Monte Carlo simulation of the compensation and critical behaviors of a ferrimagnetic core/shell nanoparticle Ising model

A Monte Carlo simulation has been used to study the magnetic properties and the critical behaviors of a single spherical nanoparticle, consisting of a ferromagnetic core of spins surrounded by a ferromagnetic shell of S=±1, 0 or , spins with antiferromagnetic interface coupling, located on a simple cubic lattice. A number of characteristic phenomena has been found. In particular, the effects of the shell coupling and the interface coupling on both the critical and compensation temperatures are investigated. We have found that, for appropriate values of the system parameters, two compensation temperatures may occur in the present system.     

Compensation temperature and hysteresis behaviors of a graphene-like bilayer: Monte Carlo Study

Using the Monte Carlo method, the compensation temperature and hysteresis loops of a ferrimagnetic mixed spin-3/2 and spin-5/2 Ising-type graphene-like bilayer are investigated induced by different physical parameters such as crystal field, exchange coupling, external magnetic field, and temperature. The variations of magnetization, magnetic susceptibility,specific heat, and internal energy with the change of temperature are discussed. In addition, we also plot the phase diagrams including trans...     

Magnetic properties of a ferrimagnetic core/shell nanocube Ising model: A Monte Carlo simulation study

Monte Carlo simulation has been used to study the magnetic properties and hysteresis loops of a single nanocube, consisting of a ferromagnetic core of spin- surrounded by a ferromagnetic shell of spin-1 with antiferromagnetic interface coupling. We find a number of characteristic phenomena. In particular, the effects of the shell coupling and the interface coupling on both the compensation temperature and the magnetization profiles are investigated. The effects of the interface coupling on the hysteresis loops are also examined.     

Monte Carlo study of the two dimensional quadratic ising ferromagnet with mixed spins ofS=1/2 andS=1

The single-spin-flip Metropolis algorithm is applied to an Ising ferromagnet with mixed spins ofS=1/2 andS=1 on the square lattice. The critical temperature obtained from our Monte Carlo simulation is very close to the high temperature series expansion result. The finite size scaling results for the exponents yield the two dimensional Ising values, which are in good agreement with those suggested by the universality hypothesis.     

A kinetic Monte Carlo study for stripe-like magnetic domains in ferrimagnetic thin films

The topology and dynamics of stripe-like magnetic domains obtained in a ferrimagnetic garnet subjected to a time-dependent external magnetic field is studied experimentally and theoretically. Experiments are performed on a commercially available magnetic bubble apparatus, allowing the observation of the time-evolution of the magnetic domain structure. The system is modeled by a meso-scale Ising-type lattice model. Exchange and dipolar interactions between the spins, and interaction of the spins with the external magnetic field are considered. The model is investigated by kinetic Monte Carlo simulations with time-varying transition rates. In the limit of low temperatures the elaborated model leads to a magnetic domain topology and dynamics that is similar to the ones observed in the experiments. In the highly non-equilibrium limit with a high driving frequency the model reproduces the experimentally recorded hysteresis loops as well.     

Green's function study of a mixed spin-1 and spin-3/2 Heisenberg ferrimagnetic system

The magnetic properties of a mixed spin-1 and spin-3/2 Heisenberg ferrimagnetic system on a square lattice are investigated by using the double-time temperature-dependent Green's function technique. In order to decouple the higher order Green's functions, Anderson and Callen's decoupling and random phase approximations have been used. The nearest- and next-nearest-neighbor interactions and the single-ion anisotropies are considered and their effects on compensation and critical temperature are studied.     

Hysteresis loop behaviors of ferroelectric thin films: A Monte Carlo simulation study

The ferroelectric response of bismuth titanate Bi4Ti3O12(BIT) thin film is studied through a Monte Carlo simulation of hysteresis loops. The ferroelectric system is described by using a Diffour Hamiltonian with three terms: the electric field applied in the z direction, the nearest dipole–dipole interaction in the transversal(x–y) direction, and the nearest dipole–dipole interaction in the direction perpendicular to the thin film(the z axis). In the sample construction, we take into consideration the dipole orientations of the monoclinic and orthorhombic structures that can appear in BIT at low temperature in the ferroelectric state. The effects of temperature, stress, and the concentration of pinned dipole defects are assessed by using the hysteresis loops. The results indicate the changes in the hysteresis area with temperature and stress, and the asymmetric hysteresis loops exhibit evidence of the imprint failure mechanism with the emergence of pinned dipolar defects. The simulated shift in the hysteresis loops conforms to the experimental ferroelectric response.     

Bond dilution in the 3D Ising model: a Monte Carlo study

We study by Monte Carlo simulations the influence of bond dilution on the three-dimensional Ising model. This paradigmatic model in its pure version displays a second-order phase transition with a positive specific heat critical exponent . According to the Harris criterion disorder should hence lead to a new fixed point characterized by new critical exponents. We have determined the phase diagram of the diluted model, starting from the pure model limit down to the neighbourhood of the percolation threshold. For the estimation of critical exponents, we have first performed a finite-size scaling study, where we concentrated on three different dilutions to check the stability of the disorder fixed point. We emphasize in this work the great influence of the cross-over phenomena between the pure, disorder and percolation fixed points which lead to effective critical exponents dependent on the concentration. In a second set of simulations, the temperature behaviour of physical quantities has been studied in order to characterize the disorder fixed point more accurately. In particular this allowed us to estimate ratios of some critical amplitudes. In accord with previous observations for other models this provides stronger evidence for the existence of the disorder fixed point since the amplitude ratios are more sensitive to the universality class than the critical exponents. Moreover, the question of non-self-averaging at the disorder fixed point is investigated and compared with recent results for the bond-diluted q = 4 Potts model. Overall our numerical results provide evidence that, as expected on theoretical grounds, the critical behaviour of the bond-diluted model is indeed governed by the same universality class as the site-diluted model.Received: 24 February 2004, Published online: 28 May 2004PACS: 05.40.-a Fluctuation phenomena, random processes, noise, and Brownian motion - 64.60.Fr Equilibrium properties near critical points, critical exponents - 75.10.Hk Classical spin models     

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.     

Monte Carlo study on dynamical behaviour of spins and spin clusters in the 2D ± J Ising spin-glass

To investigate dynamical (time-dependent) behavior of spins and spin clusters in the two-dimensional ± J Ising spin-glass, we have evaluated the relaxation time τ_i of individual spins by the Monte Carlo simulation. An interesting finding is that, at least in the temperature range $\text{T ? 0.8J}$, most of τ_i are described by $\text{τ}{}_{\mathrm{i}}\text{∝}\text{exp}\text{{}\text{E}{}_{\mathrm{i}}\text{(T ? T}{}_0\text{}}$ with T₀?0.5J (E_i being a constant), which suggests that the system undergoes a glass transition obeying Fulcher's law.     

Monte Carlo simulation of the SU(3) spin model with chemical potential in a flux representation

We present a simulation of the SU(3) spin model with chemical potential using a recently proposed flux representation. In this representation the complex phase problem is avoided and a Monte Carlo simulation in terms of the fluxes becomes possible. We explore the phase diagram of the model as a function of temperature and chemical potential.     

(3 1)-Dimensional Quantum Mechanics from Monte Carlo Hamiltonian: Harmonic Oscillator

In Lagrangian formulation, it is extremely difficult to compute the excited spectrum and wavefunctions ora quantum theory via Monte Carlo methods. Recently, we developed a Monte Carlo Hamiltonian method for investigating this hard problem and tested the algorithm in quantum-mechanical systems in 1 1 and 2t1 dimensions. In this paper we apply it to the study of thelow-energy quantum physics of the (3 1)-dimensional harmonic oscillator.``     

Dielectric properties and compensation behaviors of a monolayer naphthalene-like nanoisland: Monte Carlo simulations

In this paper, we propose a model to study the thermal and dielectric properties of a monolayer naphthalene-like nanoisland. The ferrielectric mixed spin (1, 7/2) Blume-Capel Ising model has been studied, using Monte Carlo simulations under the Metropolis algorithm. The ground state phase diagrams, in the absence of temperature, are investigated. Moreover, we illustrated the thermal and dielectric properties as a function of temperature, for fixed values of the other parameters. On the other hand, we examine the effects of the exchange coupling interactions on the compensation and blocking temperatures. In addition, we explore the effects of crystal and external longitudinal electric fields on total polarizations.     

U(3) four-dimensional lattice gauge theory and Monte Carlo calculations

Monte Carlo results for the pure U(3) lattice gauge theory on a 6⁴ lattice are reported. Wilson loops and the string tension are presented. The first-order phase transition in U(3) is reflected quite clearly in a discontinuity in the string tension at β = β_c. The U(1) factor of U(3) is extracted using the determinant of the Wilson loops. As expected, the U(1) component appears to deconfine at the phase transition..     

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.