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[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] 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] 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] By using the effective field theory (EFT), the mixed spin-1/2 and spin-3/2 Ising ferrimagnetic model adapted to a double perovskite structure has been studied. The EFT calculations have been carried out from Ising Hamiltonian by taking into account first and second nearest-neighbors interactions and the crystal and external magnetic fields. Both first- and second-order phase transitions have been found in phase diagrams of interest. Depending on crystal-field values, the thermodynamic behavior of total magnetization indicated the compensation phenomenon existence. The hysteresis behaviors are studied by investigating the reduced magnetic field dependence of total magnetization and a series of hysteresis loops are shown for different reduced temperatures around the critical one. - Highlights: • Magnetic properties of double perovskite Structure have been studied. • Compensation temperature has been observed below the critical temperature. • Hysteresis behaviors have been studied.
[en] On the basis of ab-initio calculations and Monte Carlo simulations the magnetic and electronic properties of Gallium nitride (GaN) doped with the transition metal Manganese (Mn) were studied. The ab initio calculations were done using the AKAI–KKR–CPA method within the Local Density Approximation (LDA) approximation. We doped our Diluted Magnetic Semiconductor (DMS), with different concentrations of magnetic impurities Mn and plotted the density of state (DOS) for each one. Showing a half-metallic behavior and ferromagnetic state especially for Ga_0_._9_5Mn_0_._0_5N making this DMS a strong candidate for spintronic applications. Moreover, the magnetization and susceptibility of our system as a function of the temperature has been calculated and give for various system size L to study the size effect. In addition, the transition temperature was deduced from the peak of the susceptibility. The Ab initio results are in good agreement with literature especially for (x=0.05) of Mn which gives the most interesting results. - Highlights: • The AKAI–KKR–CPA method has been applied to study the doped compound GaN:Mn. • The local density approximation (LDA) has been applied. • The ab-initio calculations have been performed. • The density of states (DOS) have been plotted for differents doping concentrations, using Monte Carlo simulations
[en] The exchange interactions and the magnetic exchange energies are calculated by using the mean field theory and the probability law of Zn1-xMnxCr2O4 nanoparticles. The high-temperature series expansions have been applied in the spinels Zn1-xMnxCr2O4 systems, combined with the Pade approximants method, to determine the magnetic phase diagram, i.e. TC versus dilution x. The critical exponent associated with the magnetic susceptibility (γ) is deduced. The obtained value of γ is insensitive to the dilution ratio x and may be compared with other theoretical results based on the 3D Heisenberg model.