Results 1 - 10 of 65
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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 cationic substitutions effects at different concentrations, within the ZnxCd1-xCr2Se4 lattice (0.35≤x≤0.58), on the electronic transfer in chromium's ion are investigated. For this purpose, after taking into account the exchange integrals, we use the Anderson-Kanamori theory of superexchange to determine the transfer integrals bσσ and bπσ. Using the conventional molecular orbital model, the covalency-mixing parameters λσ and λπ, the electron spin transfer coefficients fi(i=σ, π, s) and the total charge C are evaluated. The bσσ and bσπ variation with x, agrees with the passage of the system from ferromagnetic to antiferromagnetic order. For the compound presenting re-entrant behaviour (0.45≤x≤0.58), fs exhibits anomalies. The C increasing with x is consistent with the expected decrease in covalency of the non-magnetic cation in a tetrahedral site
[en] Magnetic, electronic and structural properties of titanium dioxide material with different structural defects are studied using the first-principles ab-initio calculations and the Korringa–Kohn–Rostoker method (KKR) combined with the coherent potential approximation (CPA) method in connection with the local density approximation (LDA). We investigated all structural defects in rutile TiO_2 such as Titanium interstitial (Ti_i), Titanium anti-sites (Ti_o), Titanium vacancies (V_T_i), Oxygen interstitial (O_i), Oxygen anti-sites (O_T_i) and oxygen vacancies (V_o). Mechanisms of hybridization and interaction between magnetic atoms are investigated. The transition temperature is computed using the Mean Field Approximation (MFA).Magnetic stability energy of ferromagnetic and disordered local moment states is calculated to determine the most stable state. Titanium anti-sites have a half-metallic aspect. We also studied the change type caused by structural defects in this material. - Highlights: • Green function technique is used to study disordered systems. • We used DFT to study electronic structure of TiO_2 perturbed by defects. • TiO_2 with titanium antisite defect posesses a magnetic behavior. • The transition temperature is computed using the Mean Field Approximation.
[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] 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] Highlights: • Magnetic properties of the 3d-4d double perovskite Sr2CrMoO6 have been studied. • The system is stabilized at ground state by the ferromagnetic phase of core spins. • Effective spins Hamiltonian and the exchange coupling effects have been investigated. - Abstract: In this work, we use the exact diagonalization and Monte Carlo calculations to study magnetic behaviors of the 3d-4d double perovskite Sr2CrMoO6. The model is described by a quantum Hamiltonian induced by the hybridization mechanism in Sr2CrMoO6 via the double exchange, considering the transition metal Mo5+ (σ = 1/2, 4d1) cation totally non-magnetic and classical core spins S = 3/2 located at sites of Cr3+ (S = 3/2, 3d3) cations. We have defined a Hamiltonian matrix and determined eigen-energies which are functions of core spins interactions. At ground state, we have found that the ferromagnetic phase of core spins stabilizes the system for the electronic density n = 0.25. To study magnetic properties at finite temperature, we have defined an effective magnetic Hamiltonian for spins, approving the Monte Carlo simulations for systems of high sizes. Thus, the exchange coupling effect, the magnetization and the magnetic susceptibility are investigated for different sizes, and the critical temperature is determined.
[en] Using mean field theory and high-temperature series expansions (HTSEs), extrapolated with the Pade approximants method, the effect of Zn doping on magnetic properties of NiFe2O4 ferrite spinel has been studied. The nearest neighbour super-exchange interactions for intra-site (JAA, JBB) and inter-site (JAB) of the ZnxNi1-xFe2O4 ferrites spinels, in the range 0≤x≤1, have been computed using the probability approach, based on Moessbauer data. The paramagnetic Curie-Weiss temperature θ and the Curie temperature TC are calculated as a function of Zn concentration. The critical exponent γ associated with magnetic susceptibility is calculated. The spin correlation functions intra-plane and inter-plane have been also computed and compared with exchange couplings. The obtained theoretical results are in good agreement with experimental ones obtained by magnetic measurements and Moessbauer spectroscopy.
[en] The magnetic properties of antiferromagnetic CoO compound have been studied using Monte Carlo simulations within the Ising model framework. The thermal magnetizations and magnetic susceptibilities are computed for a fixed size. In addition, the Néel temperature is deduced. The magnetization versus the reduced exchange interactions and crystal field are studied for a fixed system size, N = 10 nm particles. The magnetic hysteresis cycle versus temperature is also established. (author)
[en] A Monte Carlo model was developed to study the ferroelectric and energy storage properties of PbZr1−xTixO3 (PZT). The proposed model aims to calculate the exchange coupling constants in ferroelectric PbZr1−xTixO3 thin films system, useful for Monte Carlo simulation within metropolis algorithm. Thus, the effect of temperature on the ferroelectric properties of the PZT thin films, such as, hysteresis loops, polarization and coercive field were investigated. Moreover, the phase diagram as a function of x values of Ti in PbZr1−xTixO3 was studied. The obtained P-E hysteresis loops permitted to predicte the energy storage properties of the studied system. A maximum of the recoverable energy density of 13.93 J cm−3 was obtained with the energy density efficiency of 79% for x = 0. The obtained results are in good agreement with the reported experimental data for the same material. (paper)
[en] In this work, using the ab-initio calculations, we have investigated the phantom magnetism when the diamagnetic solids, carbon and nitrogen with d doped CdTe. We have applied in these calculations the combination between the Korringa-Kohn-Rostoker and coherent potential approximation method within the local density approximation and generalized gradient approximation (GGA). In this study, the doped compound presents a metallic behavior for both the approximations characterized by a small moment of about 0.299/0.326 and 0.249/0.266 μ for 24% of C and N, respectively. The polarization has shown a low and decreasing value from 43.73/59.56 to 0.29/2.26% for 9% and 24% of C impurity concentration, respectively. Unlike for the case of N, this parameter varies from 76.7/72.86 to 85.29/83.63% for 9% and 24% concentration, respectively. In addition, we have determined the mechanism of ferromagnetic coupling for the C- and N-doped CdTe. Furthermore, the stability of the compound is investigated by comparing the energy difference between the spin glass and ferromagnetic states. It is found that below the percolation threshold, contrary to the case of doping by N except for 20% using GGA, the C impurities lead to the most ferromagnetic stable phase. While the system changes its stability above this threshold when doped by the C impurities. Finally, we have estimated and discussed the Curie temperature using the mean field approximation.