Results 21 - 30 of 152
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[en] Two non perturbative methods by means of the Transfer-Matrix Finite-Size-Scaling (TMFSS) and Monte Carlo (MC) simulations are used to investigate the spin-1 Ashkin-Teller model (A.T.M.). We have obtained rich phase diagrams with first and second order phase transitions with several multicritical points of higher order. Also this model exhibits a new partially ordered phase PO2 which does not exist in the spin-1/2 Ashkin-Teller model (A.T.M.). Finally, the critical behaviour of this model is discussed. (author)
[en] In this paper, we aimed to study the effect of doping of the compound CdTe with the cobalt impurity (Co), as well as the vacancy defects in Cd sites. On one hand, this leads to the investigation of the magnetic properties and the Faraday rotation effect for the studied alloy, doped with different concentrations of cobalt (0.01, 0.05, 0.1, 0.15, 0.2, 0.25). On the other hand, we have created 0.01 of vacancy defects in Cd sites. Then, we raised the vacancy defect concentration to 0.05, keeping the same concentrations of cobalt. As a result, we have investigated that there is a magnetism appearing with the cobalt doping, while the vacancy defects in Cd sites affect the stability of the magnetic states. In general, it improves the ferromagnetic state that will be well explained in the discussion. Our calculations were performed using the KKR-CPA method within the spin-polarized density functional theory (DFT) with the local density approximation (LDA). The curves of the density of states (DOS) illustrate the results of this study which has been discussed, analyzed, and explained below. In addition, the energy of each case was calculated and given in the tables below for both the DLM (half of the cobalt spins are up while the other half are down) and the ferromagnetic state in order to confirm which one of them is stable.
[en] Using mean-field theory, we have studied the effect of quantum transverse anisotropies with RKKY interaction on the multi-layer transition and magnetic properties of the spin-1 Blume—Capel model of a system formed by two magnetic multi-layer materials, of different thicknesses, separated by a non-magnetic spacer of thickness M. It is found that the multilayer magnetic order—disorder transition temperature depends strongly on the value of the transverse anisotropy. The multilayer transition temperature decreases when increasing the transverse anisotropy. Furthermore, there exists a critical quantum transverse anisotropy ΔxL beyond which the separate transitions occur in the two magnetic layers. The critical transverse anisotropy ΔxL decreases (increases) on increasing the non-magnetic spacer of thickness M (on increasing the crystal field), and ΔxL undergoes oscillations as a function of the Fermi level. (condensed matter: electronic structure, electrical, magnetic, and optical properties)
[en] The effect of the transverse magnetic field on the phase diagrams structures of the Ising model in a random longitudinal magnetic field with a trimodal symmetric distribution is investigated within a finite cluster approximation. We find that a small magnetizations ordered phase (small ordered phase) disappears completely for a sufficiently large value of the transverse field or/and large value of the concentration of the disorder of the magnetic field. Multicritical behaviour and reentrant phenomena are discussed. The regions where the tricritical, reentrant phenomena and the small ordered phase persist are delimited as a function of the transverse field and the concentration p. Longitudinal magnetizations are also presented. (author). 33 refs, 6 figs
[en] Using the effective field theory with a probability distribution technique that accounts for the self-spin correlation functions, the phase transitions in the semi-infinite anisotropic spin-1/2 Heisenberg ferromagnet on a simple cubic lattice are examined. For fixed values of the reduced exchange anisotropic parameter, the critical temperature of the system is studied as a function of the ratio R of the surface exchange couplings to the bulk ones. It was found that if R ≤ Rc, the system orders at the bulk critical temperature TBc/J and if R ≥ Rc, the system exhibits two successive transitions. The surface orders at the surface critical temperature TSc/J which is higher than TBc/J and as the temperature is lowered, in the presence of ordered surface, the bulk orders at TBc/J. (author)
[en] Many magnetic measurements have been accomplished in order to improve the phase diagrams of the systems Zn1-x Cd Cr2 S4 and Zn1-xCdxCr2Se4 particularly in the region of the intermediate concentration with mixed structure. We have computed, in this paper, the exchange integrals for the first and second coordination sphere (J and K) versus to dilution for the three systems: Zn1-x Cd Cr2 S4, Zn1-xCdxCr2Se4 and Zn 1-xHgxCr2Se4. The evolution of the structure and the magnetic properties with the dilution of diamagnetic ions in tetraedric site is the results of the competition between the ferromagnetic and antiferromagnetic interactions. Besides we have presented a compared magnetic properties with the phase diagrams (T,X) of these systems. Because of its complicated antiferromagnetic structure, the sulphur compound seems to have particular characteristics among the mixed spinel systems. (author). 12 refs, 7 figs, 1 tab
[en] The effects of two different single-ion anisotropies on the magnetization of the mixed (7/2, 5/2) Ising ferrimagnetic system on a square lattice are investigated by the use of Monte Carlo simulation. The ground-state phase diagram is obtained. Compensation points are detected for certain values of the crystal fields. Different types of magnetization curves are available.
[en] The control of vehicles in urban traffic is a requirement to maximize the flow and to ensure the safety of traffic. Using the cellular automata Nagel–Schreckenberg (NaSch) model within a parallel dynamic update, we studied the effect of the intersection of two symmetrical roads, with typical periodic boundary conditions. It is found that the fundamental diagram depends strongly on the probability P of priority and the probability P1 of changing the road at the intersection. Beside the free flow, the platoon and the jamming phases, the fundamental diagram exhibits a fourth new phase occurring for any value of P ≠ 0.5, which disappears gradually as one increases the probability P, and disappears completely for P = 0.5. The effects of the braking probability Pb on the fundamental diagram and space time structures are also computed for different values of maximal velocities. (papers)
[en] The magnetic properties of square Ising nanowire on the Bethe lattice with core–shell structure consisting of spin-2 at the center and four spin-3/2 at the corners are studied by Monte Carlo simulation. The core–shell structured model is studied using exchange interactions between surface spins ( Jss ), between core spins ( J σσ ) and between surface and core spins ( J S σ ) and crystal field interaction (Δ) at the sites of spin-2 and spin-3/2. The critical temperature is deduced for different layers ( N ) and different shell–shell exchange interactions. The variation of magnetization with the reduced crystal field and exchange interactions on square Ising nanowire on the Bethe lattice has been studied with effect of other physical parameters. The magnetic hysteresis cycle is studied with different parameters such as: temperature, crystal field and shell–shell exchange interactions. The multiple hysteresis cycles are found. (paper)
[en] Within the framework of effective-field theory, we investigate the effects of an interface singe-ion anisotropy on the magnetic properties in the simple cubic spin-1/2 Ising model. We find a new tricritical behaviour of the interface magnetization depending on the strength of the ferromagnetic perpendicular exchange interaction the spin between the spins at the interface and their neighbors in the next layers in the right and left sides. The magnetic behaviour of an interface coupled antiferromagnetically to the two sides is also examined. (author). 25 refs, 7 figs