ and Fe content concentration x are revealed. The equilibrium vacuum gap Dmin and the equilibrium film thickness hmin are determined from the minimum of the total interfacial energy. The energy and magnetic characteristics of FexNi1-x films on close-packed W and Cu surfaces are calculated in dependence on concentration x for different temperatures.
Abstract: It is predicted that in 2D magnonic crystals the edge rotational magnons of forward vol-ume magnetostatic spin waves can exist. Under certain conditions locally bounded magnons may ap-pear within the crystal consisting of the ferromagnetic matrix and periodically inserted magnetic/non-magnetic inclusions. It is also shown that interplay of different resonances in 2D magnonic crystalmay provide conditions for spin wave modes existence with negative group velocity.