Search Results (13)

In this paper, we explore the magnetization dynamics in a long ferromagnetic nanostripe with finite width in the presence of antisymmetric Dzyaloshinskii–Moriya exchange interactions (DMIs). It is known that DMIs, which are currently of great interest because they give rise to chiral and nonreciprocal properties and influence surface topologies, can be enhanced by interfacing the nanostripe with a heavy metal. Our theoretical approach employs a microscopic (or Hamiltonian-based) analysis that includes symmetric bilinear exchange, antisymmetric DMI, long-range dipole–dipole interactions, and Zeeman energy due to an external magnetic field applied out of the plane of the nanostripe. In this geometry, we calculate the frequencies and amplitudes of the discrete spin-wave modes that have a standing-wave character across the finite width of the stripe and a propagating character (with wavenumber k) along the stripe length. The individual spin-wave modes display nonreciprocal propagation in their dispersion relations due to DMI. We also find that there may be localized edge spin waves with amplitudes that undergo spatial decay near the stripe edges. Full article

Recently, significant effort has been devoted to enhancing magnetic anisotropy energy (MAE) and the Dzyaloshinskii–Moriya interaction (DMI) in two-dimensional (2D) ferromagnetic materials through various tuning approaches. Among these methods, defect engineering is one of the most effective strategies. However, the influence of these charged defects on the MAE and DMI is unclear. Therefore, we systematically investigate the defect effect on the MAE and DMI of I vacancy-doped (v_I-CrI₃), 3d-transition-metal-doped (TM = Ti, V, Cr, Mn, Fe, Co, Ni, Cu, and Zn) (3d-TM_i@CrI₃), and v_I-TM co-doped (3d-TM_i@v_I-CrI₃) monolayer CrI₃ using first-principles calculations. Our results indicate that Cr-rich conditions can promote the defect formation of v_I-CrI₃, 3d-TM_i@CrI₃, and 3d-TM_i@v_I-CrI₃ systems and demonstrate that 49 types of charged systems are stable. Among these systems, the Cu_i@v_I-CrI₃ in the +1 charge state (Cu_i^•@v_I-CrI₃) system has a smaller defect formation energy, exhibiting a large MAE exceeding 30 meV, and the ratio (D/J) of the antisymmetric magnetic exchange parameter (D) to the Heisenberg exchange parameter (J) reaches 1.04. The large MAE originates from the transition from single-ion anisotropy (SIA) to covalent interaction anisotropy (CIA) due to the coupling variation between the p_y and p_x orbitals of I atoms near the Fermi level caused by charge states. The enhancement of the DMI is due to the electrostatic potential differences between the I-top and I-bottom layers, which are conducive to forming stable chiral spin textures. This study provides insight into the defect charge state modulating the magnetism of 2D magnetic materials. Full article

In this study, we investigate the magnetic properties of the molecular compound [Mn(mal)(H₂O)]ₙ (mal = dianion of malonic acid) by integrating microscopic and macroscopic characterization, combining unpolarized neutron diffraction and magnetometry measurements. Neutron diffraction, though non-commonly applied to molecular compounds, proved essential for fully resolving the magnetic structure, as well as overcoming challenges such as hydrogen-related incoherent scattering and difficulties in accurately locating light atoms. Our neutron data provided critical structural details, including the precise location of hydrogen atoms, especially those associated with crystallization water molecules. By conducting low-temperature measurements below the magnetic ordering temperature, we identified the correct Shubnikov space group (Pc’a2₁’) and established a magnetic model consistent with the observed weak ferromagnetism. Our findings reveal that the compound presents a spin-canted structure with a weak ferromagnetic signal along the b-axis. This signal originates primarily from antisymmetric exchange interactions rather than single-ion anisotropy, consistent with the isotropic nature of the Mn(II) (⁶A₁g) ground state. The combined neutron diffraction and magnetometry results provide a comprehensive understanding of how structural and symmetry factors influence the magnetic properties of malonate-based manganese compounds. Full article

Based on the proposed microscopic model, we investigate the multiferroic characteristics of VOX₂ (X = Cl, Br, I) monolayers using a Green’s function method. The dependence of the microscopic parameters of the ferroelectric system (pseudo-spin arrangement and flipping rate) on the magnitude and sign of the exchange magnetic interaction along the b-axis and the value of the Dzyaloshinskii–Moria vector have been investigated and qualitatively explained. The possibility of observing a spin-reorientation transition with a change in the character of spin ordering from antiferromagnetic to ferromagnetic is investigated. It is found that the antisymmetric magnetoelectric interaction may be responsible for the spin-reorientation transition without a change in the ordering of magnetic moments. Changing the sign of the exchange magnetic interaction along the b-axis leads to ferromagnetic ordering without observing a spin-reorientation transition. The dependence of isotropic and antisymmetric magnetic interactions on the microscopic parameters of the ferroelectric system is qualitatively explained. A mechanism for the occurrence of the spin-reorientation transition is presented based on the proposed microscopic model. The obtained results qualitatively coincide with Density Functional Theory calculations. Full article

A theory is presented to study the effect of interfacial Dzyaloshinskii–Moriya interactions (DMIs) on the static and dynamic magnetic properties in single-layered ferromagnetic nanorings. A microscopic (Hamiltonian-based) approach is used that also includes the antisymmetric DMI besides the competing symmetric (bilinear) exchange interactions, magnetic dipole–dipole interactions, and an applied magnetic field. Here, the axial vector of the DMI is taken to be in the plane of the nanoring (by contrast with earlier studies) and we explore cases where it is either parallel or perpendicular to the in-plane magnetic field. Significantly, with this orientation for the DMI axial vector, the inhomogeneous static magnetization is tilted to have a component perpendicular to the plane giving a surface texture. This effect is studied in both the low-field vortex and high-field onion states. There is a consequent modification to the discrete set of spin-wave modes in both states through their frequencies and spatial amplitudes. We present combined analytical and numerical results for the static properties and dynamical magnetization in ferromagnetic nanorings, including the variation with applied field. Full article

The synthesis and structural characterization of a new triangular Cu₃–μ₃OH pyrazolato complex of formula, [Cu₃(μ₃−OH)(pz)₃(Hpz)₃][BF₄]₂ (1−Cu₃), Hpz = pyrazole, is presented. The triangular unit forms a quasi-isosceles triangle with Cu–Cu distances of 3.3739(9), 3.3571(9), and 3.370(1) Å. This complex is isostructural to the hexanuclear complex [Cu₃(μ₃−OH)(pz)₃(Hpz)₃](ClO₄)₂]₂ (QOPJIP). A comparative structural analysis with other reported triangular Cu₃–μ₃OH pyrazolato complexes has been carried out, showing that, depending on the pyrazolato derivative, an auxiliary ligand or counter-anion can affect the nuclearity and/or the dimensionality of the system. The magnetic properties of 1−Cu₃ are analyzed using experimental data and DFT calculation. A detailed analysis was performed on the magnetic properties, comparing experimental and theoretical data of other molecular triangular Cu₃–μ₃OH complexes, showing that the displacement of the μ₃−OH⁻ from the Cu₃ plane, together with the type of organic ligands, influences the nature of the magnetic exchange interaction between the spin-carrier centers, since it affects the overlap of the magnetic orbitals involved in the exchange pathways. Finally, a detailed comparison of the magnetic properties of 1−Cu₃ and QOPJIP was carried out, which allowed us to understand the differences in their magnetic properties. Full article

The dynamic electrical conductivity of dense Zr plasma near melting is calculated using ab initio molecular dynamics and the Kubo–Greenwood formula. The antisymmetrization of the electronic wave function is considered with the determinant of one-electron wave functions; exchange and correlation effects are treated via an exchange–correlation functional. Optical properties are restored using the Kramers–Kronig transformation. The influence of computational parameters and inner shell electrons on the results is thoroughly investigated. We demonstrate the convergence of our computations and analyze comparison with experimental data. Full article

Ni-H-Beta catalysts for ethylene oligomerization (EO) were prepared by ion exchange of NH₄-Beta and H-Beta zeolites with aqueous Ni(NO₃)₂ and characterized by H₂-temperature-programmed reduction (TPR), NH₃-temperature-programmed desorption (TPD), and diffuse-reflectance infrared Fourier-transform spectroscopy (DRIFTS). Quadruple exchange of NH₄-Beta at 70 °C resulted in 2.5 wt.% Ni loading corresponding to a Ni²⁺/framework aluminum (FAl) molar ratio of 0.52. [NiOH]⁺ and H⁺ are the primary charge-compensating cations in the uncalcined catalyst, as evidenced by TPR and DRIFTS. Subsequent calcination at 550 °C in air yielded a Ni-H-Beta catalyst containing primarily bare Ni²⁺ ions bonded to framework oxygens. Quadruple exchange of H-Beta at 70 °C gave 2.0 wt.% Ni loading (Ni²⁺/FAl = 0.41). After calcination at 550 °C, the resulting Ni-H-Beta catalyst comprises a mixture of bare Ni²⁺ ions: [NiOH]⁺ and NiO species. The relative abundance of [NiOH]⁺ increases with the number of exchanges. In situ pretreatment at 500 °C in flowing He converted the [NiOH]⁺ species to bare Ni²⁺ ions via dehydration. The bare Ni²⁺ ions interact strongly with the Beta framework as evidenced by a perturbed antisymmetric T-O-T vibration at 945 cm⁻¹. DRIFT spectra of CO adsorbed at 20 °C indicate that the Ni²⁺ ions occupy two distinct exchange positions. The results of EO testing at 225 °C and 11 bar (ethylene) suggested that the specific Ni²⁺ species initially presented (e.g., bare Ni²⁺, [NiOH]⁺) did not significantly affect the catalytic performance. Full article

Symmetries in the Physical Laws of Nature lead to observable effects. Beyond the regularities and conserved magnitudes, the last few decades in particle physics have seen the identification of symmetries, and their well-defined breaking, as the guiding principle for the elementary constituents of matter and their interactions. Flavour SU(3) symmetry of hadrons led to the Quark Model and the antisymmetric requirement under exchange of identical fermions led to the colour degree of freedom. Colour became the generating charge for flavour-independent strong interactions of quarks and gluons in the exact colour SU(3) local gauge symmetry. Parity Violation in weak interactions led us to consider the chiral fields of fermions as the objects with definite transformation properties under the weak isospin SU(2) gauge group of the Unifying Electro-Weak SU(2) × U(1) symmetry, which predicted novel weak neutral current interactions. CP-Violation led to three families of quarks opening the field of Flavour Physics. Time-reversal violation has recently been observed with entangled neutral mesons, compatible with CPT-invariance. The cancellation of gauge anomalies, which would invalidate the gauge symmetry of the quantum field theory, led to Quark–Lepton Symmetry. Neutrinos were postulated in order to save the conservation laws of energy and angular momentum in nuclear beta decay. After the ups and downs of their mass, neutrino oscillations were discovered in 1998, opening a new era about their origin of mass, mixing, discrete symmetries and the possibility of global lepton-number violation through Majorana mass terms and Leptogenesis as the source of the matter–antimatter asymmetry in the universe. The experimental discovery of quarks and leptons and the mediators of their interactions, with physical observables in spectacular agreement with this Standard Theory, is the triumph of Symmetries. The gauge symmetry is exact only when the particles are massless. One needs a subtle breaking of the symmetry, providing the origin of mass without affecting the excellent description of the interactions. This is the Brout–Englert–Higgs Mechanism, which produces the Higgs Boson as a remnant, discovered at CERN in 2012. Open present problems are addressed with by searching the New Physics Beyond-the-Standard-Model. Full article

Two anionic complexes, {[Cu₃(µ₃-OH)(µ-4-Ph-pz)₃Cl₃]₂[Cu(4-Ph-pzH)₄](µ-Cl)₂}²⁻ (1) and [Cu₃(µ₃-OH)(µ-pz)₃(µ_1,1-N₃)₂(N₃)]⁻ (2), crystallize as one-dimensional polymers, held together by weak Cu-(µ-Cl) and Cu-(µ-N₃) interactions, respectively. Variable temperature magnetic susceptibility analyses determined the dominant antiferromagnetic intra-Cu₃ exchange parameters in the solid state for both complexes, whereas the weak ferromagnetic inter-Cu₃ interactions manifested also in the solid state EPR spectra, are absent in the corresponding frozen solution spectra. DFT calculations were employed to support the results of the magnetic susceptibility analyses. Full article

We present an overview of the microscopic theory of the Dzyaloshinskii–Moriya (DM) coupling in strongly correlated 3d compounds. Most attention in the paper centers around the derivation of the Dzyaloshinskii vector, its value, orientation, and sense (sign) under different types of the (super)exchange interaction and crystal field. We consider both the Moriya mechanism of the antisymmetric interaction and novel contributions, in particular, that of spin–orbital coupling on the intermediate ligand ions. We have predicted a novel magnetic phenomenon, weak ferrimagnetism in mixed weak ferromagnets with competing signs of Dzyaloshinskii vectors. We revisit a problem of the DM coupling for a single bond in cuprates specifying the local spin–orbital contributions to the Dzyaloshinskii vector focusing on the oxygen term. We predict a novel puzzling effect of the on-site staggered spin polarization to be a result of the on-site spin–orbital coupling and the cation-ligand spin density transfer. The intermediate ligand nuclear magnetic resonance (NMR) measurements are shown to be an effective tool to inspect the effects of the DM coupling in an external magnetic field. We predict the effect of a strong oxygen-weak antiferromagnetism in edge-shared CuO ${}_2$ chains due to uncompensated oxygen Dzyaloshinskii vectors. We revisit the effects of symmetric spin anisotropy directly induced by the DM coupling. A critical analysis will be given of different approaches to exchange-relativistic coupling based on the cluster and the DFT (density functional theory) based calculations. Theoretical results are applied to different classes of 3d compounds from conventional weak ferromagnets ( $\alpha$ -Fe ${}_2$ O ${}_3$ , FeBO ${}_3$ , FeF ${}_3$ , RFeO ${}_3$ , RCrO ${}_3$ , …) to unconventional systems such as weak ferrimagnets (e.g., RFe ${}_{1-x}$ Cr ${}_x$ O ${}_3$ ), helimagnets (e.g., CsCuCl ${}_3$ ), and parent cuprates (La ${}_2$ CuO ${}_4$ , …). Full article

Two new dicarboxylate-based three-dimensional cobalt coordination polymers, [Co(Me₂mal)(bpe)_0.5(H₂O)]_n (1) and [Co(Me₂mal)(bpe)_0.5]_n (2), were synthesized from dimethylmalonic acid (H₂-Me₂mal) in temperature-controlled solvothermal reactions. Lower temperatures (60–80 °C) favored the formation of 1, while higher temperatures (120 °C) favored the production of 2. Compound 1 is comprised of Co(II) corrugated layers linked by syn–anti carboxylate bridges from the Me₂mal²⁻ ligands and pillared through bis-monodentate bpe groups. Compound 2 is comprised of a three-dimensional network involving one-dimensional Co–carboxylate chains bonded by antisymmetric µ₄-Me₂mal²⁻ ligands and aligned parallel to the [001] direction. The solvothermal retreatment of crystalline samples of 1 in a DMF/H₂O solvent at 120 °C allowed the structural reassembly, with complete conversion within 2 over 48 h. Magnetic analyses revealed that compound 1 exhibits both spin-orbital coupling and antiferromagnetic interactions through a syn–anti carboxylate (Me₂mal²⁻) bridge exchange pathway [Co–Co separation of 5.478 Å] and compound 2 showed a ferromagnetic interaction resulting from the short Co–Co separation (3.150 Å) and the small Co–O–Co bridging angles (98.5° and 95.3°) exchange pathway which was provided by µ₄-Me₂mal²⁻ bridging ligand. Full article

This paper analyzes the structural, energetic and mechanical properties of carbon dioxide hydrate clathrates calculated using finite cluster and periodic ab initio density-functional theory methodologies. Intermolecular interactions are described by the exchange-hole dipole moment method. The stability, gas saturation energetics, guest–host interactions, cage deformations, vibrational frequencies, and equation of state parameters for the low-pressure sI cubic phase of the CO₂@H₂O clathrate hydrate are presented. Our results reveal that: (i) the gas saturation process energetically favors complete filling; (ii) carbon dioxide molecules prefer to occupy the larger of the two cages in the sI structure; (iii) blue shifts occur in both the symmetric and antisymmetric stretching frequencies of CO₂ upon encapsulation; and (iv) free rotation of guest molecules is restricted to a plane parallel to the hexagonal faces of the large cages. In addition, we calculate the librational frequency of the hindered rotation of the guest molecule in the plane perpendicular to the hexagonal faces. Our calculated spectroscopic data can be used as signatures for the detection of clathrate hydrates in planetary environments. Full article