L. Bahmad

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.

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.

Wetting and layering transitions in a nano-dendrimer PAMAM structure: Monte Carlo study

This study is based on a nano-model of the dendrimer polyamidoamine (PAMAM). The idea is to examine the magnetic properties of such models in the context of wetting and the layering transitions. The studied system consists of spins Ising ferromagnetic in real nanostructure found in different scientific domains. To study this system, we perform Monte Carlo simulations leading to interesting results recapitulated in two classes. The former is the ground state phase diagrams study. The latter is the magnetic properties at non null temperatures. Also, we analyzed the effect of the terms present in the Hamiltonian governing our system such as the external magnetic field and the exchange couplings interactions.

First principle calculations with SIC correction of Fe-doped CuO compound

In this work the electronic properties of Fe doped CuO thin films are studied by using a standard density functional theory. This approach is based on the abinitio calculations under the Korringa Kohn Rostoker coherent potential approximation. We carried out our investigations in the framework of the general gradient approximation and self interaction corrected. The density of states in the energy diagrams are presented and discussed. The computed electronic properties of the studied compound confirm the half metalicity nature of this material. In addition, the absorption spectra of the studied compound within the Generalized Gradient Approximation, as proposed by Perdew Burke Ernzerhof approximations are examined. When compared with the pure CuO, the Fermi levels of doped structures are found to move to the higher energy directions. To complete this study, the effect of Fe doping method in CuO has transformed the material to half metallic one. We found a high wide impurity band in two cases of approximations methods.

Magnetic properties of a Lie symmetry double square nanostructure: Monte Carlo study

Abstract Inspired by Lie symmetries and motivated by the existence of the double hexagonal geometry in the two-dimensional material physics, we study a nanosystem based on the double square structure, which we refer as the B2 nanostructure, known as SO(5) Lie algebra. This structure involves two squares of unequal side lengths rotated by angle 45°. The model contains eight atoms instead of four ones appearing in the single square structure. The number of atoms is identified with the non- null roots of the B2 Lie algebra. In particular, we propose a nanolattice model consisting of particles with and S = 1 spins, consisting of two and three states, respectively. Theses spins are placed at the double square sites producing a new geometry formed by and (1×1) structures. More precisely, we study the effect of σ–σ and σ–S coupling exchange interactions. This investigation is made in terms of both external and crystal magnetic fields. First, we elaborate analytically the corresponding ground-state phase diagrams. Second, we investigate the magnetic properties using Monte Carlo method.