H. Ez-Zahraouy

Order–disorder layering transitions of a spin-1 Ising model in a variable crystal field

Abstract The magnetic order–disorder layering transitions of a spin-1 Ising model are investigated, under the effect of a variable surface crystal field Δ s , 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 Δ k = Δ s / k α , 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 k 0 layers. This reentrant phenomenon is absent for the remaining N − k 0 layers, but the tricritical 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 Δ s and selected values of the parameter α are established.

Order-disorder layering transitions in a variable transverse field

Abstract The effect of a variable surface transverse magnetic field Ω s , 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)=Ω s /z α , (z=1, …, 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 .

A Monte Carlo study of random surface field effect on layering transitions

The effect of a random surface field, within the bimodal distribution, on the layering transitions in a spin-12 Ising thin film is investigated using Monte Carlo simulations. It is found that the layering transitions depend strongly on the concentration p of the disorder of the surface magnetic field, for a fixed temperature, surface and external magnetic fields. Indeed, the critical concentration pc(k) at which the magnetisation of each layer k changes the sign discontinuously, decreases with increase in applied surface magnetic field, for fixed values of the temperature T and the external magnetic field H. Moreover, the behaviour of the layer magnetisations as well as the distribution of positive and negative spins in each layer, are also established for specific values of Hs, H, p and temperature T.

First-principles study and electronic structures of Mn-doped ultrathin ZnO nanofilms

The first-principles density functional calculation is used to investigate the electronic structures and magnetic properties of Mn-doped and N-co-doped ZnO nanofilms. The band structure calculation shows that the band gaps of ZnO films with 2, 4, and 6 layers are larger than the band gap of the bulk with wurtzite structure and decrease with the increase of film thickness. However, the four-layer ZnO nanofilms exhibit ferromagnetic phases for Mn concentrations less than 24% and 12% for Mn-doping performed in the whole layers and two layers of the film respectively, while they exhibit spin glass phases for higher Mn concentrations. It is also found, on the one hand, that the spin glass phase turns into the ferromagnetic one, with the substitution of nitrogen atoms for oxygen atoms, for nitrogen concentrations higher than 16% and 5% for Mn-doping performed in the whole layers and two layers of the film respectively. On the other hand, the spin-glass state is more stable for ZnO bulk containing 5% of Mn impurities, while the ferromagnetic phase is stable by introducing the p-type carriers into the bulk system. Moreover, it is shown that using the effective field theory for ferromagnetic system, the Curie temperature is close to the room temperature for the undamped Ruderman?Kittel?Kasuya?Yoshida (RKKY) interaction.

Wetting of a corrugated surface 3D Ising model

The wetting of a corrugated surface, 3D spin-12 Ising model, in the presence of longitudinal and uniform surface fields is studied using mean field theory and Monte Carlo simulations. On the one hand, the prewetting phenomena and layering transitions occur at T=0 under the corrugation effect. Such result has not been obtained in the case of a perfect surface. On the other hand, it is found that the wetting temperature Tw depends weakly on the surface corrugation degree n for fixed values of the surface field.

Electronic and magnetic structures of V-doped zinc blende Zn 1−x V x N y O 1−y and Zn 1−x V x P y O 1−y

Electronic and magnetic structures of zinc blende ZnO doped with V impurities are studied by first-principles calculations based on the Korringa—Kohn—Rostoker (KKR) method combined with the coherent potential approximation (CPA). Calculations for the substitution of O by N or P are performed and the magnetic moment is found to be sensitive to the N or P content. Furthermore, the system exhibits a half-metallic band structure accompanied by the broadening of vanadium bands. The mechanism responsible for ferromagnetism is also discussed and the stability of the ferromagnetic state compared with that of the paramagnetic state is systematically investigated by calculating the total energy difference between them by using supercell method.

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.

Using a parametric study to analyse the performance of wood solar dryers with glazed walls

Parameters characterizing wood and air were studied in order to analyze the performance of a wood solar dryer functioning under Moroccan climate. A mathematical model based on the climate data of Rabat city was used to investigate theoretically the wood drying process. Two wood speciesthuya(Tetraclinis articulate) and pine (Pinus pinaster)-were examined in the present study. The results obtained by computer simulations are in good agreement with the experimental values. Furthermore, our findings indicate that the use of a global mass transfer coefficient for low temperature convectiveand homogenous drying conditions allows the influence of the principal operating parameters (wood thickness, wood density, air temperature, air velocity and ventilation mode) on the drying time to be estimated with great accuracy. Analyses pertaining to the two studied wood species revealed that(1)pine dries more quickly than thuya, (2) the drying process is faster in the summer relative to other seasons, (3) increasing the air velocity by 100% results in a 20% reduction in the drying time, and (4) continued ventilation reduces the drying time by 43%.