Values Of Exchange Interactions Research Articles

TRANSITION TEMPERATURE DEPENDENCE ON ANISOTROPY IN THE Ј1-Ј2-Јc-Јp HEISENBERG MODEL WITH APPLICATION TO 122-TYPE OF IRON PNICTIDES

Recently there has been a considerable interest in studying iron-based superconductors called pnictides. The aim of this paper is to give a contribution to better understanding of magnetic properties of the 122 class of iron pnictides. We use anisotropic J1-J2-Jc-Jp Heisenberg model. The method of spin Green’s functions in the Tyablikov’s random phase approximation (RPA) decoupling scheme is used to determine the transition temperature. Based on the obtained expression for the transition temperature, we study its dependence on the spin anisotropy. Furthermore, we compare our model results with the experimentally available data for the transition temperature. In addition, we test our model predictions of the transition temperature using the exchange interaction values from other articles. Results presented here should give additional contribution to understanding the magnetic properties of the 122 class of iron-based superconductors.

Spin reorientation in CoV2O4 thin film: A first principles study

Spinel vanadates (AV2O4) are a class of materials where an interesting interplay between spin and orbital degrees of freedom on a frustrated lattice is manifested in the multiple structural and magnetic phase transitions. CoV2O4 is unique among the spinel vanadates as it shows no or very weak cubic to tetragonal structural phase transition but two magnetic (paramagnetic to collinear ferrimagnetic and collinear to noncollinear ferrimagnetic) phase transitions at lower temperatures in its bulk form. In a recent experiment (Thompson et al., Phys. Rev. Mater. 2 (2018) 104411), an epitaxial thin film of CoV2O4 grown on the SrTiO3 (001) substrate, is observed to have an orthorhombic structural phase accompanied by a noncollinear magnetic state where the spin moments of Co and V are reoriented from [001] (seen in the bulk form) to the [110] direction. In this work, we have explored the mechanism behind this complex spin-reorientation and noncollinear magnetic ground state by investigating the electronic structure, magnetic, and structural properties of CoV2O4 in its orthorhombic phase and compared with the corresponding properties of the cubic phase using the first principles density functional theory. Our GGA+U+SO calculations on orthorhombic CoV2O4 reveal that spin moments prefer to lie in the ab-plane with the crystallographic b-direction being the preferred direction in the collinear ferrimagnetic state where it has a larger band gap and orbital moment (of V) as compared to the cases with spin moments aligned parallel to a and c-directions. We further observe competing magnetic exchange interaction values in orthorhombic phase when the spins lie in the ab-plane giving rise to more frustration and hence more canting in the non-collinear magnetic ground stateas compared to the cubic phase, consistent with experiment. With the help of source-free exchange–correlation (XC) functional we were able to capture the non-collinear magnetic ground state and the spin-reorientation in this system. We also discussed the possible orbital ordering in the orthorhombic phase of CoV2O4.

Effect of the chemical structure of a tetradentate equatorial ligand on the spin-crossover properties of the Fe (III) complexes chain structures: Electron paramagnetic resonance study.

The effect of the chemical structure of the equatorial ligand on the spin state of the Fe (III) ion in a series of 1-D chain complexes of the general formula [Fe(L)(tvp)]BPh4 ·nCH3 OH, where L = dianions of Schiff base containing a different number of aromatic groups: N,N'-ethylenebis (salicylaldimine) (salen) 1, N,N'-ethylenebis (acetylacetone)2,2'-imine (acen) 2, N,N'-ethylenebis (benzoylacetylacetone)2,2'-imine (bzacen) 3, and tvp = 1,2-di(4-pyridyl)ethylene, was studied by ultraviolet-visible (UV-vis) and electron paramagnetic resonance (EPR) methods. The values of exchange interactions, thermodynamic parameters of spin-crossover, and electronic structure features of the Fe (III) complexes were estimated from the EPR data. The substitution of a fragment of the equatorial ligand L in the series salen-acen-bzacen changes the local symmetry of the complex in the 1-D chain, thereby affecting the spin variable properties.

Mixed spin-3/2 and spin-2 nanowire: magnetic properties and hysteresis behaviors

PurposeThe purpose of this article is to investigate the magnetic properties and the hysteresis loops behavior of a ferrimagnetic cubic nanowire with mixed spins SA = 3/2 and SB = 2.Design/methodology/approachWe have used the Monte Carlo simulation to examine the influences of the exchange interaction JB, the crystal field ∆ and the temperature on the magnetic properties and hysteresis loops of the nanowire. More exactly, we have shown the temperature dependence of the sublattice magnetizations (mA and mB) and the total magnetization (M) for several values of the Hamiltonian parameters, as well as the corresponding phase diagrams. Finally, the effect of an external magnetic field is studied by plotting the hysteresis loops of the system for different values of exchange interaction, crystal field and temperature.FindingsThe obtained results show the existence of second-order phase transitions, as well as the compensation behavior. Moreover, according to the values of the Hamiltonian parameters, the system can exhibit one, two or three hysteresis loops.Originality/valueThe magnetic nanowires are of great interest in experimental works, but without theoretical explanations, the experimental results cannot be clarified in depth. For this, we contribute through this theoretical study to understand the nanowires, especially those with mixed spins (2, 3/2).

Experimental and theoretical study of magnetization and magnetocaloric effect in Nd1−xSmxFeO3

We have experimentally and theoretically investigated the magnetic properties and magnetocaloric effect in doped orthoferrite series Nd1−xSmxFeO3 (x = 0, 0.1, and 0.4). Our magnetization measurements reveal that the spin-reorientation transition temperatures of the Fe3+ moments increase as a function of Sm doping. Our measurements on the magnetocaloric effect indicate that the effect [measured by the maximum change in magnetic entropy (−ΔSM)] reduces with Sm doping. Non-collinear calculations including the effect of Coulomb correlation (U) and spin–orbit interaction (SO) were performed within the GGA + U + SO approximation using the first-principles density functional theory (DFT) to establish the magnetic structure above and below the spin-reorientation transition. Calculations of the exchange interaction strengths using DFT reveal a higher strength of Sm–Fe exchange interaction than that of Nd–Fe interaction, which explains the higher spin-reorientation transition temperatures observed in the doped compounds. Using Monte Carlo calculations on the Ising model with large moments of Nd/Sm, we have extracted the effective nearest neighbor exchange interaction values between rare-earth moments, which provide good agreement with the corresponding experimental results on −ΔSM around and below the transition temperature.

Entanglement dynamics and fractional quantum state transport in the spin-[formula omitted] triangular plaquette

The dramatic growth of research areas within the province of quantum state transmission (QST) is rapidly accelerating. An important insight to understand the process of QST can be fulfilled by considering the dynamical behavior of its entanglement content. One well-established approach to continuously transfer quantum states is utilizing spin structures. Here, from the view of entanglement propagation, we disclose the signature of fractional QST possibilities. In the present work, we proposed a form of spin-12 triangular plaquette whose Hamiltonian entails Heisenberg interaction on the legs and Dzyaloshinskii–Moriya type of coupling over the rungs. The feature of such a system is that the time instants of QST emerge in a discrete fashion, thereby the values of exchange interactions associated with these moments behave fractionally. Importantly, it is found that for special values of magnetic interaction, QST has singularity i.e., the entanglement propagation is forbidden for such values. In addition, the finite-size nature of this system makes it possible for us to read the nexus between time crystallinity and symmetry breaking. The development of our knowledge about time crystalline symmetry and its breaking helps us to understand the defined concept and fundamental physics of this phenomenon.

Magnetic compensation phenomena and paramagnetic behavior on coronene -Like Superlattice: A Monte Carlo study

The magnetic properties of Coronene -Like Superlattice (CLS) have been investigated via Monte Carlo study. The system has been described by Hamiltonian contain the (S–S), (σ-σ), (σ-S) spins, external magnetic and the crystal field applied on the two spins S and σ. The several more stable magnetic phases of CLS are obtained under effect of crystal field, exchange interactions and magnetic field. The critical and compensation temperatures of CLS are found for a several exchange interactions between the spins (S–S, σ-S) and crystal fields for a fixed value of exchange interactions between spins σ-σ. Magnetic hysteresis cycles of CLS are obtained for a several temperatures, exchanges interactions (S–S), (σ-σ) and crystal filed Dσ. In particular, the results show that the effect of: temperature, exchange coupling, magnetic field and crystallized field, play an important role in the presence of the more saturated magnetic phases and their effect on the magnetic hysteresis cycle.

Monte Carlo study combined with Blume-Capel model of ferroelectric phase transitions of a naphthalene-like structure with defects

We have used the Monte Carlo simulations combined with Blume-Capel model to investigate the ferroelectric phase transitions of a naphthalene-like structure with defects. The ferroelectric Curie temperature has been deduced from the variation of partial and total polarizations for the pure and the defects impurities on naphthalene-like nanostructure. The effect of crystal field and defects impurities on our system were studied. The variation of partial and total polarizations with temperature of naphthalene-like nanostructure are obtained for several exchange interaction values. The ferroelectric hysteresis cycles are given for different values of impurities, temperatures, exchange interactions (S-S) and crystal fields. The nanoparticles possess paraelectric features around the ferroelectric Curie temperature.

Ground States Phase Diagrams and Magnetizations Properties of Ferrimagnetic Model on Decorated Hexagonal Nanolattice: Monte Carlo Study

The ground states phase diagrams and magnetizations properties of ferrimagnetic model on decorated hexagonal nanolattice have been studied via Monte Carlo simulations. The shape of the phase diagrams depends of the values of the magnetic parameters given in the Hamiltonian. The phase diagrams show some key features: coexistence between regions, points where two, three, four and five and states can coexist. This richness of the ground state phase diagrams can serve as a guide for exploring interesting regions of the model’s finite temperature phase diagrams. The transition temperatures have been deduced for different values of exchange interactions between the atoms of spins moments(S). The effect of crystal field and exchange interactions on magnetization has been investigated. The magnetic coercive field and remanent magnetization are obtained for different parameters such as: exchange couplings and temperatures values. The superparamagnetic behavior has been found at the transition temperature.

Reentrant spin-glass behaviour in highly frustrated Mn-rich spinel zinc manganate

The present work offers an insight into the magnetic properties of Mn-rich spinel zinc manganate. Rietveld refinement reveals the formation of Zn0.67Mn2.33O4, where Zn2+ and Mn2+ ions are randomly distributed in the tetrahedral sublattice. DC and AC susceptibility measurement of Zn0.67Mn2.33O4 infers the occurrence of two kinds of transition below 11 K. Paramagnetic to ferrimagnetic transition occur at 10.7 K and ferrimagnetic to spin glass-like transition occurs at 5.8 K. The long-range canted ferrimagnetic ordering is corroborated using modified Lotgering model and calculated the exchange interaction values (JAA = 5.2 K,JAB = 5.3 K, JBB = 14.8 K). Further, the observed shift in freezing temperature with DC magnetic field obeys Almeida–Thouless behaviour and frequency dependence of AC susceptibility follows Vogel–Fulcher law. However, the complete establishment of the canonical spin glass state is denied since the manganese ions occupied at the tetrahedral site is equal to the percolation threshold (=33%). Subsequently, the observed spin glass-like behaviour (5.8 K) below Curie temperature (10.7 K) evidences the reentrant spin glass nature. Similarly, the strong frustration in Zn0.67Mn2.33O4 lattice is observed through the substantial negative value of Curie–Weiss temperature (∼−599 K) and very high frustration factor (f = 56). Overall, the chosen Zn0.67Mn2.33O4 is a highly frustrated magnetic system revealing re-entrant spin-glass behaviour.

Magnetic Properties and Spin-Wave Excitations in Co/Mn Multilayers

Co/Mn multilayers were prepared using evaporation technique under ultra-high vacuum conditions and their structural and magnetic properties were studied. The temperature dependence of the spontaneous magnetization [Formula: see text] was recorded for different cobalt layer thickness ([Formula: see text]) ranging from 19[Formula: see text]Å to 70[Formula: see text]Å. Bloch’s law describes satisfactory the thermal change of magnetization for all samples. In addition, the change of the spin-wave (SW) parameter [Formula: see text] versus [Formula: see text] was discussed. The torque magnetometry was utilized to extract the effective anisotropy ([Formula: see text]) and the interlayer coupling constant ([Formula: see text]) was determined from the in-plan M–H loops. An updated model based on SW theory was applied to describe the [Formula: see text] variation and to calculate the approximate values of exchange interactions for samples with different Co-layer thicknesses.

O-Quinone phenalenyl derivatives as expedient ligands for the design of magnetically active metal complexes: A computational study

The DFT calculations of the electronic structure, energy characteristic and spin densities of cobalt(II) diketonate adducts with polycyclic triangular hydrocarbons functionalized with o-benzoquinone groups aimed at search for spin-state switchable compounds have been performed. It is shown that by varying substituents in the ancillary diketone ligands one can control energies of the isomeric forms, while values of exchange interactions are determined by the number of six-membered cycles in hydrocarbon moiety. The polystable paramagnetic adducts capable of stepwise intramolecular electron transfers between the metal ions and redox-active fragments possessing the properties of efficient magnetically responsive molecular switches have been computationally found.

Magnetic properties of binary axb1−x hexagonal monolayers: a Monte Carlo study

ABSTRACTThe magnetic properties of binary Ax(S = 3/2)B1−x(σ=2) hexagonal monolayers (A and B are the magnetic ions with spins S and σ, respectively) are studied via Monte Carlo simulations. The partial, total magnetizations and magnetic susceptibilities of binary Ax(S)B1−x(σ) hexagonal monolayers have been found for one proportion of spin-σ (x = 0.3). Magnetic phase diagrams of spin-σ (tC−x) of our system were deduced. The effect of crystal field on the total magnetizations has been studied for different proportions of spin-σ and for different values of exchange interactions between half-integer spins. The coercive field and remanent magnetization have been established for different proportions, x, exchange interactions R, exchange interactions between spins(σ–σ) and temperatures values.

Phase Transition in Mixed Spin Ising Nanoparticles

The phase transitions of two mixed-spin (1/2,1) Ising nanoparticles with n = 4.0 and n = 6.0 are investigated by using the effective-field theory with correlations (EFT). The spin-1 atom with a single-ion anisotropy is decorated at the center of regular n-polygon (or nanoparticle with a value of n) and the spin-1/2 n atoms are put at the perimeter n sites. The phase transition in the nanoparticle with n = 6.0 is rather different from that in the nanoparticle with n = 4.0. It is shown that the number of n plays just like the coordination number z in the bulk mixed-spin Ising systems: the hexagonal nanoparticle with n = 6.0 has exhibited the first-order transition and the tricritical behavior, although the tetragonal nanoparticle with n = 3.0 did not show such behaviors, depending on the values of exchange interactions.

A New Chiral Ni4O4 Distorted Cube: Synthesis, Structure, and Magneto‐Structural Correlation

A new chiral tetranuclear Ni4O4 complex has been developed with distorted cubane‐type structure. The structure was determined by single‐crystal X‐ray diffraction. The new tetradentate Schiff‐base ligand in this compound was synthesized via the condensation of 2′‐hydroxyacetophenone and (R)‐3‐amino‐1,2‐propanediol. Temperature dependent magnetization measurements showed a predominant ferromagnetic interaction among the four Ni centers. The best fit of the χT vs. T curve results in a solution with two exchange interaction values, J1=12.7 cm–1, and J2=1.6 cm–1 which are appreciably correlated with the structure. Ac susceptibility data were collected between 2 and 28 K and it is found that the data display a small frequency dependence with the peaks shifting to higher temperatures with increasing frequencies.

Magnetic excitations in the quasi-two-dimensional ferromagnet Fe3−xGeTe2 measured with inelastic neutron scattering

Fe3-xGeTe2 is an itinerant ferromagnet composed of two-dimensional layers weakly connected by van der Waals bonding that shows a variety of intriguing phenomena. Inelastic neutron scattering measurements on bulk single crystals of Fe2.75GeTe2 were performed to quantify the magnetic exchange interaction energies and anisotropy. The observed inelastic excitations are indicative of dominant in-plane correlations with negligible magnetic interactions between the layers. A spin-gap of 3.9 meV is observed allowing a measure of the magnetic anisotropy. As the excitations disperse to their maximum energy (~65 meV) they become highly damped, reflective of both the magnetic site occupancy reduction of 25{\%} on one Fe sublattice and the itinerant interactions. A minimal model is employed to describe the excitation spectra and extract nearest neighbor magnetic exchange interaction values. The temperature evolution of the excitations are probed and correlations shown to persist above Tc, indicative of low dimensional magnetism.

Исследование фазовых переходов и критических свойств модели Гейзенберга на объемно-центрированной кубической решетке

The phase transitions and critical phenomena of the three-dimensional antiferromagnetic Heisenberg model on a body-centered cubic lattice with next and next-nearest neighbor interactions are studied using the replica Monte Carlo algorithm. Investigations are carried out for relations of exchange interaction values of next and next-nearest neighbors in the range of k values [0.0, 0.6]. A behavior of phase transitions is analyzed by the histogram method. A whole set of main static critical exponents is estimated within the finite-size scaling theory. The universality class of the model critical behavior is shown to be unchanged in the considered interval of k value.

Magnetic properties of mixed spin-5/2 and spin-2 Ising model on a decorated square lattice: A Monte Carlo simulation

The magnetic properties of mixed spin-5/2 and spin-2 Ising model on a decorated square lattice are studied using the Monte Carlo simulations. The ground state phase diagrams of mixed spin-5/2 and spin-2 Ising model on a decorated square lattice are obtained. The reduced transition temperature is obtained for different values of exchange interactions between mixed spins-5/2 and 2. The effect of reduced crystal field on the system are studied. The magnetic hysteresis cycles of the mixed spin-5/2 and spin-2 Ising model on a decorated square lattice for a several values of exchange interactions and reduced crystal field values have been found. Finally, the system exhibit the superparamagnetism behavior for d=-4 and for this value the total magnetization is equal to zero.

Influence of non-bridging donor atoms on the value of exchange interaction in binuclear CuII complexes with bis-hydrazones based on 2,6-diformylphenol

The quantum chemical simulation of exchange interaction according to the broken symmetry approach was performed for a series of binuclear CuII complexes containing the asymmetric exchange moiety based on bis-hydrazones of 2,6-diformylphenol. The calculated values were compared between themselves and with the experimental values of the exchange parameter 2J for the binuclear copper(II) complexes with bis-hydrazones based on 2,6-diformylphenol in order to reveal the influence pattern of the nature of nonbridging donor atoms X (O, N, S, and Se) onto the exchange interaction. The strength of exchange interaction of the antiferromagnetic type depends significantly on the nature of nonbridging donor atoms, increasing in the series of X = O, NH, S, and Se.

Magnetic properties of magnetic bilayer Kekulene structure: A Monte Carlo study

In the present work, we have studied the magnetic properties of magnetic bilayer Kekulene structure with mixed spin-5/2 and spin-2 Ising model using Monte Carlo study. The magnetic phase diagrams of mixed spins Ising model have been given. The thermal total, partial magnetization and magnetic susceptibilities of the mixed spin-5/2 and spin-2 Ising model on a magnetic bilayer Kekulene structure are obtained. The transition temperature has been deduced. The effect of crystal field and exchange interactions on the this bilayers has been studied. The partial and total magnetic hysteresis cycles of the mixed spin-5/2 and spin-2 Ising model on a magnetic bilayer Kekulene structure have been given. The superparamagnetism behavior is observed in magnetic bilayer Kekulene structure. The magnetic coercive field decreases with increasing the exchange interactions between σ-σ and temperatures values and increases with increasing the absolute value of exchange interactions between σ-S. The multiple hysteresis behavior appears.