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.     

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.     

Dilution effect on the compensation temperature in a honeycomb nano-lattice: Monte Carlo study

In this work, we use Monte Carlo simulations with the Metropolis algorithm to study the dilution effect on the magnetic properties in a honeycomb nano-lattice. The geometry of the system is formed by two sub-lattices of 45+45 = 90 atoms consisting of the spins σ = 3/2 and S = 5/2. The ground state phase diagrams at zero absolute temperature are presented. Also, the variation of the total magnetization with reduced temperature for several values of the reduced exchange coupling are discussed. It is found that the compensation temperature depends on the reduced exchange coupling parameters. Furthermore, the dilution of the S spins affects the compensation temperature and hysteresis loops of the studied system.     

Magnetic properties of naphthalene-like nano-structure with RKKY interactions: Monte Carlo simulations

In this paper, we have studied the magnetic properties of a bi-layer naphthalene-like nano-structure with RKKY (Ruderman-Kittel-Kasuya-Yoshida) interactions, using Monte Carlo simulations. The system consists of two naphthalene-like magnetic planes, with mixed spins σ = 7/2 and S = 1, separated by L non-magnetic layers. In the first part of this study, the ground state phase diagrams for zero temperature are reported and discussed. In the second part, we investigate the effect of the RKKY interactions on the magnetizations and susceptibilities of the system, for non-zero temperature values. It is found that the value of the blocking temperature decreases when increasing the number of non-magnetic layers. Furthermore, the effect of the ferrimagnetic parameter (J_Sσ) on the blocking temperature behavior has been investigated. Finally, the effect of different physical parameters, namely the number of non-magnetic layers, the ferrimagnetic parameter, the crystal field and the temperature, on the magnetic hysteresis cycles are illustrated.     

Ground state phase diagrams for the mixed Ising 3/2 and 5/2 spin model

We calculate the ground state phase diagrams of a mixed Ising model on a square lattice where spins S (± 3/2, ± 1/2) in one sublattice are in alternating sites with spins Q (± 5/2, ± 3/2, ± 1/2), located on the other sublattice. The Hamiltonian of the model includes first neighbor interactions between the S and Q spins, next-nearest-neighbor interactions between the S spins, and between the Q spins, and crystal field. The topologies of the phase diagrams depend on the values of the parameters in the Hamiltonian. The diagrams show some key features: coexistence between regions, points where two, three, four, five and six states can coexist. Besides being very useful as a way to check the low temperature limit of the finite-temperature phase diagram, often obtained by mean-field theories, the richness of the ground state diagrams for certain combinations of parameters can be used as a guide to explore interesting regions of the finite-temperature phase diagram of the model.     

Mean field study of the mixed Ising model in a random crystal field

The magnetic properties of a mixed Ising ferrimagnetic system, in which the two interacting sublattices have spins σ, (±1/2) and spins S, (±1,0) in the presence of a random crystal field, have been studied 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, partly ferrimagnetic phase and consequently the existence of four topologically different types of phase diagrams. Furthermore, compensation behaviour and re-entrant phenomenon are found for appropriate ranges of crystal field. Thermal magnetization behaviour and phase diagrams have been discussed in detail.     

The critical magnetic behavior of the new Heusler CoXO2 alloys (X=Cu or Mn): Monte Carlo Study

In this paper, we study the effect of different physical parameters on the critical magnetic behavior of the new full Heusler CoXO2 (X=Cu or Mn) alloys, using Monte Carlo simulations (MCS) under the Metropolis algorithm.To reach this goal, we have proposed a Hamiltonian describing and modeling these compounds. The CoCuO2 alloy is formed with the spin variables of cobalt Co atoms (S_i=±2, ±1 or 0) and the copper Cu atoms modeled by the spins σ_i=±1/2. While, the compound CoMnO2 is modeled the spins of Co atoms (Si=±2, ±1 or 0) and the Mn atoms represented by the spins (Q_i=±5/2, ±3/2 or ±1/2).In fact, we have studied and discussed the ground state phase diagrams of the new Heusler alloys CoXO2(X=Cu or Mn) in different physical parameters. Moreover, we have presented and discussed the thermal behavior of the total magnetizations and the magnetic susceptibilities, for non-null temperature values.To complete this study, we have elaborated and discussed the hysteresis loops of the studied alloys when fixing the temperature values.     

Phase diagrams of a nonequilibrium mixed spin-3/2 and spin-2 Ising system in an oscillating magnetic field

The phase diagrams of the nonequilibrium mixed spin-3/2 and spin-2 Ising ferrimagnetic system on square lattice under a time-dependent external magnetic field are presented by using the Glauber-type stochastic dynamics. The model system consists of two interpenetrating sublattices of spins σ=3/2 and S=2, and we take only nearest-neighbor interactions between pairs of spins. The system is in contact with a heat bath at absolute temperature T_abs and the exchange of energy with the heat bath occurs via one-spin flip of the Glauber dynamics. First, we investigate the time variations of average order parameters to find the phases in the system and then the thermal behavior of the dynamic order parameters to obtain the dynamic phase transition (DPT) points as well as to characterize the nature (first- or second-order) phase transitions. The dynamic phase diagrams are presented in two different planes. Phase diagrams contain paramagnetic (p), ferrimagnetic (i₁, i₂, i₃) phases, and three coexistence or mixed phase regions, namely i₁+p, i₂+p and i₃+p mixed phases that strongly depend on interaction parameters.     

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.     

n + p → d + γ in effective field theory

The radiative capture process n + p → d + γ provides clear evidence for meson exchange currents in nuclear physics. We compute this process at low energies using a recently developed power counting for the effective field theory that describes nucleon-nucleon interactions. The leading order contribution to this process comes from the photon coupling to the nucleon magnetic moments. At subleading order there are other contributions. Among these are graphs where the photon couples directly to pions, i.e. meson exchange currents. These diagrams are divergent and require the presence of a local four-nucleon-one-photon counterterm. The coefficient of this operator is determined by the measured cross section, σ^expt = 334.2±0.5 mb, for incident neutrons with speed |v| = 2200 m/s.     

Random crystal-field effects in a mixed spin-1/2 and spin-2 ferrimagnetic Ising system

The magnetic properties of a mixed spin ferrimagnetic system σ?=?1/2 and S?=?2 at the sites of a square lattice with a random crystal field are studied by the mean field approximation. The ground state of the system is determined and the total magnetization is plotted according to the model parameters. Different behaviors for the thermal dependence of the magnetization are highlighted. In particular, one sees the appearance of one or two compensating points and many types of phase diagrams with first and second order phase transition lines as well as isolated critical and tricritical points.     

Verification of a new quantum simulation approach through its application to two-dimensional Ising lattices

A new quantum simulation approach has been applied in the present work to the two-dimensional (2D) ferromagnetic and antiferromagnetic Ising lattices to calculate their magnetic structures, magnetizations, free energies and specific heats in the absence of an external magnetic field. Surprisingly, no size effects could be observed in our simulations performed for the Ising lattices of different sizes. Most importantly, our calculated spontaneous thermally averaged spins for the two kinds of systems are exactly same as those evaluated with quantum mean field theory, and the magnetic structures simulated at all chosen temperatures are perfectly ferromagnetic or antiferromagnetic, verifying the correctness and applicability of our quantum model and computational algorithm. On the other hand, if the classical Monte Carlo (CMC) method is applied to the ferromagnetic 2D Ising lattice with S=1, it is able to generate correct magnetization well consistent with Onsager's theory; but in the case of S=1/2, the computational results of CMC are incomparable to those predicted with the quantum mean field theory, giving rise to very much reduced magnetization and considerably underestimated Curie temperature. The difficulty met by the CMC method is mainly caused by its improperly calculated exchange energy of the randomly selected spin in every simulation step, especially immediately below the transition temperature, where the thermal averages of spins are much less than 1/2, however they are assigned to ±1/2 by CMC to evaluate the exchange energies of the spins, such improper manipulation is obviously impossible to lead the code to converge to the right equilibrium states of the spin systems.     

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.     

Phase diagrams,electronic and magnetic properties of the quaternary Heusler alloy NbRhCrAl

In this paper, we are studying the quaternary Heusler alloy NbRhCrAl, using the full potential linearized augmented plane wave and the generalized gradient approximation of the exchange and correlation potential methods. For this purpose, we established the electronic and magnetic properties of this quaternary Heusler alloy. We investigated the density of states (DOS) and the band structure of this Heusler compound.This material contains three magnetic atoms: Nb, Rh and Cr modeled by the spin moments S = 1, P = 5/2 and Q = 3/2, respectively. In a first step, we elaborated the ground state phase diagrams in different physical parameter planes to explore the existing stable configurations. On the other hand, we used the MONTE CARLO simulations (MCS) to investigate the thermal behavior of the magnetizations of this system. Also, we have investigated the effect of varying the crystal field and the external magnetic field on the total magnetizations. To complete this study, we illustrated and discussed the hysteresis loops as a function of the external magnetic field, when fixing the values of the studied physical parameters.     

Dynamic magnetic properties in the kinetic mixed spin-2 and spin-5/2 Ising model under a time-dependent magnetic field

We extend the recent paper [W. Jiang, V-C. Lo, B-D. Bai, J. Yang, Physica A 389 (2010) 2227-2233] to present a study, within a mean-field approach, the dynamic magnetic properties of the mixed spin-2 and spin-5/2 Ising ferrimagnetic system, which corresponds the molecular-based magnetic materials AFe^IIFe^III(C₂O₄)₃ [ A=N(n-C_nH_2n+1)₄, n=3-5], by using the Glauber-type stochastic dynamics. This mixed Ising ferrimagnetic system is used on a layered honeycomb lattice in which Fe^II (S=5/2) and Fe^III (σ=2) occupy sites. First, we investigate the time variations of average order parameters to find the phases in the system and then the thermal behavior of the dynamic order parameters to obtain the dynamic phase transition (DPT) points as well as to characterize the nature (first-or second-order) phase transitions. We also present the dynamic phase diagrams and study the dynamic magnetic hysteresis loop behaviors of the kinetic mixed spin-2 and spin-5/2 Ising ferrimagnetic system. The results are compared with some experimental and theoretical works and a good overall agreement is found.     

Compensation temperatures,magnetic susceptibilities and phase diagrams of a mixed ferrimagnetic ternary system on the Bethe lattice

Compensation behaviors, magnetic susceptibilities and the phase diagrams of the ternary system of the type ABC consisting of Ising spins σ = 1/2, S = 3/2 and m = 5/2 in the presence of a single-ion anisotropy are studied on the Bethe lattice within the framework of the exact recursion relations. Both ferromagnetic and antiferromagnetic exchange interactions are considered. The exact expressions for sublattice magnetizations and magnetic susceptibilities are obtained, and then thermal behaviors of the sublattice magnetizations, total magnetization, magnetic sublattice susceptibilities and total susceptibility are investigated. We find that the system only undergoes a second order phase transition for the different and same bilinear nearest-neighbor exchange interaction parameters, but displays compensation behaviors for only different bilinear interaction parameters. We also present the phase diagrams for the different and same bilinear nearest-neighbor exchange interaction parameters. A comparison is made with the other ternary system of the type ABC consisting of different spin values.     

Effect of magnetic,crystal field and exchange interactions on graphene system: a Monte Carlo study

In this article, we employ the classical Monte Carlo approach to study the magnetic properties of graphene system. We analyze the ground-state phase diagrams in the presence of external magnetic and crystal fields under effect of the exchange interactions. The critical temperature is deduced. It is proven that the model exhibits the second-order phase transitions at the transition temperature. The total magnetization with the exchange interactions has studied under the temperatures effect. The total magnetization with the crystal field has been established under effect of exchange interactions and temperatures effect. The magnetic hysteresis cycles of graphene system is deduced under effect of temperatures and crystal field. The observations are in good agreement with related experiments and the other theoretical results. It is proven that the graphene system exhibits the superparamagnetic at the transition temperature and a specific value of reduced crystal field.     

Behaviour at ok and at different magnetic fields of the double magnetic helix

We study the behaviour at 0K and at all magnetic field values up to the magnetic saturation of the magnetic helix in which two distinct magnetic ions of spin lengths S₁, and S₂ are acted on by two unequal molecular fields. Although limited to the case of zigzag and collinear chains on which only the magnetic interactions J between first nearest neighbours (n.n) and J₁, or J₁ between second n.n. are considered, complex phase diagrams are obtained. The most disclosing features show up by taking S₁, S₂, J₁,J₂ as parameters and h, J as variables. Whereas the biconical configuration appears as the most general solution, triangular and oblique configurations also exist in finite field in all (h, J) diagrams. In addition, a collinear ferrimagnetic configuration is found to be stable for definite h and J values. We finally observe that according to the ratio values S₁S₂ and $\text{(J}{}_2\text{J}{}_1\text{)}\text{1}\text{2}$, eight types of (h, J) diagrams exist which differ from each other by distinct approaches to the ferri- or ferromagnetic alignment.     

Dynamic compensation temperature in the mixed spin-1 and spin-2 Ising model in an oscillating field on alternate layers of a hexagonal lattice

The dynamic behavior of a mixed spin-1 and spin-2 Ising system with a crystal-field interaction in the presence of a time-dependent oscillating external magnetic field on a hexagonal lattice is studied by using the Glauber-type stochastic dynamics. The lattice is formed by alternate layers of spins σ=1$\sigma =1$ and S=2. The Hamiltonian model includes intersublattice, intrasublattice and crystal-field interactions. The set of mean-field dynamic equations is obtained by employing the Glauber transition rates. Firstly, we study time variations of the average sublattice magnetizations in order to find the phases in the system, and the thermal behavior of the average sublattice magnetizations in a period or the dynamic sublattice magnetizations to obtain the dynamic phase transition points as well as to characterize the nature (continuous and discontinuous) of transitions. Then, the behavior of the dynamic total magnetization as a function of the temperature is investigated to find the dynamic compensation points as well as determine the type of behavior. We also present the dynamic phase diagrams for both presence and absence of the dynamic compensation temperatures in the nine different planes. According to the values of Hamiltonian parameters, besides the paramagnetic (p), antiferromagnetic (af), ferrimagnetic (i) and non-magnetic (nm) fundamental phases, eight different mixed phases and the compensation temperature or L- and N-types behavior in the Néel classification nomenclature exist in the system.     

Mictomagnetic State in an EuBaCo<Subscript>2–<Emphasis Type="Italic">x</Emphasis></Subscript>O<Subscript>5.5–δ</Subscript> Single Crystal

The magnetic properties of an EuBaCo_1.9O_5.36 single crystal are studied in the temperature range T = 2–300 K and the magnetic field range H ≤ 90 kOe. This binary layered cobaltite single crystal has vacancies in the cobalt and oxygen sublattices, in contrast to the stoichiometric EuBaCo₂O_5.5 composition. All cobalt ions in EuBaCo1.9O5.36 are in a trivalent state. The single crystal has an orthorhombic structure with space group Pmmm, and its unit cell parameters are a = 3.883 Å, b = 7.833 Å, and c = 7.551 Å. The field and temperature dependences of the magnetization of the single crystal demonstrate that it is ferrimagnet below T_C = 242 K. At T < 300 K, all three spin states of the Co³⁺ ions are present. The nearest-neighbor interactions give antiferromagnetic (AFM) and ferromagnetic (FM) contributions to the exchange energy. The ratio of the AFM to the FM contributions changes when temperature decreases because of a change in the spin state of the Co³⁺ ions. The single crystal exhibits signs of mictomagnetism at low temperatures in high magnetic fields. At T = 2 K and H = 90 kOe, the zero-field and nonzero-field magnetizations are strongly different because of a uniaxial magnetic anisotropy, which tends to set magnetization along the magnetic field applied in cooling throughout the crystal volume. As a result, a complex ferrimagnetic structure with a noncollinear direction of Co³⁺ spins appears. The following phenomena characteristic of mictomagnets are also observed in the EuBaCo_1.9O_5.36 single crystal: a shift in a magnetization hysteresis loop when temperature decreases, retained hysteretic phenomena and no magnetization saturation in high magnetic fields, and an orientation transition. The mictomagnetic state in EuBaCo_1.9O_5.36 is shown to be caused by the structural distortions induced by vacancies in the cobalt and oxygen sublattices and by the frustration of AFM and FM exchange interactions.