Monte Carlo study of nanowire magnetic properties

In this work, we use Monte Carlo simulations to study the magnetic properties of a nanowire system based on a honeycomb lattice, in the absence as well as in the presence of both an external magnetic field and crystal field. The system is formed with NL layers having spins that can take the values σ = ±1/2 and S = ±1, 0. The blocking temperature is deduced, for each spin configuration, depending on the crystal field Δ. The effect of the exchange interaction coupling Jp between the spin configurations σ and S is studied for different values of temperature at fixed crystal field. The established ground-state phase diagram, in the plane (Jp ,Δ), shows that the only stable configurations are: (1/2, 0), (1/2, +1), and (1/2,-1). The thermal magnetization and susceptibility are investigated for the two spin configurations, in the absence as well as in the presence of a crystal field. Finally, we establish the hysteresis cycle for different temperature values, showing that there is almost no remaining magnetization in the absence of the external magnetic field, and that the studied system exhibits a super-paramagnetic behavior.     

Monte Carlo study on bilayer thin films magnetic properties dependence with discontinouos anisotropy parameters

In the last few years there has been significant interest in the field of thin films, due to numerous specific phenomena related to the low dimension of these systems, and to the large opportunities in development of high technologies based on their specific magnetic and electronic properties. When dealing with systems of reduced dimensionality it is important to take into account the influence of magnetic anisotropies. In this paper we investigate the magnetic properties of bilayer thin film. This behavior is modeled using Monte Carlo simulations, in the Extended Anisotropic Heisenberg Model. The magnetization, out-of-plane and in-plane magnetic susceptibilities, and also the specific heat bearings according to temperature are investigated in order to find the potential magnetic ordering phases and the critical temperatures, for two sets parameter assignments. For quasi-uniform anisotropy parameters of the film we detect the ferromagnetism-paramagnetism transition and then, by changing the model parameters values, we relieve a short range ferromagnetic ordering phase arising from the antiferromagnetic base layer coupling influence and from easy-plane anisotropy discontinuity on the layers interface.      

Monte Carlo study of a superlattice with alternate layers

A Monte Carlo (MC) simulation was used to observe the magnetic behavior of a superlattice Ising Model, in the presence of both an external and crystal magnetic fields. The system is made up to layers σ=±1/2$\sigma =\pm 1/2$ and S=±1,0$S=\pm 1,0$. The effect of the exchange interaction coupling _Jp$J_p$ between the spin configurations σ$\sigma$ and S$S$ is investigated for different values of temperature at fixed values of the crystal field. We found that this parameter increases the magnetization of the system at high temperature. Also, the critical temperature is calculated, for each spin configuration as function of temperature using the MC technique. The thermal behavior magnetizations and susceptibilities are studied. Finally, the response of the magnetization to the field shows a hysteresis behavior.     

Effect of electric field and magnetic field on spin transport in bilayer graphene armchair nanoribbons: A Monte Carlo simulation study

In this article we study the effect of external magnetic field and electric field on spin transport in bilayer armchair graphene nanoribbons (GNR) by employing semiclassical Monte Carlo approach. We include D'yakonov-Perel' (DP) relaxation due to structural inversion asymmetry (Rashba spin-orbit coupling) and Elliott-Yafet (EY) relaxation to model spin dephasing. In the model we neglect the effect of local magnetic moments due to adatoms and vacancies. We have considered injection polarization along z-direction perpendicular to the plane of graphene and the magnitude of ensemble averaged spin variation is studied along the x-direction which is the transport direction. To the best of our knowledge there has been no theoretical investigation of the effects of external magnetic field on spin transport in graphene nanoribbons. This theoretical investigation is important in order to identify the factors responsible for experimentally observed spin relaxation length in graphene GNRs.     

Dendrimer-magnetic nanostructure: a Monte Carlo simulation

In this paper, the magnetic properties of ternary mixed spins (σ,S,q) Ising model on a dendrimer nanostructure are studied using Monte Carlo simulations. The ground state phase diagrams of dendrimer nanostructure with ternary mixed spins σ = 1/2, S = 1 and q = 3/2 Ising model are found. The variation of the thermal total and partial magnetizations with the different exchange interactions, the external magnetic fields and the crystal fields have been also studied. The reduced critical temperatures have been deduced. The magnetic hysteresis cycles have been discussed. In particular, the corresponding magnetic coercive filed values have been deduced. The multiples hysteresis cycles are found. The dendrimer nanostructure has several applications in the medicine.     

Critical dynamics of the three-dimensional Ising model: A Monte Carlo study

Extensive Monte Carlo simulations have been performed to analyze the dynamical behavior of the three-dimensional Ising model with local dynamics. We have studied the equilibrium correlation functions and the power spectral densities of odd and even observables. The exponential relaxation times have been calculated in the asymptotic one-exponential time region. We find that the critical exponentz=2.09 ±0.02 characterizes the algebraic divergence with lattice size for all observables. The influence of scaling corrections has been analyzed. We have determined integrated relaxation times as well. Their dynamical exponentz _int agrees withz for correlations of the magnetization and its absolute value, but it is different for energy correlations. We have applied a scaling method to analyze the behavior of the correlation functions. This method verifies excellent scaling behavior and yields a dynamical exponentz _scal which perfectly agrees withz.     

Monte Carlo study of percolation on disordered triangular lattices

A simple model for amorphous solids, consisting of a mixed bond triangular lattice with a fraction of attenuated bonds randomly distributed (which simulate the presence of defects in the surface), is studied here by using computational simulation. The degree of disorder of the surface is tunable by selecting the values of (1) the fraction of regular [attenuated] bonds ρ [1−ρ] (0≤ρ≤1) and (2) the factor r, which is defined as the ratio between the value of the conductivity associated to an attenuated bond and that corresponding to a regular bond (0≤r≤1). The results obtained show how the percolation properties of the disordered system are modified with respect to the standard random bond percolation problem (r=0).     

Magnetic properties of Ni/Au core/shell studied by Monte Carlo simulations

The magnetic properties of ferromagnetic Ni/Au core/shell have been studied using Monte Carlo simulations within the Ising model framework. The considered Hamiltonian includes the exchange interactions between Ni–Ni, Au–Au and Ni–Au and the external magnetic field. The thermal total magnetizations and total magnetic susceptibilities of core/shell Ni/Au are computed. The critical temperature is deduced. The exchange interaction between Ni and Au atoms is obtained. In addition, the total magnetizations versus the external magnetic field and crystal filed for different temperature are also established.     

Magnetic properties for cobalt nanorings: Monte Carlo simulation

In this paper, two structure models of cobalt nanoring cells (double-nanorings and four-nanorings, named as D-rings and F-rings, respectively) have been considered. Base on Monte Carlo simulation, the magnetic properties of the D-rings and F-rings, such as hysteresis loops, spin configuration, coercivity, etc., have been studied. The simulated results indicate that both D-rings and F-rings with different inner radius (r) and separation of ring centers (d) display interesting magnetization behavior and spin configurations (onion-, vortex- and crescent shape vortex-type states) in magnetization process. Moreover, it is found that the overlap between the nearest single nanorings connect can result in the deviation of the vortex-type states in the connected regions. Therefore, the appropriate d should be well considered in the design of nanoring device. The simulated results can be explained by the competition between exchange energy and dipolar energy in Co nanorings system. Furthermore, it is found that the simulated temperature dependence of the coercivity for the D-rings with different d can be well described by H_c=H₀ exp[−(T/T₀)^p].     

Magnetocaloric effect and magnetic properties in YMnO3 perovskite

The magnetic properties and magnetocaloric effect in YMnO₃ have been investigated using Monte Carlo simulations. The thermal magnetization, specific heat and magnetic entropy have been obtained for different values of exchange interactions and for a several external magnetic field values. The variation of adiabatic temperature change with the temperatures has been obtained for several values of external magnetic field. It has been found that the sample exhibited a paramagnetic to ferromagnetic phase transition at 30 K. The transition temperature of YMnO₃ has been deduced for different values of size (1/L) and different values of exchange interactions. The relative cooling power with several values of external magnetic field has been established.     

Two-dimensional Monte Carlo simulations of a suspension comprised of magnetic and nonmagnetic particles in gradient magnetic fields

Distributions of particles in a suspension comprised of magnetic particles (MPs) and nonmagnetic particles (NPs) under gradient magnetic fields are vitally important for the preparation of magnetic-nonmagnetic functionally graded materials (FGMs). In the present study, the effects of magnetic field gradient, magnetic interaction between MPs and concentration of NPs on the distributions of particles in the suspension are investigated using a two-dimensional Monte Carlo simulation. The results show that a gradient distribution of MPs is formed under gradient magnetic fields and increases with increasing the field gradient. However, as the interaction between MPs increases, the distribution gradient decreases, accompanied by the formation of chain-like MP clusters. Moreover, NPs are found to hinder the translation of MPs along the field direction. As the NP concentration increases, the translation of MPs becomes difficult.     

Ab initio and Monte Carlo studies of phase transitions and magnetic properties of YCrO3: Heisenberg model

By using the density functional theory (DFT) and Monte Carlo simulations (MCS) with the Heisenberg model, we have studied magnetic properties of the bulk perovskite YCrO₃. The exchange couplings of the Heisenberg model and the magnetic anisotropy are investigated. The 110 direction in the crystalline structure of the compound has shown the minimum energy, it is the easy magnetic direction. Using Monte Carlo simulations, the magnetizations behavior, the effects of system parameters and the critical exponents of the compound YCrO₃ are implemented. It is shown that the bulk perovskite YCrO₃ belongs to the 3D Heisenberg universality class.     

Monte Carlo simulation of magnetic multi-core nanoparticles

In this paper, a Monte Carlo simulation is carried out to evaluate the equilibrium magnetization of magnetic multi-core nanoparticles in a liquid and subjected to a static magnetic field. The particles contain a magnetic multi-core consisting of a cluster of magnetic single-domains of magnetite. We show that the magnetization of multi-core nanoparticles cannot be fully described by a Langevin model. Inter-domain dipolar interactions and domain magnetic anisotropy contribute to decrease the magnetization of the particles, whereas the single-domain size distribution yields an increase in magnetization. Also, we show that the interactions affect the effective magnetic moment of the multi-core nanoparticles.     

Phase transition and magnetic properties of a decorated Ising film: Monte Carlo and effective field treatments

A Monte Carlo Simulation (MCS) has been used to study critical behavior and magnetic properties of a decorated Ising film on a cubic lattice structure. The system consists of two magnetic atoms species A and B with and , where the B sites decorate the bonds between A sites. The dependence of the phase diagram on the axial zero-field splitting parameter, the exchange interactions and the thickness of the film are studied. We found that the system exhibits a double reentrant phenomenon and one or two compensation points. MCS results are compared to those obtained by using the effective field theory (EFT) based on the use of a probability distribution technique. However, MCS predicts the same topology of the phase diagrams as the effective field theory.     

Two-dimensional Monte Carlo simulations of structures of a suspension comprised of magnetic and nonmagnetic particles in uniform magnetic fields

The structures of suspensions comprised of magnetic and nonmagnetic particles in magnetic fields are studied using two-dimensional Monte Carlo simulations. The magnetic interaction among magnetic particles, magnetic field strength, and concentrations of both magnetic and nonmagnetic particles are considered as key influencing factors in the present work. The results show that chain-like clusters of magnetic particles are formed along the field direction. The size of the clusters increases with increasing magnetic interaction between magnetic particles, while it keeps nearly unchanged as the field strength increases. As the concentration of magnetic particles increases, both the number and size of the clusters increase. Moreover, nonmagnetic particles are found to hinder the migration of magnetic ones. As the concentration of nonmagnetic particles increases, the hindrance on migration of magnetic particles is enhanced.     

Thermodynamic properties of water in confined environments: a Monte Carlo study

Monte Carlo simulations of Mercedes-Benz water in a crowded environment were performed. The simulated systems are representative of both composite, porous or sintered materials and living cells with typical matrix packings. We studied the influence of overall temperature as well as the density and size of matrix particles on water density, particle distributions, hydrogen bond formation and thermodynamic quantities. Interestingly, temperature and space occupancy of matrix exhibit a similar effect on water properties following the competition between the kinetic and the potential energy of the system, whereby temperature increases the kinetic and matrix packing decreases the potential contribution. A novel thermodynamic decomposition approach was applied to gain insight into individual contributions of different types of inter-particle interactions. This decomposition proved to be useful and in good agreement with the total thermodynamic quantities especially at higher temperatures and matrix packings, where higher-order potential-energy mixing terms lose their importance.     

The phase diagrams of a finite superlattice with disordered interface: A Monte Carlo study

Monte Carlo Simulation (MCS) has been used to study critical and compensation behavior of a ferrimagnetic superlattice on a simple cubic lattice. The superlattice consists of k unit cells, where the unit cell contains L layers of spin −1/2 A atoms, L layers of spin −1 B atoms and a disordered interface in between that is characterized by a random arrangement of A and B atoms of A_pB_1−p type and a negative A-B coupling. We investigate the finite and the infinite superlattices and we found that the existence and the number of the compensation points depend strongly on the thickness of the superlattice (number of unit cells).     

Magnetocaloric effect and magnetic properties in NdMnO3 perovskite: A Monte Carlo approach

The magnetic properties and magnetocaloric effect in $\text{NdMn}{\text{O}}_3$ have been investigated, as well as the effect of magnetic anisotropy in the magnetocaloric properties of the perovskite-type compound. A classical Heisenberg Hamiltonian with nearest and next-nearest neighbors interactions was implemented. Hamiltonian parameters were fitted in order to reproduce experimental results. Magnetic field dependence on the magnetization, for isothermal processes, was performed. In this way, the magnetic entropy change $\left(\mathrm{\Delta }{S}_m\right)$ was computed as well as the relative cooling power (RCP). Results show that as the magnetic anisotropy constant increases, there was not only a sharpening, but also an increase in peak height of $?\mathrm{\Delta }{S}_m$. Finally, the magnetic field and anisotropy dependences on the RCP were obtained, showing that the highest values of the RCP were found for low anisotropy values.     

Nano-graphene monolayer with higher-order exchange couplings: Monte Carlo study

We have used the Monte Carlo simulation to study the phase diagrams and the magnetic properties of a single nano-graphene layer with next-nearest neighbors coupling $J_2$ and four-spin interaction $J_4$. Interesting behaviors have been found. In particular, the nanographene can present tricritical and triple points for appropriate system parameters.