Solid State Phenomena Vol. 190

Abstract: The ageing phenomena in two-dimensional XY model at the low temperatures are investigated by Monte-Carlo method. The two-time correlation function and dynamic susceptibility are measured. Violations of the fluctuation-dissipation theorem are investigated.

Abstract: We investigate a two–dimensional single-band Hubbard model with a nearest–neighbor hopping. We treat a commensurate collinear order as well as incommensurate spiral magnetic phases at a finite temperature using a Hubbard–Stratonovich transformation with a two–field representation and solve this problem in a static approximation. We argue that temperature dramatically influence the collinear and spiral magnetic phases, phase separation in the vicinity of half–filling. The results imply a possible interpretation of unusual behavior of magnetic properties of single–layer cuprates.

Abstract: The magnetic structure of the iron monoarsenide FeAs is studied using first-principlescalculations. We consider the collinear and non-collinear (spin-spiral wave) magnetic orderingand magnetic anisotropy. It is analitically shown that a magnetic triaxial anisotropy resultsin a sum of two spin-spiral waves with opposite directions of wave vectors and different spinamplitudes, so that the magnetic moments in two perpendicular directions do not equal eachother.

Abstract: We have studied the collective excitations (spin-wave energies) in the ground state of a two-dimensional (2D) electron system using the random phase approximation (RPA). To do so, we have calculated the magnetic transverse susceptibility in 2D electron system by Hartree-Fock (HF) approximation of Hubbard model at T=0 which is generally referred to as the Stoner model. The spin-waves dispersion of three-dimensional (3D) electron system due to its quadratic behavior starts from zero at q0 while our numerical results show that the spin-waves dispersion of 2D electron system starts from the value Δ, known as exchange splitting energy, even at q=0.

Abstract: The motion equation for magnetization in locally non-equilibrium media is derived. It is shown that in non-ordered media locally non-equilibrium excitations have peculiarities. There exist coupled waves of the magnetization and magnetic vector potential. Dispersion relation of the waves have frequencies corresponding to undamped waves or constant-phase damped waves and frequencies corresponding to change of real and imaginary parts of the wave number relation. Spectrum of the non-equilibrium fluctuations exhibits some pair of spin-electromagnetic modes without energy gap and electromagnetic-spin modes with energy gap branches.

Abstract: Renormalization-group approach is applied to investigate the short-time nonequilibriumcritical behavior of pure and diluted spin systems with nonmagnetic point-like impurities.The dissipative relaxation dynamics with non-conserved order parameter is considered. For thefirst time the description of the order parameter growth induced by the initial nonequilibriumstate of system is carried out at fixed dimension d = 3 without use of ε-expansion.

Abstract: At present time, a large number of experimental works have been devoted to the study of magnetic ordering in Fe, Co, and Ni ultrathin films [. The investigation of a nature of magnetism in these structures has a large fundamental interest through an observable dimensionality crossover of magnetic characteristics from three dimensional values for films with thickness d 10 nm to two dimensional values for films with thickness d 1-2 nm. It has been established that the long-range ferromagnetic order arises in films at some effective film thickness. However, the nature and regularities of this phenomenon remain not quite clear. In our paper [ it was developed in terms of the spin-density functional theory the description of influence of the temperature and ferromagnetic ordering on the adsorption of Fe, Co, and Ni transition metal ions on a nonmagnetic substrate with the formation of a submonolayer films. For case of nonactivated adsorption the conditions for the formation of magnetic monoatomic films stable with respect to island adsorption with a change in the coverage parameter θ are revealed. It was demonstrated that the inclusion of the ferromagnetic ordering substantially affects the adsorption energy and leads to its considerable increase.

Abstract: The simultaneous effect of non-equilibrium initial states and correlation betweendefects of the structure on the evolution of anisotropic disordered systems at the critical pointwas analyzed. The field theory description of the non-equilibrium critical behavior of three-dimensional disordered systems with the long-range correlated defects was given and the dy-namical critical exponent of the short-time evolution was calculated in the two-loop approxima-tion without the use of the "-expansion. The values of the dynamical critical exponent obtainedby using various methods for summing asymptotic series were compared with the results ofthe computer simulation of the non-equilibrium critical behavior of the three-dimensional dis-ordered Ising model in the short-time regime.

Abstract: We report on ab initio density-functional calculations of a novel spintronics-related compound CrGa₂Sb₂ recently synthesized under high pressure. The effect of Cr deficiency on the electronic, magnetic and optical properties of CrGa₂Sb₂ is considered. New X-ray structural measurements up to high pressure of 9 GPa are presented.