Exchange Interaction Parameters Research Articles

Pyridyl-substituted nitronyl nitroxides: comprehensive magnetochemical and quantum chemical study.

A series of pyridyl-substituted nitronyl nitroxides was synthesized and structurally characterized. A comprehensive magnetochemical and quantum chemical study of extended raw of the nitroxides with different substituents R in the pyridine fragment was performed. It was shown, that temperature-dependent magnetic properties are determined by the short contacts between nitroxide groups of adjacent molecules as well as between nitroxide group and methyl substituents in the pseudo axial positions of imidazoline fragments. Quantum chemistry allows to select the appropriate model of exchange cluster for analysis of experimental magnetic data and evaluation of the exchange interaction parameters. For NN-PyCl the "order-disorder" transition was detected by means of low temperature XRD. The difference in the experimental and calculated exchange interaction energies may serve as an indicator of temperature-induced structural rearrangements. For instance, for methyl substituted nitronyl nitroxide NN-PyMe structural transformations and significant changes in exchange interaction energies were observed.

Magnetic ordering and the role of superexchange Ni-O-B-O-Ni upon the formation of magnetic order in ludwigite Ni2MnBO5 from first-principal calculations.

The energies of various magnetically ordered structures for ludwigite Ni2MnBO5 have been calculated in the framework of the first-principles approach using the Wien2K program package, with the parameters of exchange interactions being determined. Two subsystems can be distinguished in the magnetic system, which are associated with the triads 4-2-4 and 3-1-3. The magnetic moments of the ions in both triads are antiferromagnetically oriented. The analysis of the exchange contribution to the energy shows that there occurs an increase of the magnetic cell relative to the crystallographic one due to antiferromagnetic ordering of the magnetic moments along the c axis in three-legged ladders formed by 4-2-4 triads. However, in three-legged ladders formed by 3-1-3 triads, the magnetic moments of the ions are ordered along the c-axis ferromagnetically. The predicted type of magnetic ordering in Ni2MnBO5 is similar to magnetic ordering in Fe3BO5. Non-collinear ordering of the magnetic moments of the subsystems at different temperatures is also likely to be observed in Ni2MnBO5, as is the case in Fe3BO5. Superexchange (indirect) interactions (No-O-B-O-Ni) influence the orientation of the magnetic moments of two 3-1-3 (4-2-4) triads within the unit cell. It is these interactions that may be responsible for the formation of long-range magnetic order in Co3BO5 and ferrimagnetic-antiferromagnetic spin-reorientation transition in Fe3BO5.

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.

Kinetic phase phenomena of model with the chemical formula [formula omitted] on honeycomb lattice

We investigate the ferromagnetic–ferrimagnetic ternary alloy model with the chemical formula ABpC1−p with mean-field theory based on Glauber-type stochastic dynamics, namely dynamic mean-field theory, on a honeycomb lattice consisting of spins SiA=1/2, SjB=3/2 and SjC=5/2. The model includes a selective site disorder, meaning that SiA can randomly have SjC as its nearest neighbor or SjB with probability (1−p). The mean-field dynamic equations were obtained and numerically solved. To identify the type of dynamic phase transitions (first- or second-order), the thermal behavior of dynamic magnetizations was examined. For specified values of the crystal field parameter, exchange interaction parameter, and concentration ratio, the phase diagrams are calculated in the (d−T) and (T−h) planes. The system's fundamental and mixed-phase areas were identified. The system displayed A,TP,QP,Z special points and dynamic tricritical points. It was found that the content of the ternary alloy continuously affected the critical temperature in our dynamic system. Reliable agreements are observed when our findings are compared to those in the literature.

The XYZ model in the mean-field approximation in terms of Pauli spin matrices

The mean-field approximation (MFA) of spin-1/2 XYZ model was studied by using the Pauli spin matrices and their exponentials which led to very nice hyperbolic tangent functions in nonlinear form. The magnetization components Mx, My and Mz were obtained for the ferromagnetic (FM) case and then it was modified for the antiferromagnetic (AFM) case by introducing the sublattices. The bilinear exchange interaction parameters Jx, Jy and Jz, and the external magnetic fields Hx, Hy and Hz were considered along the three-dimensions for various coordination numbers q = 3, 4 and 6. The thermal variations of the magnetizations and thus the phase diagrams were obtained to illustrate the behaviours of phase transition lines in the AFM case.

Pressure-dependent magnetic properties of FeNi alloy: Theoretical study

The influence of external pressures on the structural and magnetic properties of the FeNi alloy is elucidated using density functional theory (DFT) and Monte Carlo simulation. Utilizing a range of pressures, the study showcases compressive behavior in lattice constants, with both a and c exhibiting reductions. A sharp decline in magnetic moments is recorded at a critical pressure of 54 GPa. Indicating a magnetic phase transition from ferro to paramagnetic phase. Beyond this critical pressure, dynamical instability becomes evident, attributable to the aforementioned magnetic phase transition. Further examination of exchange interaction parameters reveals that while Fe–Fe interactions exhibited a consistent decrease with pressure, the FeNi and NiNi interactions remained relatively stable across the examined pressure range. At pressures close to 0 GPa, the Curie temperature (TC) was found to be around 838 K. As the external pressure is applied, a consequential decline in TC is witnessed, suggesting a significant sensitivity of the magnetic phase transition to external pressures. In particular, this study indicates that the FeNi alloy is sensitive to external pressures, emphasizing some relation between structural and magnetic properties.

MXene Fe2C as a promising candidate for the 2D XY ferromagnet

Monolayer Fe2C is expected to possess strong electronic correlations, which can significantly contribute to electronic and magnetic properties. In this study we consider electronic and magnetic properties of MXene Fe2C within the DFT+DMFT approach. We establish the existence of local magnetic moments in this compound, characterized by sufficiently long lifetime of fs. We also calculate exchange interaction parameters accounting for electronic correlations using the recently developed approach for paramagnetic phase. At low temperatures, we obtain the strongest exchange interaction 11 meV between next nearest neighboring Fe atoms, located above (or below) the carbon plane, and the subleading interaction 6 meV between the next to next nearest neighboring atoms, located across the carbon plane. The resulting dependence of the Berzinskii–Kosterlitz–Thouless (BKT) and Curie temperatures on magnetic anisotropy is obtained. The BKT temperature for the pristine Fe2C is K, which makes this compound a good candidate for the two-dimensional ferromagnet with XY anisotropy.

Thermodynamic Properties and DFT Study on Highly Frustrated Cr3BO6: Coexistence of Spin-Singlets with Long-Range Magnetic Order.

The triangle-based magnetic subsystem of borates with the mineral norbergite structure M3BO6 (M = Fe, Cr, V) makes these compounds unique to investigate rare quantum ground states influenced by strong magnetic frustration. In this work, we investigated the thermal and magnetic properties of Cr3BO6 to find that despite very large negative Weiss temperature Θ = -160.7 K, it orders only at TN = 4.5 K and experiences a spin-flop transition at µ0H = 5 T. Density functional theory (DFT) calculations of exchange interaction parameters allow for suggesting the model of magnetic subsystem in chromium borate Cr3BO6. The results prove the decisive role of magnetic frustration on the formation of long-range order, providing therefore a basis for future study. Both experimental data and first-principles calculations point to the coexistence of chromium spin-singlets with long-range antiferromagnetic order.

Influence of substitutional defects on the structural and magnetic properties of barium hexaferrites

A computational analysis is conducted to understand the impact of Al and Sr substitutions on the structural and magnetic properties of M-type barium hexaferrite (BaFe12O19 or BFO). Al substitutions in BFO led to concentration-dependent increases in lattice parameters and magnetocrystalline anisotropy energy (), while the magnetic moment showed a decline. In contrast, Sr substitutions influenced the lattice differently, reducing parameters but enhancing both and the spin magnetic moment, especially under varying Coulomb interaction () conditions. It’s noteworthy that the value reached approximately 1.03 meV with full Sr substitution. Furthermore, SrFe12O19 (SFO) consistently outperformed BFO in terms of exchange interaction parameters. These findings offer valuable insights for the development and optimization of next-generation magnetic materials.

Effect of Ga Partial Substitution on the Structural and Magnetic Properties of Heusler Ni–Mn–Ga Alloys

Abstract—The effect of doping with Cu and Zn atoms on the magnetic and structural properties of the cubic and tetragonal phase of the Heusler Ni2Mn1.125Ga0.875 alloy has been investigated in the framework of density functional theory. Partial substitution of Ga atoms by Cu and Zn atoms leads to a decrease in the energy difference between the ferromagnetic and ferrimagnetic states of the cubic phase and an increase in the energy barrier between the cubic and tetragonal phase. The latter observation indirectly points to the increase in the structural transition temperature observed experimentally. Within the framework of calculations of exchange interaction parameters from first principles and Monte Carlo modeling of temperature dependences of magnetization, a phase T–x diagram is constructed. It is shown that the Curie temperature of the tetragonal phase decreases significantly compared to the Curie temperature of the cubic phase with increasing Cu and Zn content due to an increase in the inter-sublattice antiferromagnetic interaction between Mn atoms.

Influence of Al and Si concentration on the structural, electronic, magnetic, and mechanical properties of Fe2CoAl1−xSix (x = 0, 0.25, 0.5, 0.75, and 1.0) full heusler alloys: First principles calculations and Monte Carlo simulation

A comprehensive investigation into the structural, electronic, magnetic, mechanical, and thermomagnetic properties of Fe2CoAl1−xSix (x = 0, 0.25, 0.5, 0.75, and 1.0) full Heusler alloy is performed using density functional theory and Monte Carlo simulation. Pristine and doped structures are found to adopt the XA prototype, and the lattice parameter is observed to decrease with increasing Si concentration. An increase in the bulk modulus is noticed with the increase of Si concentration, the resulted values range from 160 to 240 GPa. The electronic properties reveal a band gap in the spin-down states of the pristine and doped structures using mBJ approximation. The magnetic moments of the doped structures are found to be similar to those of the pristine structures, and the exchange interaction parameters highlight the Fe-Fe interaction as the strongest in the doped structures. The resulting Curie temperature using Monte Carlo simulation ranged between 792 K and 1029 K. Finally, understanding the magnetic properties of these Heusler alloys could pave the way for spintronics and high-temperature magnetic applications.

Indirect Exchange Interaction Leads to Large Lattice Contribution to Magnetocaloric Entropy Change.

Materials with a large magnetocaloric response are highly desirable for magnetic cooling applications. It is suggested that a strong spin-lattice coupling tends to generate a large magnetocaloric effect, but no microscopic mechanism has been proposed. In this Letter, we use spin-lattice dynamics simulation to examine the lattice contribution to the magnetocaloric entropy change in bcc iron (Fe) and hcp gadolinium (Gd) with exchange interaction parameters determined from abinitio calculations. We find that indirect Ruderman-Kittel-Kasuya-Yosida (RKKY) exchange interaction in hcp Gd leads to longer-range spin-lattice coupling and more strongly influences the low-frequency long-wavelength phonons. This results in a higher lattice contribution toward the total magnetocaloric entropy change as compared to bcc Fe with short-range direct exchange interactions. Our analysis provides a framework for understanding the magnetocaloric effect in magnetic materials with strong spin-lattice couplings. Our finding suggests that long-range indirect RKKY-type exchange gives rise to a larger lattice contribution to the magnetocaloric entropy change and is, thus, beneficial for magnetocaloric materials.

A comparative study of structural, electronic, magnetic, and pressure-dependent properties of Full-Heusler Fe2XSi (X=Mn and Co): first-principles calculation and Monte Carlo Simulation

This study investigates the structural, electronic, magnetic, and pressure-dependent properties of Fe2XSi (X=Mn and Co) full-Heusler alloy through a comprehensive computational approach. The structural stability of these alloys confirmed L21 and XA prototypes as favorable structures for Fe2MnSi and Fe2CoSi, respectively. In addition, Fe2MnSi exhibits 100% spin polarization suggesting its potential suitability for spintronics applications. Conversely, Fe2CoSi does not exhibit similar behavior. The calculated magnetic moments of these alloys revealed an increase due to Coulomb interaction (U). The magnetic moments satisfy the Slater-Pauling relation, with calculated values of 3 for Fe2MnSi and 5 for Fe2CoSi. Heisenberg exchange interaction parameters are calculated to illustrate distinct magnetic interactions within the alloys. The study further explores thermomagnetic properties through Monte Carlo simulations, including thermal magnetization, thermal magnetic susceptibility, and Curie temperature The calculated were found to be 294 K for Fe2MnSi and 1029 K for Fe2CoSi, showing close agreement with experimental data. We additionally investigate the pressure-dependent properties of these alloys, revealing a non-linear dependence of the band gap on pressure for Fe2MnSi, with values decreasing from 0.668 eV to 0.259 eV as pressure increased from −15.917 to 23.712 GPa. For Fe2CoSi, the band gap remained zero across varying pressure conditions, indicating its metallic conducting behavior.

Moment canting and domain effects in antiferromagnetic DyRh2Si2

A combined experimental and theoretical study of the layered antiferromagnetic compound ${\mathrm{DyRh}}_{2}{\mathrm{Si}}_{2}$ in the ${\mathrm{ThCr}}_{2}{\mathrm{Si}}_{2}$-type structure is presented. The heat capacity shows two transitions upon cooling: The first one at the N\'eel temperature ${T}_{\mathrm{N}}=55\phantom{\rule{4pt}{0ex}}\mathrm{K}$ and a second one at ${T}_{\mathrm{N}2}=12\phantom{\rule{4pt}{0ex}}\mathrm{K}$. Using magnetization measurements, we study the canting process of the Dy moments upon changing the temperature and can assign ${T}_{\mathrm{N}2}$ to the onset of the canting of the magnetic moments towards the [100] direction away from the $c$ axis. Furthermore, we found that the field dependence of the magnetization is highly anisotropic and shows a two-step process for $H\ensuremath{\parallel}001$. We used a mean-field model to determine the crystalline electric field as well as the exchange interaction parameters. Our magnetization data together with the calculations reveal a moment orientation close to the [101] direction in the tetragonal structure at low temperatures and fields. Applying photoemission electron microscopy, we explore the (001) surface of the cleaved ${\mathrm{DyRh}}_{2}{\mathrm{Si}}_{2}$ single crystal and visualize Si- and Dy-terminated surfaces. Our results indicate that the Si-Rh-Si surface protects the deeper lying magnetically active Dy layers and is thus attractive for investigation of magnetic domains and their properties in the large family of ${\mathrm{LnT}}_{2}{\mathrm{Si}}_{2}$ materials.

Influence of Mo Doping on Electronic and Magnetic Properties of Monolayer CrX3: A First-Principles Study

In recent years two dimensional (2D) materials with intrinsic magnetism have drawn intense research interest due to their potential application in spintronic devices. Among them, chromium trihalide family has received much attention due to the exhibition of wide range of electronic and magnetic properties. Aiming to improve their spintronic properties, we have investigated the electronic and magnetic properties of mono-layer (ML) Mo-doped CrX3 (X = Cl, Br and I) using density functional theory (DFT) + Hubbard U approach. Our results show that substitution of Mo atom at the Cr site is energetically favorable. Magnetic ground state of all the members is found to be ferromagnetic (FM) which undergoes a transition to antiferromagnetic (AFM) state by applying both the compressive as well as the tensile strain. FM exchange interaction parameter and the estimated Curie temperature (T c ) of ML CrMoX6 are found to be significantly enhanced compared to the corresponding values of pristine CrX3. All the members of ML CrX3 show magneto-crystalline anisotropy energy (MAE) favoring an out-of-plane easy axis of magnetization. Our calculations further reveal that all three compounds exhibit semiconducting properties with reduced bandgap compared to the ML CrX3 family. This property can make them useful in spintronic applications.

An EPR Study on Highly Stable Nitroxyl-Nitroxyl Biradicals for Dynamic Nuclear Polarization Applications at High Magnetic Fields.

Nitroxide biradicals are efficient polarizing agents in dynamic nuclear polarization (DNP) solid-state nuclear magnetic resonance. Many recently reported radicals possess substantial DNP efficiency in organic solvents but have poor solubility in water media which is unfavorable for biological applications. In this paper, we report DNP efficiency at a high magnetic field for two water-soluble biradicals resistant to reducing media. Water solubility was achieved by obtaining the radicals in the form of quaternary ammonium salts. Parameters of hyperfine interaction and exchange interaction were quantified by EPR spectroscopy, and their influence on the DNP effect was determined. The resistance of the biradicals to strongly reducing media was characterized. High stability was achieved using tetraethyl substituents and pyrrolidine moieties.

Magnetic properties and coupled spin-phonon behavior in quasi-one-dimensional screw-chain compound BaMn2V2O8

Spin-chain compounds are known to exhibit fascinating magnetic properties, which mostly display magnetic ordering at very low temperatures or remain dynamic even at 0 K. In contrast, the present quasi-one-dimensional spin-chain system $\mathrm{Ba}{\mathrm{Mn}}_{2}{\mathrm{V}}_{2}{\mathrm{O}}_{8}$ exhibits a collinear antiferromagnetic (AFM) long-range ordering at a relatively higher temperature ${T}_{\mathrm{N}}\ensuremath{\sim}37\phantom{\rule{0.16em}{0ex}}\mathrm{K}$, wherein the nearest-neighbor spins have AFM coupling along the spin chain, i.e., along the $c$ axis. The present study also reveals a short-range magnetic ordering prevailing at considerably elevated temperatures above its ${T}_{\mathrm{N}}$. Temperature-dependent Raman spectroscopy demonstrates an occurrence of spin-phonon coupling below ${T}_{\mathrm{N}}$ at least for two phonon modes, whereas the study also shows an unusual thermal evolution of the Raman modes above ${T}_{\mathrm{N}}$, which is apparently associated to the short-range magnetic ordering. Furthermore, extensive ab initio density functional theory calculations accompanied with classical Heisenberg model based theoretical calculations of various exchange interaction parameters $({J}_{0}--{J}_{5})$ suggest an AFM ground state, which matches well with the experimentally obtained spin structure.

Magnetic properties of the quaternary alloy with the mixtures of integer and half-integer spins

ABSTRACT The ferrimagnetic-ferromagnetic-ferrimagnetic quaternary alloy consisting of the spins , , and in the form of with B, C and D being the possible nearest-neighbors of A with the probabilities p, q and r, respectively, is investigated in terms of the mean field theory (MFT) based on the Bogoliubov inequality for free energy. The thermal behaviors of magnetizations belonging to the individual spins and total magnetization are examined in detail to map the possible phase diagrams on various planes. It was found that the model only exhibits second-order phase transitions in addition to one compensation temperature for appropriate values of the system parameters. In order to support the existence of the compensation temperatures, we have also carried out the Monte Carlo (MC) simulations and an agreement was shown with the predicted result of the MFT approach. We have also analyzed the effects of the bilinear interaction parameter between type-A atoms and the exchange interaction parameter ratio ( is the bilinear interaction parameter between type-A atoms and type-X atoms with X = C or D) on compensation temperatures. Our findings are compared to those reported in the literature and suitable agreements are observed.

Anhydrous copper tellurite disulfate Cu3TeO3(SO4)2 featuring the coexistence of spin singlets and a long-range antiferromagnetic order.

Anhydrous copper tellurite sulfate, Cu3TeO3(SO4)2, has been synthesized via vapor transport reactions in sealed silica glass ampoules. In measurements of magnetization M, magnetic susceptibility χ, specific heat Cp and X-band electron spin resonance, a long-range antiferromagnetic order at TN = 13 K and an H-T magnetic phase diagram have been established. One-third of Cu2+ ions were found to form magnetically silent dimers. A peak in dielectric permittivity ε, which accompanies the Néel order, allows considering Cu3TeO3(SO4)2 as a magnetoelectric multiferroic material of the second type. Density functional theory calculations provided estimations of leading exchange interaction parameters.

The spin-1 XYZ model with Dzyaloshinskii–Moriya interaction

The spin-1 XYZ model is studied in terms of a mean-field (MF) approach by introducing the spin operators on honeycomb lattice. The considered Hamiltonian consists of a bilinear exchange interaction parameter (Jz) and a single-ion anisotropy (Dz) only along the z-axis, in addition to the Dzyaloshinskii–Moriya interaction (Δm). The external magnetic field is introduced in all three dimensions and their behaviors are examined on the components of magnetization. Then the thermal behaviors of magnetizaitons are investigated in detail to obtain the phase diagrams on the (Dz/J,kT/J) and (Δm/J,kT/J) planes when the external magnetic field components are turned off. It is found that the model gives both first- and second-order phase transitions in addition to a few types critical points including the tricritical and bicritical points.