Magnetic properties in different fullerenes Xn nano-structures: Monte Carlo study

The aim of this work is to study the magnetic properties in deferent fullerenes X_n like nano-structures, where the symbol X can be assigned to any magnetic atom and “n” is the number of these magnetic atoms. Our study is based on the fullerenes: C₂₀, C₆₀ and C₇₀, formed by atoms with mixed spins σ = 1 and S = 1/2. We focus our interest on the examination of the influence of the coupling exchange interactions between atoms, the external magnetic and the crystal fields. Also, we present and discuss the nature of the corresponding ground state phase diagrams. The effect of the compensation and critical temperatures is also presented for different magnetization profiles by using the Monte Carlo simulations. The behavior of the total magnetizations as a function of the physical system parameters is also analyzed and discussed, in this study.     

Exchange bias and asymmetric hysteresis loops from a microscopic model of core/shell nanoparticles

We present Monte Carlo simulations of hysteresis loops of a model of a magnetic nanoparticle with a ferromagnetic core and an antiferromagnetic shell with varying values of the core/shell interface exchange coupling which aim to clarify the microscopic origin of exchange bias observed experimentally. We have found loop shifts in the field direction as well as displacements along the magnetization axis that increase in magnitude when increasing the interfacial exchange coupling. Overlap functions computed from the spin configurations along the loops have been obtained to explain the origin and magnitude of these features microscopically.     

Unified nonequilibrium dynamical theory for exchange bias and training effects

We have investigated the exchange bias and training effect in the ferromagnetic/antiferromagnetic (FM/AF) heterostructures using a unified Monte Carlo dynamical approach. The magnetization of the uncompensated AF layer is still open after the first field cycling is finished. Our simulated results show obvious shift of hysteresis loops (exchange bias) and cycling dependence of exchange bias (training effect) when the temperature is below 45~K. The exchange bias field decreases with decreasing cooling rate or increasing temperature and the number of the field cycling. Essentially, these two effects can be explained on the basis of the microscopical coexistence of both reversible and irreversible moment reversals of the AF domains. Our simulations are useful to understand the real magnetization dynamics of such magnetic heterostructures.     

Effect of exchange interaction in ferromagnetic superlattices:A Monte Carlo study

The Monte Carlo simulation is used to investigate the magnetic properties of ferromagnetic superlattices through the Ising model. The reduced critical temperatures of the ferromagnetic superlattices are studied each as a function of layer thickness for different values of exchange interaction. The exchange interaction in each layer within the interface and the crystal field in the unit cell are studied. The magnetic coercive fields and magnetization remnants are obtained for different values of exchange interaction, different values of temperature and crystal field with fixed values of physical parameters.     

Low-temperature metastable states in a stacked triangular Ising antiferromagnet

We study low-temperature magnetization processes in a stacked triangular Ising antiferromagnet by Monte Carlo simulations. In increasing and decreasing magnetic fields we observe multiple steps and hysteresis corresponding to formation of different metastable states. Besides the equidistant threefold splitting of the 1/3 ferrimagnetic plateau, we additionally confirm a fourth plateau in the field-increasing branch and a sizable remanence when the field is decreased to zero. The newly observed plateau only appears at sufficiently low temperature and sufficiently large exchange interaction in the stacking direction. These observations reasonably reproduce low-temperatures measurements on the spin-chain compound Ca₃Co₂O₆.     

铁磁/反铁磁双层膜中的磁锻炼效应

采用Monte Carlo 方法,研究铁磁/反铁磁双层膜中的磁锻炼效应.结果表明,反铁磁层中冷场诱发的界面净磁化(钉扎效应)的磁弛豫可导致系统中的交换偏置场的磁锻炼效应.进一步研究表明,反铁磁层中掺杂可调控交换偏置场的磁锻炼效应,原因在于反铁磁层中掺杂能有效地改变冷场诱发的净磁化的磁弛豫过程.     

Compensation behaviors in a tri-layers nano-dicoronylene like-structure with ferrimagnetic mixed spins (3/2,1): Monte Carlo study

This paper outlines the magnetic properties of a tri-layers nano-dicoronylene like-structure, using Monte Carlo simulations under the Metropolis algorithm in the Blume Capel model. The top and bottom layers are composed by S-spins (S = 0, ±1), While the middle layer is composed by σ-spins (σ=±3/2, ±1/2). Each layer contains 48 atoms. We establish, in six various planes, the ground state phase diagrams. Still, we present the thermal variation of partials and total magnetization and susceptibility for several values of exchange coupling interactions and crystal field. Also, the hysteresis cycles have been reported for selected values of temperature and exchange coupling interaction. The compensation and blocking temperatures values increases when increasing the exchange coupling parameter in the extreme layers.     

Effective-field and Monte Carlo studies of mixed spin-2 and spin-1/2 Ising diamond chain

The magnetic properties of a mixed spin-2 and spin-1/2 ferromagnetic diamond chain are studied by effective-field theory and Monte Carlo(MC) simulation based on the Ising model.The temperature dependences of magnetization,magnetic susceptibility,internal energy,and specific heat are studied,respectively.The exchange interaction dependences of magnetization and the critical temperature are calculated by MC simulation.The changes of magnetization depending on the field increasing and then the field decreasing under steady-static conditions are also given.     

Monte Carlo simulations of donor band exchange in (Zn,Co)O

We present results of a Monte Carlo study over the ferromagnetism of Co-doped ZnO. The magnetic interaction has the form of the donor impurity band exchange model, where the Co magnetic moments are exchange coupled to band electrons. These are assumed to occupy large hydrogenic orbitals and originate from shallow intrinsic ZnO defects. A number of parameters of this model remain uncertain and here we investigate the dependence of the Curie temperature on the strength of the magnetic coupling. We find an unusual concave upward shape in the magnetization curves consistent with other Monte Carlo studies for dilute systems and we predict high temperature ferromagnetism for sufficiently strong coupling.     

Optimization of the magnetocaloric effect in Ni-Mn-In alloys: A theoretical study

Based on ab initio and Monte Carlo simulations, we study the influence of the strength of the magnetic exchange parameters on the inverse and conventional magnetocaloric effect in the Ni₅₀Mn₃₄In₁₆ Heusler alloy using the mixed Potts and Blume-Emery-Griffiths model Hamiltonian. Within the proposed model, the temperature dependences of the magnetization, tetragonal deformation, and adiabatic temperature changes for magnetic field variation are obtained. It is first shown that a decrease in the magnetic exchange interactions leads to increased values of the magnetocaloric effect. We suppose that a reduction of the exchange interactions in the Ni-Mn-In alloy can be realized by the doping with nonmagnetic atoms such as B, Si, Zn, Cu, etc.     

Monte Carlo simulation of Prussian blue analogs described by Heisenberg ternary alloy model

Within the framework of Monte Carlo simulation technique, we simulate magnetic behavior of Prussian blue analogs based on Heisenberg ternary alloy model. We present phase diagrams in various parameter spaces, and we compare some of our results with those based on Ising counterparts. We clarify the variations of transition temperature and compensation phenomenon with mixing ratio of magnetic ions, exchange interactions, and exchange anisotropy in the present ferro-ferrimagnetic Heisenberg system. According to our results, thermal variation of the total magnetization curves may exhibit N, L, P, Q, R type behaviors based on the Néel classification scheme.     

Intrinsic ferromagnetism in CoBr2 nanolayers: a DFT+U and Monte Carlo study

The intrinsic ferromagnetism of CoBr₂ bulk was investigated using DFT (density functional theory) combined with the full potential linear augmented plane wave method and Monte Carlo simulations. The ground state of CoBr₂ exhibits ferromagnetic behavior and a semiconductor character. We used the generalized gradient approximation (GGA) and GGA+U (Hubbard correction) approximations to determinate the magnetic moment. The magnetic moment reached the experimental value and was in good agreement with the other theoretical values. The value obtained was used as an input to a Monte Carlo study to calculate the thermal magnetization and magnetic hysteresis cycles. Ferromagnetic behavior was observed and was found to be due to an positive exchange interaction. These results lead us to believe that this material could be a promising spintronic material.     

Effects of Cu dilution in IrMn on the exchange bias of CoFe/IrMn bilayers

The effect of nonmagnetic dilution in metallic antiferromagnets (AFMs) on the exchange bias (EB) has been investigated from a structural, magnetic, and Monte Carlo simulation point of view in bilayers of CoFe/(IrMn)1-xCux. Dilution by Cu atoms throughout the volume of the AFM IrMn gives rise to an enhanced EB field (HEB) for 5 K相似文献   

Monte Carlo study of the magnetic properties and magnetocaloric effect of an AFM/FM BiFeO_3/Co bilayer

The magnetic properties and magnetocaloric effect of an antiferromagnetic/ferromagnetic(AFM/FM)BiFeO_3/Co bilayer with mixed-spin(5/2,3/2) have been studied based on Monte Carlo simulation.The magnetization, susceptibility, and critical temperature are investigated under various exchange couplings and an external magnetic field. In particular, the influence of exchange couplings and an external magnetic field on the magnetic entropy change, adiabatic temperature change, and the relative cooling power(RCP) are studied. The simulation results indicated that the decrease of the exchange coupling and the increase of external magnetic fields can cause an increase of magnetic entropy change, adiabatic temperature change, and RCP. In addition, the hysteresis loops of the system are presented for different exchange couplings and temperatures.     

Functional-integral study of spin fluctuations in small Fe clusters

Finite temperature magnetic properties of small Fe_N clusters (N ≤6) are determined in the framework of a spin-fluctuation itinerant-electron theory based on a functional integral formulation of the canonical partition function and derived statistical averages. The free energy associated to each configuration of the exchange fields throughout the cluster are calculated by using Haydock-Heine-Kellys recursion method. The statistical averages of physical interest are obtained by performing parallel-tempering Monte Carlo simulations. Representative results are discussed for the average magnetization per atom as a function of temperature. The interplay between local environment and magnetization curves is analyzed by considering the low-temperature limit of the local spin-fluctuations energies ΔF_l(ξ) at different atoms l. The electronic calculations are contrasted with the predictions of simple of phenomenological Heisenberg-like models.     

Size effect on magnetic properties of a nano-graphene bilayer structure: A Monte Carlo study

In this paper we use the Monte Carlo simulations to investigate the magnetic properties of an Ising ferromagnetic–antiferromagnetic model. The system is based on a nano-graphene structure-like bilayer with two bloc sizes: N=24 and 42 spins. For each size N, the upper layer A is formed with spin −3/2, whereas the lower layer B is composed of spin −5/2. We only consider the first nearest-neighbor interactions between the sites i and j. The magnetic properties are studied, in the absence as well as in the presence of a crystal magnetic field, and an external magnetic field. The increasing temperature and crystal field as well as the inter-layer coupling constant, are also studied for this system sizes N=24 and 42 spins. The zero-field-cooled and the field cooled magnetization behaviors are investigated for different values of external magnetic field and a fixed value of exchange interaction between the two blocs. The magnetizations as well as the magnetic susceptibilities versus the temperature are used in order to obtain blocking temperature.     

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.     

Striped phase in a quantum XY model with ring exchange

We present quantum Monte Carlo results for a square-lattice S=1/2 XY model with a standard nearest-neighbor coupling J and a four-spin ring exchange term K. Increasing K/J, we find that the ground state spin stiffness vanishes at a critical point at which a spin gap opens and a striped bond-plaquette order emerges. At still higher K/J, this phase becomes unstable and the system develops a staggered magnetization. We discuss the quantum phase transitions between these phases.     

Computer simulation of magnetization relaxation in amorphous magnetics with random anisotropy

On the basis of Heisenberg model, the magnetic properties of amorphous terbium were studied by the Monte Carlo method. The temperature dependences of spontaneous magnetization and magnetic susceptibility were plotted as functions of the fraction of the constant of anisotropy to the exchange constant D/J ₀. The behavior of magnetization in an external magnetic field was studied, and the dependence of a coercive field and residual magnetization on the value D/J ₀ was found. The relaxation of magnetization was investigated after the external magnetic field was switched off.