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[en] The magnetic order-disorder layering transitions of a spin-1 Ising model are investigated, under the effect of a variable surface crystal field Δs, using the mean-field theory. Each layer k, of the film formed with N layers, disorders at a finite surface crystal field distributed according to the law Δk=Δs/kα, k=1,2,...,N and α being a positive constant. We have established the temperature-crystal field phase diagrams and found a constant tricritical point and a reentrant phenomenon for the first k0 layers. This reentrant phenomenon is absent for the remaining N-k0 layers, but the cricritical points subsist and depend not only on the film thickness but also on the exponent α. On the other hand, the thermal behaviour of the surface magnetisation for a fixed value of the surface crystal field Δs and selected values of the parameter α are established
[en] The effect of a variable surface transverse magnetic field Ωs, on the order-disorder layering transitions of an Ising spin-((1)/(2)) model is investigated using mean field theory (MF) and finite cluster approximation (FC). For a film with finite thickness formed with N layers, each layer z transits at a fixed temperature from the ordered to the disordered phase, under the effect of the transverse magnetic field Ω(z)=Ωs/zα, (z=1, triple dot, N). These order-disorder layering transitions are absent for α=0 even if increasing the temperature and/or the surface transverse magnetic field. On the other hand, the dependency of longitudinal and transverse magnetisations on temperature and surface transverse magnetic field are illustrated for a fixed value of the exponent α
[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] The kinetics of the semi-infinite Ising model in the presence of a time-dependent oscillating external field is studied within the framework of the mean-field approach. We use the Glauber-type stochastic dynamics to describe the evolution time of this system. We present a variety of phase in a semi-infinite cubic ferromagnet with spin-1 in two different, planes; phase diagrams contain (BF, S 1P, SP) phase, (BF, S 1F, SP) phase, completely ordered phase (BF, S 1F, SF) and completely disordered phase (BP, S 1P, SP), which strongly depend on interaction parameters. The nature first and second orders of the transitions is characterized by studying the thermal behaviors of the dynamic magnetizations. Furthermore, the system exhibits dynamical tricritical phenomenon and the reentrant behaviors. The magnetizations and phase space trajectories are given and discussed.
[en] The effect of a random transverse field (RTF) on the wetting and layering transitions of a spin-1/2 Ising model, in the presence of bulk and surface fields, is studied within an effective field theory by using the differential operator technique. Indeed, the dependencies of the wetting temperature and wetting transverse field on the probability of the presence of a transverse field are established. For specific values of the surface field we show the existence of a critical probability p, above which wetting and layering transitions disappear. (author)
[en] Using the mean field theory, we investigate the effect of the random crystal-field on the spin-3/2 Blume-Capel model. Several new features are found including the appearance of new order phase with the magnetization m=1 at low temperature and consequently a rich ground-state phase diagram. At finite temperature, new types of phase diagrams appear. Furthermore, we show that at low temperature a first-order transition line terminated at isolated critical points, between the ferromagnetic phases. A reentrant phenomenon is also discussed
[en] In this work, we use Monte Carlo simulations to study the magnetic properties of a nanowire system based on a honeycomb lattice, in the absence as well as in the presence of both an external magnetic field and crystal field. The system is formed with NL layers having spins that can take the values σ = ±1/2 and S = ±1,0. The blocking temperature is deduced, for each spin configuration, depending on the crystal field Δ. The effect of the exchange interaction coupling Jp between the spin configurations σ and S is studied for different values of temperature at fixed crystal field. The established ground-state phase diagram, in the plane (Jp, Δ), shows that the only stable configurations are: (1/2, 0), (1/2, +1), and (1/2, −1). The thermal magnetization and susceptibility are investigated for the two spin configurations, in the absence as well as in the presence of a crystal field. Finally, we establish the hysteresis cycle for different temperature values, showing that there is almost no remaining magnetization in the absence of the external magnetic field, and that the studied system exhibits a super-paramagnetic behavior. (condensed matter: electronic structure, electrical, magnetic, and optical properties)
[en] Using Monte Carlo simulations within Ising model, we study the magnetic properties and the hysteresis loops of a core–shell nanoribbon, made of spins σ=1/2 core surrounded by spins S=1 shell with anti-ferromagnetic intermediate coupling. We analyze the ground-state phase diagrams in the presence of external magnetic and crystal fields. We show the existence of the compensation temperature and its dependence on the σ–S and S–S couplings. We investigate the effects of the crystal-field, temperature, shell interactions and intermediate coupling on the hysteresis curves. A number of characteristic behaviors are found, such as the occurrence of single and triple hysteresis loops for appropriate values of the system parameters. The obtained results are in good agreement with available experimental and theoretical works. - Highlights: • We study mixed spin (1/2, 1) core–shell nanoribbon using Monte Carlo calculations. • We show the effects of crystal and magnetic fields on thermodynamic quantities. • We give the conditions for the occurrence of compensation temperature. • We describe the influence of the system's parameters on the hysteresis curves
[en] The ferrimagnetic systems have been extensively investigated because of their fascinating properties. In this paper, the mixed spin-7/2 and spin-3 Ising system with periodic boundary conditions on a square lattice has been studied using the Monte Carlo simulation within the Metropolis algorithm; the Hamiltonian of the system includes the interaction between the first nearest neighbors and the crystal fields Δ7/2 and Δ3 generated by the spin-7/2 and spin-3, respectively. The ground-state phase diagram of the system has been established. On the other hand, the impact of the single-ion anisotropies on the compensation temperature has been shown. Several topologies of the total magnetization have been found for this system.
[en] In this paper, we used the ab-initio calculations, based on the Korringa-Kohn-Rostoker (KKR) method combined with the coherent potential approximation (CPA), to simulate the magnetic properties of ZnO, doped and co-doped with manganese and carbon, respectively. For this purpose, we have used two different approximations: the Local Density Approximation (LDA) and the Local Density Approximation-Self-Interaction Correction (LDA-SIC). Numerical results are presented for the compound Zn1 − 0.06Mn0.06O1−xCx when doping and co-doping is performed with Mn and C as doping elements. Total and partial DOSs are given for different concentrations using the two approximations, LDA and LDA-SIC. It is found that for 6% with doping by Mn the system becomes magnetic. The co-doping with carbon changes the behavior of the system : it becomes also magnetic for 4, 6 and 10% concentrations within both, LDA and LDA-SIC approximations. Furthermore, we have discussed the type of mechanism of exchange interaction and found that the double exchange is responsible for the appearing magnetism in the system, within the LDA and p-d interaction for LDA-SIC approximation. For 10% of carbon, we have found that the critical temperature approaches 280 K in the LDA approximation solely; and is about 305 K in the LDA-SIC approximation.