Effective Exchange Interaction Research Articles

Magnetic properties and compensation temperatures of a graphene bilayer by Monte Carlo simulation

We used Monte Carlo simulation to study the magnetic properties and compensation behavior of a ferrimagnetic mixed-spin (3/2; 1/2) Ising system in a graphene bilayer. The effects of exchange interactions, crystal field, external magnetic field and temperature on the total magnetization, partial magnetizations, magnetic susceptibility, phase diagram and hysteresis behavior of the system were examined. Interesting results were obtained in this work, such as the compensation behavior and the multi-loop hysteresis phenomenon for specific values of the system’s physical parameters. Our results conform to those of other theoretical studies.

Site Occupancy Preference and Magnetic Properties in Nd2(Fe,Co)14B

Partial replacement of Fe by Co is an effective method to increase Curie temperature (TC), which improves the thermal stability of magnetic properties in Nd2Fe14B-based permanent magnets. The correlation between Fe substitution and magnetic properties has been studied in Nd2(Fe,Co)14B via a first-principles calculation. The calculated Fe substitution energies indicate that the Co atoms avoid the 8j2 site, which agrees with the experiments. The Co atoms are ferromagnetically coupled with Fe sublattice and show magnetic moments of about 1.2 to 1.7 μB at different crystallographic sites, less than that of Fe (2.1–2.7 μB), resulting in the decrease in total magnetization at ground state (0 K) with increasing Co content. The effective exchange interaction parameter, derived from the energy difference between varied magnetic structures, increases from 7.8 meV to 17.0 meV with increasing Co content from x = 0 to x = 14 in Nd2Fe14−xCoxB. This change in the effective exchange interaction parameter is responsible for the enhancement of TC in Nd2(Fe,Co)14B. The total magnetization at 300 K, derived from mean-field theory, shows a peak maximum value at x = 1 in Nd2Fe14−xCoxB. The phenomenon results from the interplay between the reduction of the magnetic moment in the Fe(Co) sublattice and the enhancement of TC with increasing Co content.

Mean-field study of magnetic properties and hysteresis behavior in a bilayer graphene Ising system

Using the mean-field approximation based on the Gibbs-Bogoliubov inequality for the free energy, we conducted an investigation into the magnetic properties and hysteresis behavior of a graphene Ising bilayer, where the top and bottom layers are occupied by spins σ = 3/2 and S = 5/2, respectively. The effects of exchange interactions, crystal fields, external magnetic field and temperature on the total magnetization, partial magnetizations of each layer, total magnetic susceptibility, blocking temperature and hysteresis loops of the system were thoroughly analyzed. The variations of the blocking temperature as a function of various parameters in the system’s Hamiltonian were presented. Furthermore, we demonstrated the existence of multiple hysteresis loop behaviors under specific physical conditions.

Chirality-selective topological magnon phase transition induced by interplay of anisotropic exchange interactions in honeycomb ferromagnet

A variety of distinct anisotropic exchange interactions commonly exist in one magnetic material due to complex crystal, magnetic and orbital symmetries. Here we investigate the effects of multiple anisotropic exchange interactions on topological magnon in a honeycomb ferromagnet, and find a chirality-selective topological magnon phase transition induced by a complicated interplay of Dzyaloshinsky–Moriya interaction and pseudo-dipolar interaction, accompanied by the bulk gap close and reopen with chiral inversion. Moreover, this novel topological phase transition involves band inversion at high symmetry points K and K′, which can be regarded as a pseudo-orbital reversal, i.e. magnon valley degree of freedom, implying a new manipulation corresponding to a sign change of the magnon thermal Hall conductivity. Indeed, it can be realized in 4d or 5d correlated materials with both spin–orbit coupling and orbital localized states, such as iridates and ruthenates, etc. This novel regulation may have potential applications on magnon devices and topological magnonics.

Investigating the magnetic and hysteretic properties of graphyne-based nanostructures: a comparative study of shape effects on critical temperatures, coercivity, and remanence

Our study aims to investigate the physical properties of graphyne-based nanostructures, with a particular focus on their magnetic and hysteretic features. We analyze the influence of shape on these properties in an Ising-type graphyne structure using the effective field theory (EFT) method. Our research focuses on the critical and compensation temperatures, coercivity, and remanence of the graphyne structure in different shapes, ranging from alpha (α) to beta (β) to gamma (γ). We use the magnetization curves and hysteresis loops obtained from central and intermediate spins. We also illustrate the behavior of the coercive field and remanent magnetization as well as hysteresis loop the effect of exchange interactions r and s as well as temperature. We observe N- and R- type compensation in the β graphyne, and L- and S- type compensations in the γ graphyne. We also find that the widest hysteresis is observed in γ graphyne.

Critical behavior of a mixed-spin Ising model on a square–octagon lattice

The magnetic behavior and phase diagrams of a mixed spin-5/2 and spin-3/2 Ising system on a two-dimensional square–octagon lattice were investigated using the mean-field approach based on the statistical Bogoliubov inequality. Two ground-state phase diagrams were plotted in two different planes. We also examined the effects of exchange interactions, crystal field, and external magnetic field on the magnetizations and phase diagrams of the system. First- and second-order phase transitions and isolated critical points were obtained for certain physical conditions. The influence of various physical system parameters on the blocking temperature was also clarified.

Spatially-indirect and hybrid exciton–exciton interaction in MoS2 homobilayers

Interlayer excitons in transition-metal dichalcogenide (TMD) bilayers, alongside their interplay with direct excitonic species, are supposed to offer a pathway towards robust nonlinearity, enabling the exploration of many-body quantum effects. We present a theoretical investigation of interaction among various exciton species within these structures where Coulomb attraction and repulsion are subject to reduced screening. For a homobilayer MoS2, we examine both direct, spatially-indirect, and hybridised excitons, considering the effects of direct and exchange interaction of electrons and holes distributed across one or different layers. Similar physics arises in perfectly aligned twisted TMD heterobilayers which support the direct-to-indirect exciton hybridisation. Deriving the exciton–exciton interaction matrix elements, we unveil a distinct non-monotonic dependence of the interaction on transferred momentum, changing sign from repulsive to attractive even for ground-state excitons, and compare our results with existing calculations for monolayers. Our findings demonstrate that for large momenta involved in high-density effects (strongly correlated phases), the interaction is chiefly governed by the prevailing attractive exchange component. At the same time, at small momenta that are more relevant for rarefied systems, we find that the enhancement of the interaction constant for dipolar species compared to intralayer non-dipolar excitons may be hindered by the surrounding medium. We draw comparisons with existing experiments and discuss the implications of our findings on the collective effects in TMD-based systems of excitons and exciton-polaritons.

The resolution of the weak-exchange limit made rigorous, simple and general in binuclear complexes.

The correct interpretation of magnetic properties in the weak-exchange regime has remained a challenging task for several decades. In this regime, the effective exchange interaction between local spins is quite weak, of the same order of magnitude or smaller than the various anisotropic terms, which generates a complex set of levels characterized by spin mixing. Although the model multispin Hamiltonian in the absence of local orbital momentum, , is considered good enough to map the experimental energies at zero field and in the strong-exchange limit, theoretical works pointed out limitations of this simple model. This work revives the use of ĤMS from a new theoretical perspective, detailing point-by-point a strategy to correctly map the computational energies and wave functions onto ĤMS, thus validating it regardless of the exchange limit. We will distinguish two cases, based on experimentally characterized dicobalt(II) complexes from the literature. If centrosymmetry imposes alignment of the various rank-2 tensors constitutive of ĤMS in the first case, the absence of any symmetry element prevents such alignment in the second case. In such a context, the strategy provided herein becomes a powerful tool to rationalize the experimental magnetic data, since it is capable of fully and rigorously extracting the multispin model without any assumption on the orientation of its constitutive tensors. Furthermore, the strategy allows to question the use of the spin Hamiltonian approach by explicitly controlling the projection norms on the model space, which is showcased in the second complex where local orbital momentum could have occurred (distorted octahedra). Finally, previous theoretical data related to a known dinickel(II) complex is reinterpreted, clarifying initial wanderings regarding the weak exchange limit.

Magnetic properties of perovskites Pr0.9Sr0.1 Mn[formula omitted]Mn[formula omitted]O3: Monte Carlo simulations and experiments

This work presents the remarkable experimental magnetocaloric properties of the perovskites Pr0.9Sr0.1Mn0.93+ Mn0.14+O3, including the magnetic entropy change |ΔSm| and the Relative Cooling Power (RCP). To understand these striking properties, we elaborate in this paper a model and use Monte Carlo (MC) simulations to study it for comparison. For the model, we take into account nearest-neighbor (NN) interactions between magnetic ions Mn3+ (S=2) and Mn4+ (S=3/2) and the interactions between these Mn ions with the magnetic Pr ions. The crystal is a body-centered tetragonal lattice where the corner sites are occupied by Mn ions and the center sites by Pr and Sr ions in their respective concentrations given in the compound formula. We use an Ising-like spin model describing a strong anisotropy on the z axis. We show that pairwise interactions between ions cannot reproduce the large plateau of the magnetization experimentally observed below the phase-transition temperature. By introducing for the first time a many-spin interaction between Mn ions, we obtain an excellent agreement with experiments. Fitting the experimental Curie temperature TC with the MC transition temperature, we estimate the value of the effective exchange interaction in the system. From this value, we estimate various exchange interactions between ions: the dominant one is that between Mn3+ and Mn4+ which is at the origin of the ferromagnetic ordering below TC. We also studied the applied-field effect on the magnetization in the region below and above TC. The obtained MC results for |ΔSm| are in agreement with experiments performed for applied fields from 1 to 5 T. MC results of RCP are also shown and compared to experimental ones. Various other physical quantities obtained from MC simulations including internal energy and specific heat, versus temperature are also shown and discussed.

Effect of exchange, anisotropy, and external field interactions on the hysteresis and compensation of a two-dimensional ferrimagnet

We investigate the effects of exchange, anisotropy, and external field interactions on the of hysteresis and compensation behavior of a two-dimensional ferrimagnet with spins of values Q=7/2, and S=2 alternating on a square lattice, by means of the Monte Carlo (MC) and mean-field (MFT) simulation techniques. We analyze the temperature dependence of the magnetization and the hysteresis loops of the model for various values of the parameters in the Hamiltonian. The system experiences spin compensation, discontinuities, exchange bias, and multiple hysteresis loops when it is subjected to different values of exchange, anisotropy, and magnetic fields. The temperature range where the system experiences discontinuities in the magnetization increases with the decrease (increase) of the |D1′| modulus of the anisotropic field and with the change of direction of the h′ applied field, which is corroborated by the phase diagrams in the (D1′,T) and (h′,T) planes. We found that the exchange bias phenomenon occurs for a threshold value of the next-nearest-neighbor interaction of the S-spin (J2′=7.1). We compare our results with two investigations, one that does not include antiferromagnetic next-nearest-neighbor interactions nor external fields and another that does not consider the anisotropies of the sublattices, which are key parameters in the thermomagnetic behavior of the model.

Magnetic properties and hysteresis loops of mixed spin - (3/2, 1) in core-shell hexagonal nanotube: comparative study of mean-field theory and Monte Carlo simulation

Based on the Ising model, Monte Carlo simulation (MCS) and the Mean- field approximation (MFA) were used to examine the magnetic properties and the phase diagram of a ferrimagnetic mixed-spin (2,3/2) hexagonal Ising nanotube with core-shell structure. The effects of exchange interactions ( ) and the single-ion anisotropies ( ) on the magnetization of the total system, the two sublattices and the hysteresis loops have been discussed in detail. The phase diagrams under the effect of different parameters have been provided. The results demonstrate the emergence of compensation phenomena as well as the second-order phase transitions. Multiple behaviors of hysteresis loop, including single and triple hysteresis loops, were identified as a result of competitions between various parameters.

The Effect of Exchange Interaction on Phase Transition Behavior of Magnetic Nanotubes

In this paper, the phase diagram of the Blume-Capel model under exchange interaction between lattice points in different positions in magnetic nanotube lattice is studied with effective field theory. The research shows that the interaction between nearest neighbors and the lattice field strength strongly influences the system’s three critical points and phase transition temperature. Under certain conditions, there are three critical points in the system. That is, the relationship between the crucial points is linear from the second-order phase transition to the first-order phase transition. First-order phase transition replaces second-order phase transition in the system’s phase transition. The findings demonstrate that exchange interaction significantly affects the system’s phase transition, and they offer a theoretical framework for both industrial production and experimental study.

Effect of Exchange Interaction between the Magnetic Layers of a THz Spin-Injection Emitter on Its Operating Efficiency

Effect of Exchange Interaction between the Magnetic Layers of a THz Spin-Injection Emitter on Its Operating Efficiency

Composed Effects of Electron-Hole Exchange and Near-Field Interaction in Quantum-Dot-Confined Radiative Dipoles

Conservation of polarization is an important requirement for reliable single-photon emitters, which, in turn, are essential building blocks for light-based quantum information processing. In this work, we study the exciton-spin dynamics in a double quantum dot under the combined effects of electron-hole exchange and Förster resonance energy transfer. By means of numerical solutions of the quantum master equation, we simulate the time-dependent spin polarization for two neighboring dots. According to our results, under some conditions, the depolarization caused by the electron-hole exchange may be slowed by the near field-induced interdot energy transfer, suggesting a new mechanism to extend the exciton coherence time. This opens doors to alternative schemes for improved solid-state quantum light sources.

Magnetic characterization of Sm(Fe1-x, Cox)11Ti (x = 0, 0.1) determined by heat-capacity measurement from very low to high temperatures

The thermodynamic and magnetic properties of SmFe11Ti and Sm(Fe0.9Co0.1)11Ti, which are candidates for the main constituents of rare-earth magnets, were investigated by measuring their heat capacities in the range of 2–1050 K. The Curie temperatures, TC, of SmFe11Ti and Sm(Fe0.9Co0.1)11Ti were found to be 582.9 and 705.0 K, respectively. Their magnetic moments, β, obtained from the magnetic entropies by separating the spin and lattice vibration terms were 20.09 and 22.71 (μB (compd)-1), respectively, which are consistent with the results of direct magnetization measurements. This indicates that the 3d magnetic moment of Fe, which provides the magnetization, becomes aligned by the effective exchange interaction between Sm 4f via Sm 5d and Fe 3d in a second-order phase transition. In Sm(Fe0.9Co0.1)11Ti, Co 3d strongly exchange-couples with Fe 3d, following the Slater-Pauling curves, and TC and β simultaneously increase. The Sommerfeld parameters of SmFe11Ti and Sm(Fe0.9Co0.1)11Ti indicate the inherent valence fluctuation of Sm; that is, the Sm 4f empty density of states was found to be occupied by the electrons surrounding the Sm atom, indicating low phase stability. These findings are expected to contribute to the design of materials with intrinsic magnetic properties.

Magnetic transitions in V-Fe-Co-Ni-Cu-based high entropy alloys

FeCoNi, V0.85FeCoNi, FeCoNiCu1.15 and V0.85FeCoNiCu1.15 alloys have been synthesized by arc melting and analyzed by powder X-ray diffraction, electron microscopy, magnetic measurements, and density functional theory (DFT). The influence of each alloying element on the magnetic exchange interaction, Curie temperature (TC) and magnetocaloric effect is evaluated. The experimental results show that Cu and V “dilute” the magnetic properties and couple antiferromagnetically to Fe, Co, and Ni. Analysis of the microstructure reveals a lack of solubility between V and Cu with FeCoNi, and between themselves, thus lowering the concentration of V and Cu in the main solid solution of the 5-element alloy V0.85FeCoNiCu1.15. Tc decreases significantly from 997 K in FeCoNi to 245 K in V0.85FeCoNi and 297 K in V0.85FeCoNiCu1.15, respectively. The derivative of magnetization as a function of temperature (dM/dT) in the vicinity of Tc is drastically reduced due to the presence of V which indicates a reduced magnetocaloric effect. DFT calculations confirm antiferromagnetic coupling of V to the ferromagnetic FeCoNi-base and predict a similar behavior for other transition metal elements (e.g., Ti, Cr, Mn). This leads to a lowering of Tc, which is needed to establish the magnetocaloric effect at room temperature. However, it comes at a cost of reduced magnetic moments. Nevertheless, the use of V and Cu has shown possible routes for tuning the magnetocaloric effect in FeCoNi-based high entropy alloys.

Magnetic Anisotropy in a Verdazyl-Based Complex with Cobalt(II)

A verdazyl-based complex with Co2+ ion, (p-Py-V-p-Br)2[Co(hfac)2], was successfully synthesized. The molecular arrangement indicates that spins on the radical and Co form a one-dimensional (1D) spin chain, where the intermolecular interaction between the radicals and the intramolecular interaction between the radical and Co exhibit threefold periodicity. The strong antiferromagnetic interaction between radical spins forms a nonmagnetic singlet dimer, yielding paramagnetic behavior associated with the residual spins on the Co2+ ion in the low-temperature region. We evaluated the anisotropic g values by analyzing the electron spin resonance powder patterns. Furthermore, we considered the spin–orbit coupling and distorted crystal fields of the Co2+ ion and explained the triaxial anisotropic behavior. We also evaluated the anisotropy of the expected effective exchange interaction between the fictitious spins on the Co2+ ions, indicating the existence of an Ising-like exchange interaction. These results demonstrate that the strong spin–orbit coupling of the Co2+ ion in the present verdazyl-based compound yields an effective spin-1/2 with anisotropic g values and Ising-like exchange interactions.

Ladder-based two-dimensional spin model in a radical-Co complex

We successfully synthesize a verdazyl-based complex, ${(p\text{\ensuremath{-}}\mathrm{Py}\text{\ensuremath{-}}\mathrm{V}\text{\ensuremath{-}}p\text{\ensuremath{-}}\mathrm{F})}_{2}[\mathrm{Co}{(\mathrm{hfac})}_{2}]$. Molecular-orbital calculations of this complex indicate that spin ladders composed of radical spins are coupled to Co spins via intramolecular interactions, forming a ladder-based two-dimensional (2D) spin model. The complex exhibits paramagnetic behavior associated with Co spins and the 3/5 magnetization plateau; numerical analysis demonstrates that this originates from a rung-singlet-like ground state with fully polarized Co spins. The peculiar behavior of the linear magnetization curve after the magnetization plateau is attributed to the nature of the 2D quantum spin system, which demonstrates a quantum phase transition from the gapped quantum state to the 2D magnetic state. Furthermore, we examine the magnetic anisotropy originating from ${\mathrm{Co}}^{2+}$ in the distorted octahedral environment of the complex. The anisotropic $g$ values are determined by electron-spin resonance (ESR) powder pattern analysis. Considering the spin-orbit coupling and crystal-field distortion of the ${\mathrm{Co}}^{2+}$ ion, we explain the anisotropic $g$ values of the fictitious spin-1/2 on the ${\mathrm{Co}}^{2+}$ ion. We also evaluate the anisotropy of the effective exchange interaction between the radical spin and the fictitious spin on the ${\mathrm{Co}}^{2+}$ ion, thus revealing the existence of an Ising-like exchange interaction.

Correlation between Magnetocaloric Properties and Magnetic Exchange Interaction in Gd54Fe36B10-xSix Amorphous Alloys.

Gd54Fe36B10-xSix (x = 0, 2, 5, 8, 10) amorphous ribbons were fabricated by melt-spinning technique. Based on the molecular field theory, the magnetic exchange interaction was analyzed by constructing the two-sublattice model and deriving the exchange constants JGdGd, JGdFe and JFeFe. It was revealed that appropriate substitution content of Si for B can improve the thermal stability, maximum magnetic entropy change and widened table-like magnetocaloric effect of the alloys, while excessive Si will lead to the split of the crystallization exothermal peak, inflection-like magnetic transition and deterioration of magnetocaloric properties. These phenomena are probably correlated to the stronger atomic interaction of Fe-Si than that of Fe-B, which induced the compositional fluctuation or localized heterogeneity and then caused the different way of electron transfer and nonlinear variation in magnetic exchange constants, magnetic transition behavior and magnetocaloric performance. This work analyzes the effect of exchange interaction on magnetocaloric properties of Gd-TM amorphous alloys in detail.

Effect of spin polarization on electron acceleration in magnetized quantum plasma by a surface plasma wave.

The effect of spin-up and spin-down exchange interaction on electron acceleration by a surface plasma wave (SPW) propagating in magnetized quantum plasma has been studied. The SPW was excited over the metal-vacuum interface with maximum amplitude at the interface. The effective dielectric constant was evaluated, considering the effects of quantum Bohm potential, degenerate Fermi pressure, and electron spin. The externally applied magnetic field perturbed the densities of the oppositely spinning electrons, which resulted in spin polarization. The dispersion relation and energy exchange mechanism for the electron acceleration was built, incorporating the effects of spin polarization in the wave-plasma interaction. The energy gain was found to increase with the spin polarization.