A. El Kenz

Tricritical behaviour in diluted mixed spin-1 and spin-12 on square lattice

Abstract An effective-field theory with correlations is developed for a diluted mixed sping- 1 2 and spin-1 Ising ferromagnetic system with a crystal-field interaction D in a square lattice (Z = 4). The phase diagrams in temperature-concentration of magnetic atoms (p) and in temperature-crystal field interaction planes exhibit a variety of interesting phenomena such as second order phase transitions, tricritical points and first order phase transitions. The reentrant phenomena are also observed.

First-principles prediction of the magnetism of 4f rare-earth-metal-doped wurtzite zinc oxide

Abstract Electronic structure and magnetic properties of wurtzite ZnO semiconductor doped with rare earth (RE=La, Ce, Pr, Pm, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm and Yb) atoms were studied using spin-polarized density functional theory based on the full-potential linear augmented plane wave (FP-LAPW) method as implemented in the Wien2k code. In this approach the generalized gradient approximation (GGA) was used for the exchange-correlation (XC) potential. Our results showed that the substitution of RE ions in ZnO induced spins polarized localized states in the band gap. Moreover, the studied DMSs compounds retained half metallicity at dopant concentration x =0.625% for most of the studied elements, with 100% spin polarization at the Fermi level ( E F ). The total magnetic moments of these compounds existed due to RE 4f states present at E F , while small induced magnetic moments existed on other non-magnetic atoms as well. Finally, the energy difference between far and near configurations was investigated. It was found that the room temperature ferromagnetism was possible for RE-doped ZnO at near configuration. Since the RE-RE separation was long enough (far configuration) for magnetic coupling, the system became paramagnetic or antiferromagnetic ground state.

Diluted mixed spin-1 and spin-12 on honeycomb lattice

Abstract Using an effective field theory with correlation we study a diluted mixed spin ferromagnetic system consisting of spin - 1 2 and spin -1 with a crystal field interaction D . The influence of D on magnetic properties of the honeycomb lattice is discussed and a variety of interesting phase diagrams (in T/J, p ) and ( T/J, D/J ) planes are given.

Phase diagrams of a ferromagnetic amorphous bilayer system

Abstract Using the effective-field theory, we study phase diagrams of a ferromagnetic amorphous bilayer system, consisting of two monolayers (A and B) with different spins ( S A = 1 2 and S B = 1 2 , 1) and different interaction constants coupled together with an interlayer coupling. The effects of amorphization in the monolayer B and in the interlayer coupling are investigated. The influence of the crystal-field interaction D , in the case S A = 1 2 and S B =1, is also studied. A number of interesting phenomena are obtained such as the dependence of the tricritical behavior on amorphization. Other phenomena have been given in a variety of phase diagrams. The temperature dependence of the total magnetization is also examined.

First principle calculations for improving desorption temperature in Mg16H32 doped with Ca, Sr and Ba elements

Using ab initio calculations, we predict the improvement of the desorption temperature and the hydrogen storage properties of doped Mg-based hydrides such as, Mg15AMH32 (AM = Ca, Sr and Ba) as a super cell 2 × 2 × 2 of MgH2. In particular, the electronic structure has been obtained numerically using the all-electron full-potential local-orbital minimum-basis scheme FPLO9·00-34. Then, we discuss the formation energy calculations in terms of the material stabilities and the hydrogen storage thermodynamic properties improvements. Among others, we find that the stability and the temperature of desorption decrease without reducing significantly the high storage capacity of hydrogen. Moreover, it has been observed that such a doping procedure does not affect the electronic behavior as seen in MgH2, including the insulator state in contrast with the transition metal hydrides, which modify the electronic structure of pure MgH2.

Phase Diagrams and Magnetic Behaviour of Films with Amorphous Surfaces

The phase diagrams and magnetic behaviour of thin films with two amorphized surfaces are investigated by the use of the effective field theory with correlations. The transition temperature dependence of the exchange integral at surfaces, coupling between surface and nearest-layer, film thickness, and structural fluctuations are studied. Some interesting phenomena can occur as wetting phenomenon and compensation point.

New statistical lattice model with double honeycomb symmetry

Inspired from the connection between Lie symmetries and two-dimensional materials, we propose a new statistical lattice model based on a double hexagonal structure appearing in the G2 symmetry. We first construct an Ising-1/2 model, with spin values σ = ±1, exhibiting such a symmetry. The corresponding ground state shows the ferromagnetic, the antiferromagnetic, the partial ferrimagnetic and the topological ferrimagnetic phases depending on the exchange couplings. Then, we examine the phase diagrams and the magnetization using the mean field approximation (MFA). Among others, it has been suggested that the present model could be localized between systems involving the triangular and the single hexagonal lattice geometries.

PHASE TRANSITIONS OF THE MIXED-SPIN ISING MODEL ON A DECORATED LATTICE WITH THE NEAREST-NEIGHBOR INTERACTION BETWEEN DECORATING SPINS

A mean-field approximation is developed for a decorated ferrimagnetic Ising model, in which the two magnetic atoms A and B have spins σ=1/2 and S=1, respectively. In this system, the exchange interaction between nearest-neighbors of atom B is taken into account. Some interesting phenomena, such as the appearance of three types of phase diagrams and the existence of one and two compensation points are found. Phase diagrams and temperature dependence of the magnetizations of the system are investigated in detail.

Surface magnetic properties of an anisotropic free surface in the semi-infinite Ising model

The influence of anisotropy on the phase transitions of a semi-infinite three-dimensional Ising model is investigated by the use of the effective field theory. The surface magnetism is examined as a function of Δ = Jy/J − 1 and α = Jx/Jy, where Jx and Jy are the nearest-neighbor exchange constants in the free surface and J in the volume. The critical values Δc, transition temperatures and phase diagrams for surface ordering are given as functions of α and Δ. The reduced magnetization curves of a surface and bulk are also studied.

Magnetic behavior of Mn-doped silicon carbide nanosheet

Magnetic and optical properties of (Mn, Fe)-doped SiC nanosheet (NS) are investigated using first principle calculations based on Density Functional Theory (DFT) within the Full Potential Linearized Augmented Plane Wave (FP-LAPW) method. The Generalized Gradient Approximation (GGA) shows that doping SiC NS by Mn has a half-metal ferromagnetic behavior when one Si atom is replaced by Mn or Fe. We also study the effect of (Mn, Fe) doping on optical properties of SiC NS such as absorption coefficient and optical reflectivity as function of energy. We found that doping SiC NS increases the absorption coefficient, the optical conductivity and the reflectivity in the visible region.