Results 1 - 10 of 38
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[en] A new type of renormalization group theory using the generalized Callen identities is exploited in the study of the disordered systems. Bond diluted and frustrated Ising systems on a square lattice are analyzed with this new scheme. (author). 9 refs, 2 figs, 2 tabs
[en] New types of renormalization group theory using the generalized Callen identities are exploited in the study of the Ising model. Another type of two-step renormalization is proposed. Critical couplings and critical exponents yT and yH are calculated by these methods for square and simple cubic lattices, using different size clusters. (author). 17 refs, 2 tabs
[en] The influence of frustration on the phase transitions of a semi-infinite three-dimensional Ising model is investigated. Temperature-concentration phase diagrams for fixed values of the ratio of surface and bulk interactions can exhibit five different types of phase transition. (author). 6 refs, 1 fig
[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] 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
[en] A three-dimensional ferromagnetic Ising model with an interface amorphization is investigated with the use of the effective field theory. Phase diagrams and reduced magnetization curves of interface and bulks are studied. We obtain a number of characteristic behaviour such as the possibility of the reentrant phenomena and a large depression of interface magnetization. (author). 21 refs, 5 figs
[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] 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] 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] In this study, the electronic structure of V-doped ZnO system is studied by means of density functional theory. Different concentrations of V and rising of Fermi level increase the relative occupation of majority/minority spin of 3d state and also induce strong spin-splitting. The existence of three different states of V spin moment has been confirmed and is found to be concentration dependent. We found that O p-orbitals are responsible for the origin of the magnetic moment. Ruderman–Kittel–Kasuya–Yosida mechanism and the atomic spin polarization of V are the key factors for the appearance of ferromagnetism in V-doped ZnO system. The synthesized nanoparticles exhibit hexagonal wurtzite crystal structure, where both crystallite size and lattice parameters vary with V content. Magnetic measurements at room temperature confirm the ferromagnetic behaviour of V-doped ZnO system.