Magnetic Coupling Constants Research Articles

Investigation of Magnetic Properties of Multilayer Thin Films with Spin-Wave Resonance

A theory has been developed to simulate spin-wave resonance (SWR) modes in the multilayer systems consisting of alternate magnetic and nonmagnetic layers. An equation of motion of magnetization with Gilbert-type damping parameter for simulating SWR modes was used. It has been realized that the theory developed for the magnetic multilayer films is suitable to study the spin dynamics and extract various magnetic parameters. It has been shown that SWR modes strongly depend on an effective magnetic anisotropy constant (K eff), interlayer exchange coupling constant (A 12) and effective magnetization (M eff). The nature of the effective magnetic anisotropy and interlayer exchange coupling constants has been investigated by using the developed SWR theory in detail. The separation between optic and acoustic modes strongly depends on the magnitude of the interlayer exchange coupling constant, whereas the relative position of the acoustic and optic modes depends on the sign of $A_{12}$ . With increasing the interlayer exchange coupling constant, the resonance field of the optic mode decreases (increases) for ferromagnetic (antiferromagnetic) coupling. When the effective magnetic anisotropy constant increases, the resonance field of the acoustic and optic modes increases for both the ferromagnetic and antiferromagnetic couplings. The increasing of the effective magnetization results in decreasing of the resonance field of SWR mode at parallel geometry, whereas that of SWR mode increases at the perpendicular geometry. The results are compatible to the other theories and experimental results.

Ternary diagrams of magnetic properties of Ni-Mn-Ga Heusler alloys from ab initio and Monte Carlo studies

A systematic ab initio calculations based on the DFT methodology and Spin-Polarized Relativistic Korringa-Kohn-Rostoker method are carried out to investigate magnetic properties of structure-disordered Ni-Mn-Ga Heusler alloys. In order to generate a complete set of disorder compositions, which will cover the whole area of the ternary diagram of Ni-Mn-Ga, the coherent potential approximation is used. Using our previously calculated lattice constants, the magnetic moments and exchange coupling constants are calculated for the optimized crystal structure of selected alloys. Obtained data from ab initio calculations are used as input for Monte Carlo simulations of Heisenberg Hamiltonian to compute the thermomagnetization curves. The Curie temperatures for the austenite structure of studied compositions are obtained and mapped onto the ternary diagram. The calculated data for Ni-Mn-Ga are in a good agreement with available experimental ones.

Effect of deprotonation on the magnetic exchange coupling constant of fluorene‐based verdazyl diradical: A computational study

AbstractThe magnetic behavior of the fluorene bridged verdazyl diradicals has been studied theoretically in their neutral and deprotonated states. The deprotonation of C9‐H site of the fluorene ring opens a new coupling pathway, which changes the nature of magnetic coupling. The transmission spectra analysis reveals that the transmission through fluorine has also increased due to the opening of a new coupling path after deprotonation.

Magnetic properties of Fe100−xGax: Ab initio and Monte Carlo study

This work presents a theoretical study of magnetic properties for Fe100-xGax (x = 25, 28.125%) alloys in the framework of first-principles calculations and Monte Carlo simulations. In particular, the zero-temperature ground state properties, magnetocrystalline energy, and magnetic exchange coupling constants for both D03 and L12 structures are calculated. It is shown that for both structures, the magnetocrystalline anisotropy energy change the sign from positive to negative during tetragonal distortion from c/a<1 to c/a>1, respectively. Using the exchange interaction constants within Heisenberg model, the thermomagnetization curves and Curie temperatures for compositions studied are obtained. Calculated data are in a good agreement with experimental ones.

Structural, magnetic and thermodynamic properties of Mn3-X-C (X = Ga, Sn) compounds: ab initio study

In this work, the magnetic, structural and thermodynamic properties in a series of Mn3-X-C (X = Ga, Sn) have been studied by means of first-principles based on the density-functional theory and quasi-harmonic approximations based on the Debye model. To evaluate the magnetic effects, first-principles calculations have been carried out for different magnetic spin configurations including ferro- and ferrimagnetic configurations as well as different non-colinear ones. It has been shown that for both compounds the non-colinear spin orderings with a lower magnetic moment are favorable configurations at zero-temperature as compared with the ferromagnetic one. The magnetic exchange coupling constants showing an oscillation behavior becomes weaker along the sequence of Mn3-Ga-C →Mn3-Sn-C. Using the thermodynamic approach, the magnetic phase transition between ferromagnetic and non-colinear states in Mn3-Ga-C is found. While for Mn3-Sn-C, this transition does not occur in a studied temperature range. The pressure effect on the thermodynamic properties has been estimated. The calculated results are in a good agreement with available experimental data.

Characterization of the [18.28]0--a3Δ1 (0,0) band of tantalum nitride, TaN.

The [18.28]0--a3Δ1 (0,0) band near 647 nm of tantalum nitride, TaN, has been recorded and analyzed field-free and in the presence of static electric and magnetic fields. The fine and hyperfine parameters for the a3Δ1 and [18.28]0- states are determined. The 181Ta(I = 7/2) axial nuclear electric quadrupole coupling constant, eQq0, and the effective magnetic hyperfine coupling constant, h1, are used to garner insight into the nature of electronic states. The Stark tuning of the P(2) and Q(1) lines was analyzed to determine the molecular frame permanent electric dipole moment, μ→el, for the a3Δ1 state of 4.49 ± 0.09 D. The Zeeman effect on the P(1) line was analyzed to determine an upper limit of the magnetic moment, μ→m, for the a3Δ1 state. Scalar-relativistic coupled-cluster calculations for eQq0 and μ→el of the a3Δ1 state are also reported and compared with experimental results. The determined properties are compared with the predicted values and the implication of the proposed scheme for determination of the 181Ta magnetic quadrupole moment is discussed.

Magnetic coupling constants for MnO as calculated using hybrid density functional theory

The properties of MnO have been calculated using generalised gradient approximation (GGA-) and hybrid (h-) density functional theory (DFT), specifically variants of the popular PBE and PBESol exchange–correlation functionals. The GGA approaches are shown to be poor at reproducing experimental magnetic coupling constants and rhombohedral structural distortions, with the PBESol functional performing worse than PBE. In contrast, h-DFT results are in reasonable agreement with experiment. Calculation of the Néel temperatures using the mean-field approximation gives overestimates relative to experiment, but the discrepancies are as low as 15 K for the PBE0 approach and, generally, the h-DFT results are significant improvements over previous theoretical studies. For the Curie–Weiss temperature, larger disparities are observed between the theoretical results and previous experimental results.

Liquid and gas phase NMR spectra of 13CH313CHO acetaldehyde

The gas phase NMR experiments perform a vital role in establishing the magnetic shielding and spin-spin coupling constants which are free from intermolecular interactions, equivalent to the parameter of isolated molecules. This work is concerned with an acetaldehyde molecule. Small amounts of acetaldehyde 13CH313CHO in gaseous matrices of CO2 and Xe were studied using high-precision 1H and 13C NMR measurements. Results were extrapolated to the zero-density limit permitting the determinations of the 1H and 13C absolute nuclear magnetic shielding of an isolated acetaldehyde molecule. The difference between the experimental and recent theoretical DFT results is discussed.Several samples of 13CH313CHO dissolved in popular organic and inorganic solvents were also investigated. Gas-to-solution shifts show the influence of the association process when acetaldehyde is transferred from gas to liquid state. Several spin-spin coupling constants in the gas phase and in different solvents were precisely measured.

First-principles calculations on the origin of ferromagnetism in transition-metal doped Ge

Many researchers have shown an interest in Ge-based dilute magnetic semiconductors (DMSs) due to potential advantages for semiconductor spintronics applications. There has been great discussion about mechanisms of experimentally observed ferromagnetism in (Ge,Fe) and (Ge,Mn). We investigate the electronic structures, structural stabilities, magnetic exchange coupling constants, and Curie temperature of Ge-based DMSs, and clarify origins of the ferromagnetism, on the basis of density functional theory calculations. In both the (Ge,Fe) and (Ge,Mn) cases, the inhomogeneous distribution of the magnetic impurities plays an important role to determine the magnetic states; however, physical mechanisms of the ferromagnetism in these two materials are completely different. By the spinodal nanodecomposition, the Fe impurities in Ge gather together with keeping the diamond structure, so that the number of the first-nearest-neighbor Fe pairs with strong ferromagnetic interaction increases. Therefore, the Curie temperature drastically increases with the progress of the annealing. Our cluster expansion method clearly reveals that the other ordered compounds with different crystal structures such as ${\mathrm{Ge}}_{3}{\mathrm{Mn}}_{5}$ and ${\mathrm{Ge}}_{8}{\mathrm{Mn}}_{11}$ are easily generated in the (Ge,Mn) system. The estimated Curie temperature of ${\mathrm{Ge}}_{3}{\mathrm{Mn}}_{5}$ is in agreement with the observed Curie temperature in experiments. It should be considered that the precipitation of the ferromagnetic ${\mathrm{Ge}}_{3}{\mathrm{Mn}}_{5}$ clusters is an origin of high Curie temperature in (Ge,Mn).

Protonation-Enhanced Antiferromagnetic Couplings in Azobenzene-Bridged Diradicals

Proton-induced magnetic enhancement in an organic diradical is an appealing phenomenon. Here, taking two nitroxide groups as spin sources, we predict the magnetic properties of the trans and cis forms of azobenzene (AB)-bridged diradicals in which the central −N═N– unit can undergo single protonation to convert to its protonated counterpart or vice versa. The calculated results for these two pairs of diradicals (protonated versus unprotonated trans and cis forms) indicate that the signs of their magnetic coupling constants J do not change, but the magnitudes remarkably increase after protonation from −716.4 to −1787.1 cm–1 for the trans form and from −388.1 to −1227.9 cm–1 for the cis form, respectively. Such noticeable magnetic enhancements induced by protonation are mainly attributed to the strong mediating role of the coupler AB between two radical groups through its lowest unoccupied molecular orbital (LUMO) with a lower energy level after protonation. The planar structure for the protonated trans dir...

Dynamics, magnetic properties, and electron binding energies of H2O2 in water.

Results for the magnetic properties and electron binding energies of H2O2 in liquid water are presented. The adopted methodology relies on the combination of Born-Oppenheimer molecular dynamics and electronic structure calculations. The Keal-Tozer functional was applied for predicting magnetic shieldings and H2O2 intramolecular spin-spin coupling constants. Electron binding energies were calculated with electron propagator theory. In water, H2O2 is a better proton donor than proton acceptor, and the present results indicate that this feature is important for understanding magnetic properties in solution. In comparison with the gas-phase, H2O2 atoms are deshielded in water. For oxygen atoms, the deshielding is mainly determined by structural/conformational changes. Hydrogen-bond interactions explain the deshielding of protons in water. The predicted chemical shift for the H2O2 protons in water (δ∼11.8 ppm) is in good agreement with experimental information (δ=11.2 ppm). The two lowest electron binding energies of H2O2 in water (10.7±0.5 and 11.2±0.5 eV) are in reasonable agreement with experiment. In keeping with data from photoelectron spectroscopy, an ∼1.6 eV red-shift of the two first ionisation energies relative to the gas-phase is observed in water. The strong dependence of magnetic properties on changes of the electronic density in the nuclei environment is illustrated by a correlation between the σ(17O) magnetic shielding constant and the energy gap between the [2a] lowest valence and [1a] core orbitals of H2O2.

Synthesis, crystal structures and magnetic properties of cyanide-bridged heterobimetallic trinuclear Cr 2 III MnII complexes based on the cis-dicyanidemetalate [Cr(2,2′-bipy)2(CN)2]ClO4 building block

A structurally characterized cis-dicyanidemetalate [Cr(2,2′-bipy)2(CN)2]ClO4 (1) building block and two mononuclear seven-coordinate macrocyclic manganese(II) complexes have been employed to assemble two cyanide-bridged heterometallic CrIII–MnIII complexes: {[Mn(L1)][Cr(2,2′-bipy)2(CN)2]2}{ClO4}4·H2O (2) and {[Mn(L2)][Cr(2,2′-bipy)2(CN)2]2}{ClO4}4·2H2O (3) (L1 = 2,13-dimethyl-3,6,9,12,18-pentaazabicyclo[12.3.1]octadeca-1(18),2,12,14,16-pentaene, L2 = 2,13-dimethyl-6,9-dioxa-3,12,18-triazabicyclo[12.3.1]octadeca-1(18),2,12,14,16-pentaene). Single-crystal X-ray diffraction analysis shows that 2 and 3 have similar cyanide-bridged cationic trinuclear Cr2Mn structures with free ClO4 − as balancing anions, in which the cyanide precursor acts as a monodentate ligand to connect the macrocyclic manganese(II) units. The coordination geometry of the manganese(II) centers in the two complexes is slightly distorted pentagonal-bipyramidal, with two cyanide nitrogen atoms at the trans-positions and N5 or N3O2 donor sets in the equatorial plane provided by the macrocyclic ligand. Investigation of the magnetic properties reveals antiferromagnetic magnetic coupling between the cyanide-bridged Cr(III) and Mn(II) centers. A best-fit analysis of the magnetic susceptibility leads to the magnetic coupling constants J = −1.15 and −1.08 cm−1 for complexes 2 and 3, respectively.

Dynamic Magnetic Hysteresis Behaviors in a Mixed Spin (3/2, 2) Bilayer System with Different Crystal-Field Interactions

Dynamic magnetic hysteresis (DMH) behaviors of the mixed spin-3/2 and spin-2 Ising bilayer system with different crystal-field interactions on a two-layer square lattice is studied by the use of dynamic mean field calculations based on the Glauber-type stochastic. The hysteresis loops are obtained for different reduced temperatures (T), magnetic field amplitudes (h), frequencies (w) and interlayer coupling constants (J3). Influences of the T, h, w and J3 on the DMH properties are investigated. We also study the temperature, frequency and interlayer coupling interaction dependence of the coercive field and remanent magnetization. We compare our results with some theoretical and experimental works and observe a quantitatively good agreement with some theoretical and experimental results.

Evaluation of the Magnetic Interactions in Salts Containing [Ni(dmit)2]− Radical Anions

This paper reports on a theoretical analysis of the electronic structure and magnetic properties of the [Ni(dmit)2]− radical anions in four salt compounds resulting from the combination with supramolecular cation–crown ether assemblies. The selected compounds are representative examples of the diversity of stacking patterns and magnetic behaviors found for systems based on these [Ni(dmit)2]− anions. Difference Dedicated Configuration Interaction calculations have been performed on fragments containing two neighbor [Ni(dmit)2]− units to evaluate the intradimer, interdimer, and interchain magnetic coupling constants and analyze the electronic structure of the [Ni(dmit)2]− ions. The amplitude and sign of these through-space interactions are consistent with the structure and arrangement of the [Ni(dmit)2] monomers in the chain and the overlap and transfer integral between the magnetic orbitals. They accurately simulate the thermal dependence of the magnetic susceptibility but do not agree with the reported J ...

On the performance of generalized valence bond theory in predicting magnetic exchange coupling constant in organic diradicals: A comparison with Hartree-Fock theory

We have investigated four different already synthesized meta- and para-connected diradicals to get their magnetic exchange coupling constants (J) through generalized valence bond (GVB) approach and also with Hartree-Fock (HF) theory with a view to establish the superiority of GVB method over HF theory for the prediction of J. In doing so, at first optimization of these molecules are done at B3LYP level of density functional theory using the 6-31++g(d) basis set. With these optimized geometries, J values are computed at GVB method along with TZV basis set and these values are compared with those of the J values obtained using HF/TZV level of theory. Nonetheless, J values found in both the theoretical processes are also compared with the experimental findings. Thus, the supremacy of the wavefunction based GVB method in predicting J is established in comparison with the HF method.

Three 1D cyanide-bridged M(Ni, Pd, Pt)-Mn(II) coordination polymer: synthesis, crystal structure and magnetic properties

Three tetracyanide-containing building blocks K2[M(CN)4] (M = Ni, Pd, Pt) and one semi-closed macrocycle seven-coordinated manganese(II) compound have been employed to assemble cyanide-bridged heterometallic complexes, resulting in three cyanide-bridged MII-MnII complexes: [Mn(L)][Ni(CN)4] · 2H2O (1) [Mn(L)][Pd(CN)4] (2) and [Mn(L)][Pt(CN)4] (3) (L = 2,6-bis[1-(2-(N-methylamino)ethylimino)ethyl]pyridine). Single-crystal X-ray diffraction analysis shows their similar one-dimensional structure consisting of the alternating [Mn(L)]2+ species and [M(CN)4]2- building blocks, generating a cyanide-bridged neutral polymeric chain. In all three isostructural complexes the coordination geometry of manganese ion is a slightly distorted pentagonal-bipyramidal with the two cyanide nitrogen atoms at the trans positions and N5 coordinating mode at the equatorial plane from ligand L. Investigation over magnetic properties of these complexes reveals very weak antiferromagnetic interaction between neighboring Mn(II) ions bridged by the long NC-M-CN unit. A best-fit to the magnetic susceptibility of complexes 1-3 leads to the magnetic coupling constant of J = -0.081, -0.103 and -0.14 cm-1, respectively.

Ferromagnetic model on the Apollonian packing.

This work investigates the influence of geometrical features of the Apollonian packing (AP) on the behavior of magnetic models. The proposed model differs from previous investigations on the Apollonian network (AN), where the magnetic coupling constants depend only on the properties of the network structure defined by the packing, but not on quantitative aspects of its geometry. In opposition to the exact scale invariance observed in the AN, the circle's sizes in the AP are scaled by different factors when one goes from one generation to the next, requiring a different approach for the evaluation of the model's properties. If the nearest-neighbors coupling constants are defined by J_{i,j}∼1/(r_{i}+r_{j})^{α}, where r_{i} indicates the radius of the circle i containing the node i, the results for the correlation length ξ indicate that the model's behavior depend on α. In the thermodynamic limit, the uniform model (α=0) is characterized by ξ→∞ for all T>0. Our results indicate that, on increasing α, the system changes to an uncorrelated pattern, with finite ξ at all T>0, at a value α_{c}≃0.743. For any fixed value of α, no finite temperature singularity in the specific heat is observed, indicating that changes in the magnetic ordering occur only when α is changed. This is corroborated by the results for the magnetization and magnetic susceptibility.

Communication: Viewing the ground and excited electronic structures of platinum and its ion through the window of relativistic coupled cluster method.

Highly accurate electronic structure calculations are often needed to supplement scant experimental data. We report the ground 3D3 and some selected low lying excited/ionized states of Pt and its ions obtained using the Fock space multireference coupled cluster method with four-component relativistic spinors. The present work establishes the stability of the 2S1/2 state of its negative ion and reproduces the binding energy of this state within 10 cm-1. The first ionization potential (cm-1) is estimated to be 72 005, deviating from the experiment by just 200 (0.3%). We also report the magnetic hyperfine coupling constants (A) of Pt and its ions. The present calculation provides the A value (GHz) of the 3D3 state of Pt to be 5.78 exhibiting very good agreement with the experimental data of 5.70. To our knowledge, this is the first relativistic ab initio calculation of the ionization potential and magnetic hyperfine coupling constant for the neutral and ionic states of Pt at a high level of correlation treatment.

Ferromagnetism of undoped anatase TiO2 based on the first-principles calculations

Compared with conventional semiconductors, the diluted magnetic semiconductors, in which the cations are substituted by transition metal ions, have attracted a great deal of attention due to their promising applications in spintronics. Recently, the unexpected room temperature ferromagnetism has been found in many undoped oxides. These findings challenge our understanding of magnetism in these systems, because neither cations nor anions have unpaired d or f electrons. Generally, the candidate defects responsible for the unexpected ferromagnetism must fulfill two conditions at the same time: (i) the defects should prefer a spin-polarized ground state with a nonzero local magnetic moments; (ii) the exchange interactions between local magnetic moments induced by defects should be ferromagnetic energetically. Among these oxides, TiO2 has recently attracted much attention because of its unique properties and potential applications in spintronics, laser diodes and biomaterials. In order to explore the origin of ferromagnetism in such an undoped TiO2 system, the electronic structures and magnetic properties of oxygen vacancy (VO) and Ti vacancy (VTi) in anatase TiO2 have been studied systematically by the first-principles calculation based on the density functional theory with the LDA+U method (UTi-3d = 5.8 eV). It is found that two electrons introduced by VO are captured by two neighbor Ti4+ ions, and thereby the Ti4+ ions are restored to Ti3+ ions with opposite spin orientation. Therefore, the single VO cannot induce local magnetic moment. The defect energy level locates near the Fermi level for VTi. Six oxygen atoms neighboring VTi constitute an octahedron, and the defect energy level is split into a single state A, a double state E and a triple state T in the octahedral crystal field. The occupation of four unpaired electrons introduced by six oxygen atoms is a+1t+3t-0e0 (subscripts + and - mean up-spin and down-spin, respectively), and the VTi can induce 4 B local moments. Furthermore, the magnetic coupling interaction between local magnetic moments induced by two VTi is ferromagnetic, and the magnetic coupling constant (JO) is 88.7 meV. It means the ferromagnetism can continue up to room-temperature. The VO cannot induce local magnetic moment, but it can enhance the coupling strength between two VTi, which can explain the origin of ferromagnetism observed experimentally in undoped anatase TiO2, i.e., the VTi induces local magnetic moment, while VO enhances the long range ferromagnetic coupling interaction between VTi. Especially, for the ferromagnetic coupling between local magnetic moments, we have proposed the second type direct exchange interaction model, which has been recommended in detail.

Determination of the hyperfine coupling constants of the 5D5/2 state of 85Rb atoms by using high signal-to-noise ratio electromagnetically-induced transparency spectra

We report the hyperfine splitting measurement of the 85Rb 5D5/2 state by electromagnetically induced transparency spectroscopy with high signal-to-noise ratio in the 85Rb 5S1/2-5P3/2-5D5/2 ladder-type system (m 780 nm + 776 nm). The frequency calibration is performed by employing a phase-type electro-optic modulator with a confocal Fabry-Perot cavity. From the measured hyperfine splittings among the manifolds of (F=5), (F=4) and (F=3) of the 85Rb 5D5/2 state, we determine the magnetic dipole hyperfine coupling constant (A= (-2.222 0.019) MHz) and the quadrupole coupling constant (B= (2.664 0.130) MHz) of 5D5/2 state of 85Rb atoms.