arXiv subjects

L. Bahmad

Publications and source records attributed to L. Bahmad.

At least 37 records · Page 2Linked to original sources

arXiv2019

Charactering the magnetic properties of the copper chalcopyrite semiconductor CuGaSe2 via Monte Carlo simulations

This manuscript presents a model and simulation of the copper chalcopyrite semiconductor CuGaSe2 in order to predict its magnetic properties. In the semiconductor material CuGaSe2 (CGS), the atom Cu is the only magnetic element with a magnetic moment value S = 1/2. We propose a Hamiltonian to calculate the energies corresponding to different configurations of the system. In a first step, we presented the magnetization behavior by using the Monte Carlo simulations (MCS). The magnetizations as a function of the temperature, the exchange coupling have been studied and discussed. We have also deduced and studied the magnetic susceptibilities. To complete this study, we have established and discussed the magnetic hysteresis loops of the studied compound. Also we have calculated the corresponding critical exponents and compared them with the standard 3D Ising model.

cond-mat.mtrl-sci↗

arXiv2019

Study of the Magnetic Properties of a Lieb Core-Shell Nano-Structure: Monte Carlo Simulations

The object of this work is to study the magnetic properties of the Lieb structure based on magnetic particles with core-shell nano-structure. This system is consisting of a ferromagnetic 2D Ising model formed with spins: sigma=1/2, S=1 and q=3/2. Firstly, we examine the magnetic properties of this nanostructure. On the other hand, we study the effect of the coupling exchange interactions and the crystal field. We use the Monte Carlo simulations to investigate also the behavior of the Lieb nano-structure. Finally, we study the influence of the temperature on the critical and compensation behaviors of the studied system. The obtained results are summarized in the phase diagrams.

cond-mat.str-el↗

arXiv2018

Study of the magnetic properties of the compound M_n B_i using the Monte Carlo simulations

The aim of this paper is to investigate the magnetic properties of the M_n B_i magnetic systems by using the Monte Carlo simulation. Indeed, we analyzed the effect of the number of layers on the magnetic properties of the compound M_n B_i. The magnetization, the binder cumulant, the Curie temperature and the hysteresis cycle are also calculated in this work. We used the free boundary conditions to simulate the properties of the studied system, and we presented the effect of multilayer numbers on the magnetic properties of M_n B_i films. Meanwhile, there are clear indications that coercive field increases drastically with the increases multilayer numbers is consistent with the experimental facts.

cond-mat.mtrl-sci↗

arXiv2018

Compensation Behavior and Magnetic Properties of a Core/Shell Double Fullerene Structure

In This work, we apply the Monte Carlo simulations to study the magnetic properties and compensation behavior of a core/shell double fullerene structure X_60 where the symbol X can be assigned to any magnetic atom. We focus our study on a system formed by a double sphere forming a core-shell. The two spheres are containing the spins: S=1/2 in the core; and σ=1 in the shell, respectively. In a first step, we investigate and discuss the corresponding ground state phase diagrams in different planes. Also, we illustrated the behavior of the magnetizations and the effect of the coupling exchange interactions as well as the crystal field. The effect of the external magnetic field, and the exchange coupling interactions on the hysteresis loops have been inspected. To complete this study, we showed the existence of the compensation temperature for the studied system.

cond-mat.mtrl-sci↗

arXiv2017

Monte Carlo study of a superlattice mixed spins under a crystal and external magnetic fields

In this paper, we study the effect of both an external and crystal magnetic fields for a superlattice with mixed spins (1/2, 1), using Monte Carlo simulations. The ground state phase diagrams, in the external magnetic field, coupling constant and the crystal magnetic field planes, were established. The effect of increasing temperatures has also been established showing the behavior of the magnetizations and susceptibilities, for fixed crystal field values. In particular, the obtained results as well as in the ground state phase diagrams, as for non null temperature are obtained for fixed external magnetic and crystal field values. On the other hand, the effect of the temperature on the hysteresis loops is presented showing that the decreasing temperature effect is to increase the absolute values of the corresponding coercitif magnetic field.

cond-mat.str-el↗

arXiv2017

Ground state study of a double core-shell dendrimer nanostructure

Based on a dendrimer graph theory, we investigate the ground state phase diagrams of a nanostructure formed by three spins: $σ=1/2$, S=1 and q=3/2. This nanostructure is built on a dendrimer with triangular core geometry. More precisely, we evaluate the effect of different physical parameters including the exchange interactions, the external magnetic and the crystal fields. By varying these parameters, we analytically elaborate the ground state phase diagrams. Concretely, we found that the existence of the crystal field increases the number of stable phases. The total number of possible phases is 24=2x3x4, which are: ($\pm$1/2,$\pm$1,$\pm$3/2), ($\pm$1/2,0,$\pm$3/2), ($\pm$1/2,$\pm$1,$\pm$1/2) and ($\pm$1/2,0,$\pm$1/2). All these phases are found to be stable, expect two special phases: (+1/2,0,-3/2) and (-1/2,0,+3/2), which are masked by the other phases in all the ground state phase diagrams.

cond-mat.stat-mech↗

arXiv2017

Study of a Nano-Square Structure with Defects: Monte Carlo Study

In this paper, we study the defects effect on magnetic properties of a nano-square structure. This system is composed with alternate layers consisting of the spins σ=3/2 and S=1. More precisely, we examine the effect of different physical parameters which are: the coupling exchange interactions, the external magnetic and the crystal fields applied on the studied system. The ground state phase diagrams have been deduced and discussed in different planes involving such different parameters. Also we study the behavior of this nano-structure by using the Monte Carlo simulations. The behavior of the magnetizations is studied as a function of the external magnetic field, the crystal field, the temperature and the exchange coupling interactions. The magnetizations and the hysteresis cycles are presented and discussed for fixed values of these parameters.

cond-mat.stat-mech↗

arXiv2008

Study of the mixed Ising spins (1/2,3/2) in a random crystal field

We study the magnetic properties of a mixed Ising ferrimagnetic system, in which the two interacting sublattices have spins $σ$, $(\pm 1/2)$ and spins $S$, $(\pm 3/2,\pm 1/2)$ in the presence of a random crystal field, with the mean field approach. The obtained results show the existence of some interesting phenomena, such as the appearance of a new ferrimagnetic phase namely the partly ferrimagnetic phase $(m_σ=\frac{-1}{2},m_S=+1)$ and consequently the existence of three topologically different types of phase diagrams. The effect of increasing the exchange interaction parameter $J$, at very low temperature is investigated. The transitions shown in these phase diagrams are in good agreement with those obtained in the ground state case.

cond-mat.stat-mech↗

arXiv2005

Interface disorder and layer transitions in Ising thin films

The disorder and layer transitions in the interface between an Ising spin-1/2 film denoted $(n)$, and an Ising spin-1 film denoted $(m)$, are studied using Monte Carlo simulations. The effects of both an external magnetic field, acting only on the spin-1/2 film, and a crystal magnetic field acting only on the spin-1 film, are studied for a fixed temperature and selected values of the coupling constant $J_p$ between the two films. It is found that for large values of the constant $J_p$, the layers of the film $(n)$, as well as those of the film $(m)$, undergo a first order layering transition. On the other hand, the only disordered layer of the film $(n)$ is that one belonging to the interface films $(n)/(m)$, for any values of the crystal field $Δ$. We show the existence of a critical value of the crystal field $Δ_c$, above which this particular layer of the film $(n)$ is disordered. We found that $Δ_c$ depends on the values of the constant coupling $(J_p)$ between the two films.

cond-mat.stat-mech↗

arXiv2005

Phase diagrams in the Blume-Emery-Griffiths Ising films

We study the spin-1 Ising model with bilinear and biquadratic exchange interactions and single-ion crystal field. In addition to the four usual phases: disordered DIS, ferromagnetic FER, antiquadrupolar AQU and ferrimagnetic FRI, we found three new phases in the case of a thin magnetic film, namely: the sublattice A non magnetic phase NMA, the sublattice B non magnetic phase NMB and the global non magnetic phase NMG. These phases are studied, for each layer of the film, either in the temperature-crystal field plane $(T,Δ)$ or in the biquadratic coupling-crystal field plane $(d,Δ)$, for diferent film thicknesses. On the other hand, the thermal behaviour of the layer quadrupolar moments $q_{A}, q_{B}$ and layer magnetisations $m_{A}, m_{B}$ are investigated for negative values of the biquadratic coupling and crystal field. It is also found that for fixed values of the biquadratic coupling, the temperature and the crystal field, each layer of the film can belong to a differnt phase. To illustrate this situation, an example is given for $d=-3.0$, $Δ=-3.0$ and $T=1.3$.

cond-mat.stat-mech↗

arXiv2005

Effect of the inhomogeneity of substrate on layering transitions

We study the surface substrate inhomogeneity effect on the wetting of a spin-1/2 Ising ferromagnetic film in an external magnetic field $H$, using Monte Carlo simulations. It is found the inhomogeneity leads to the formation of islands of positive spins in each layer. However, depending on the values of $H$, for a fixed surface magnetic field $H_s$, each layer exhibits three different phases; totally wet, nonwet and partially wet (PW). In the latter case, we show the existence of three distinct configurations namely: A configuration (PWTD) in which the layer is partially wet with a total disconnection between islands; (PWPD) a configuration in which the layer is partially wet with a partial disconnection between islands; (PWTC) a configuration in which the layer is partially wet and the islands are totally connected. Furthermore, we show that an increase of $H_s$ values breaks the bonds connecting some islands of the phase (PWTC) which leads to an expansion of the (PWPD) region. On the other hand, the frequency distribution and the mean size of island are investigated in the (PWTD) region for fixed values of temperature $T$, $H_s$ and $H$. Moreover, we show the existence of 3D-islands resulting from the formation of islands in each layer.

cond-mat.mtrl-sci↗

arXiv2005

Surface Effects On Wetting and Layering Transitions

Wetting phenomena plays an interesting role in the technological development of materials. Recently, much attention has been directed to the study of magnetic solid films. To understand, theoretically, the effect of surface on wetting and layering transitions in these systems, we will give in this chapter a presentation of several relevant recent theoretical works realized on wetting and layering transitions in which we explain how the nature of surface can affect the behavior of wetting and layering transitions. Using different spin systems models and different numerical and approximate methods such as mean field and effective field theories, real space renormalization group technique, transfer matrix method and Monte-Carlo simulations, it is found that the wetting and layering transitions depend on the nature of the surface magnetic field, the surface coupling strength, the surface crystal field, the geometry of surface, the in-homogeneity of substrate and the quantum fluctuations at the surface�

cond-mat.mtrl-sci↗

arXiv2004

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-1/2 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 $p_c(k)$ at which the magnetisation of each layer $k$ changes the sign discontinuously, decreases for increasing the 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 $H_s$, $H$, $p$ and the temperature $T$. \\

cond-mat.stat-mech↗

arXiv2003

Monte Carlo Study of Order-Disorder Layering Transitions in the Blume-Capel Model

The order-disorder layering transitions, of the Blume-Capel model, are studied using the Monte Carlo (MC) simulations, in the presence of a variable crystal field. For a very low temperature, the results are in good agreement with the ground state study. The first order transition line, found for low temperatures, is connected to the second order transition line, seen for higher temperatures, by a tri-critical point, for each layer. The reentrant phenomena, caused by a competition of thermal fluctuations and an inductor magnetic field created by the deeper layers, is found for the first $k_0$ layers from the surface, where $k_0$ is exactly the number of layering transitions allowed by the ground state study. The layer magnetizations $m_k$, the magnetic susceptibilities $χ_{m,k}$ and the quadrupolar magnetic susceptibilities $χ_{q,k}$ and the corresponding critical exponent, for each layer $k$, are also investigated.

cond-mat.soft↗

arXiv2003

Surface coupling effect on wetting and layering transitions

The effect of the surface coupling $J_s$ on the dependency of the layering transition temperature $T_L$ as a function of the thickness $N$, of a spin-1/2 Ising film, is studied using the mean field theory. It is found that for $J_s$ greater than a critical value ($J_{sc}=1.30$), the layering transition temperature decreases when the film thickness $N$ increases for any values of the surface magnetic field $H_s$. While, for $J_s < J_{sc}$, the behaviour of the layering transition temperature $T_L$, as a function of $N$, depends strongly on the values of $H_s$. Indeed, we show the existence of three distinct behaviours of $T_L$, as a function of the film thickness $N$, separated by two critical surface magnetic fields $H_{sc1}$ and $H_{sc2}$, namely: $(i)$ for $H_s < H_{sc1}$, $T_L$ increases with $N$; $(ii)$ for $H_{sc1} < H_s < H_{sc2}$, $T_L$ increases for small values of $N$, and decreases for large value ones; (iii) while for $H_s > H_{sc2}$, $T_L$ decreases with increasing the film thickness. Furthermore, depending on the values of $J_s$, the wetting temperature $T_w (T_w=T_L$ when $N \to \infty$ for a given material), can be greater or smaller than the layering transition temperature of a film of thickness $N$ of the same material.

cond-mat.soft↗

arXiv2002

Edge wetting of an Ising three-dimensional system

The effect of edge on wetting and layering transitions of a three-dimensional spin-1/2 Ising model is investigated, in the presence of longitudinal and surface magnetic fields, using mean field (MF) theory and Monte Carlo (MC) simulations. For T=0, the ground state phase diagram shows that there exist only three allowed transitions, namely: surface and bulk transition, surface transition and bulk transition. However, there exist a surface intra-layering temperature $T_{L}^{s}$, above which the surface and the intra-layering surface transitions occur. While the bulk layering and intra-layering transitions appear above an other finite temperature $T_{L}^{b} (\ge T_{L}^{s})$. These surface and bulk intra-layering transitions are not seen in the perfect surfaces case. Numerical values of $T_{L}^{s}$ and $T_{L}^{b}$, computed by Monte Carlo method are found to be smaller than those obtained using mean field theory. However, the results predicted by the two methods become similar, and are exactly those given by the ground state phase diagram, for very low temperatures. On the other hand, the behavior of the local magnetizations as a function of the external magnetic field, shows that the transitions are of the first order type. $T_{L}^{s}$ and $T_{L}^{b}$ decrease when increasing the system size and/or the surface magnetic field. In particular, $T_{L}^{b}$ reaches the wetting temperature $T_{w}$ for sufficiently large system sizes.

cond-mat.soft↗

arXiv2002

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

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.

cond-mat.soft↗

arXiv2002

Wetting and layering transitions of a spin-1/2 Ising model in a random transverse field

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 $p_{c}$ above which wetting and layering transitions disappear.

cond-mat.soft↗