On the cause for the no spin-gap behavior of the triangular spin tube system CsCrF4

The spin exchange interactions of the triangular spin tube system CsCrF₄ were evaluated by performing mapping analysis on the basis of density functional theory calculations. Our results show that the exchange J₁ for the triangular ring and the exchange J₂ along the chain are both antiferromagnetic with the ratio J₁/J₂≈0.5, so the spin gap of CsCrF₄ is too small to be experimentally detected. This finding is consistent with the experimental observation and theoretical analysis. The possible ways of preparing a triangular spin tube with observable spin gap was discussed.     

High-frequency tunable EPR spectroscopy of Cr3+ in synthetic forsterite

The electron paramagnetic resonance spectra of isolated and dimer impurity centers of trivalent chromium ions in the octahedral Ml sites in synthetic forsterite are studied in the frequency range of 65–90 GHz. The measurements are performed at 4.2 K in magnetic field from ?0.04 to 0.3 T. The zero-field splitting between spin doublets of the isolated Cr³⁺ ion Δ_s = 66.7 GHz and between spin sublevels of the Cr³⁺-Cr³⁺ dimer Δ_d1 = 71.5 GHz and Δ_d2 = 73.0 GHz is measured directly at zero field. The analysis of the spin Hamiltonian parameters shows that the dimer center consists of a pair of Cr³⁺ ions with an Mg²⁺ vacancy between them replacing three Mg²⁺ ions situated in a quasi-one-dimensional chain aligned parallel to the crystal c-axis. It is found that the exchange interaction in the dimer is ferromagnetic with parameters J_z = 0.47 GHz and J_t = 0.79 GHz.     

Electronic structure and magnetic coupling in CaV<Subscript>2</Subscript>O<Subscript>5</Subscript>: spin dimer versus spin ladder

The electronic structure and magnetic exchange interactions of the ladder vanadate CaV₂O₅ have been studied by ab initio electronic structure calculations based on density functional theory (DFT). Geometry optimization and electronic structure calculations are performed using spin-polarized generalized gradient approximation (GGA) exchange-correlation functionals for four possible spin-ordered states. The experimentally observed insulating behavior has been reproduced successfully in the framework of the band theory by considering the magnetic ordering. Calculated results reveal that the true magnetic ground state of CaV₂O₅ is the antiferromagnetic (AFM) state with AFM exchange interactions both inside the rungs and along the ladder legs. Calculated exchange parameters indicate that the ladder structural vanadate CaV₂O₅ should be described as weakly coupled dimer system rather than as spin ladder compound. The AFM interactions inside the dimer are crucial to the insulating ground state and magnetic characteristics of CaV₂O₅.     

阻挫三角反铁磁AgCrO2螺旋自旋序的第一性原理研究

基于密度泛函理论的广义梯度近似(GGA)和投影缀加波(PAW)方法,分别从共线和非共线磁性结构出发,研究了自旋阻挫三角反铁磁AgCrO₂的基态、磁性以及电子结构,从理论计算的角度给出了基态磁性结构.计算结果表明:AgCrO₂具有120°螺旋自旋序反铁磁基态,其自旋螺旋面平行于(110)面或(11^-0)面;由于Cr离子间的自旋几何阻挫,导致沿晶体的a,b和a+b方向上均形成了螺旋自旋转动角为120°的 关键词： 第一性原理 交换相互作用 阻挫 反铁磁     

Cooperative order and excitation spectra in the bicomponent spin networks

A ferrimagnetic spin model composed of S = 1/2 spin-dimers and S = 5/2 spin-chains is studied by combining the bond-operator representation (for S = 1/2 spin-dimers) and Holstein-Primakoff transformation (for S = 5/2 spins). A finite interaction J _DF between the spin-dimer and the spin chain makes the spin chains ordered antiferromagnetically and the spin dimers polarized. The effective interaction between the spin chains, mediated by the spin dimers, is calculated up to the third order. The staggered magnetization in the spin dimer is shown proportional to J _DF. Due to the triplon-magnon interaction, the degeneracy of the triplons is lifted and the hybridized triplon-like excitations show different behaviors near the vanishing J _DF. A mode with longitudinal polarization is identified. The hybridized magnon-like excitations are also studied. These results are compared with the experiments on Cu₂Fe₂Ge₄O₁₃.     

On the long-range magnetic order and the preferred spin orientation of the layered magnetic oxides Sr2MnSi2O7 and Ba2MnGe2O7

The causes for the three-dimensional magnetic ordering and the observed spin orientation of the layered oxides Sr₂MnSi₂O₇ and Ba₂MnGe₂O₇ were investigated by evaluating the spin exchange interactions and the preferred spin orientation on the basis of density functional calculations and by calculating the magnetic dipole–dipole interaction energies.     

Exchange interaction at the supramolecular level. EPR investigation of two copper (II) compounds: [Cu2(acac)2(phen)2(bpe)](CIO4)2·(bpe)·CH3CN·H2O and [Cu2(acac)2(phen)2(bpp)](CIO4)2·6H2O (bpe=trans-1,2-bis(4-pyridyl)ethylene, bpp=bis(4-pyridyl)propane)

Intra- and intermolecular exchange and dipole-dipole interactions in two supramolecular compounds [Cu₂(acac)₂(phen)₂(bpe)](CIO₄)₂·(bpe)·CH₃CN·H₂O (I) and [Cu₂(acac)₂(phen)₂(bpp)]× (CIO₄)₂·6H₂O (II), which are built up of binuclear fragments through π-π stacking interactions, are investigated. The electron paramagnetic resonance (EPR) spectra of the polycrystalline samples of I and II were measured in the X-band in the temperature range of 300–4.2 K, and in the Q-band atT=300 and 4.2 K. The EPR spectra were interpreted as being due to weakly interacting dimer fragments. Triplet and singlet states of dimer fragments arise from a larger interactionJS ₁ S ₂ between two nearest copper complexes of two neighboring binuclear fragments. The theoretical analysis of the EPR spectrum of the polycrystalline sample for weakly interacting triplet states is carried out. The influence of the weak interaction between triplet states with value ofJ′ is considered in the model of the frequency exchange. A special attention is focused on the presence of the additional signal due to the exchange merging in some orientations where theJ′ value exceeds the fine structure parameters of the spectrum. The analysis of the conditions for the detection of the additional signal and of the influence of this signal on the form of the EPR spectrum allows us to estimate the value of the exchange interactionJ′=o.025±0.005 cm^?1 for compounds I and II and anisotropic part of exchange interaction between two nearest copper complexes asJ _zz =?0.02 cm^?1,J _xx.yy =0.01 cm^?1 for compound I.     

双层反铁磁体K3Cu2F7 中轨道序驱动的自旋二聚化

基于自旋-轨道-晶格Hamilton量,应用团簇自洽场方法,研究了双层钙钛矿结构材料K₃Cu₂F₇基态的晶格、磁及轨道结构,发现近孤立的双层的对称破缺和Jahn-Teller晶格畸变使得Cu²⁺离子在每层内交替占据 z²-x²〉/ z²-y²〉轨道,进而导致双层的层间表现为强的反铁磁耦合,层内为弱的铁磁耦合.强反铁磁耦合导致层间     

Spin frustration,charge ordering,and enhanced antiferromagnetism in TMTTF2SbF6

We theoretically investigate the effects of charge order and spin frustration on the spin ordering in TMTTF salts. Using first-principles band calculations, we find that a diagonal inter-chain transfer integral t_q1, which causes spin frustration between the inter-chain dimers in the dimer-Mott insulating state, strongly depends on the choice of anion. Within the numerical Lanczos exact diagonalization method, we show that the ferroelectric charge order changes the role of t_q1 from the spin frustration to the enhancement of the two-dimensionality in spin sector. The results indicate that t_q1 assists the cooperative behavior between charge order and antiferromagnetic state observed in TMTTF₂SbF₆.     

The exact ground state and the origin of the spin gap in the distorted mixed spin (1, 1/2) diamond chain

By using the method of exact diagonalization, we investigated the properties of the distorted mixed spin (1, 1/2) diamond chain with antiferromagnetic interactions along the rung and leg. The ground states of this model contain the sawtooth chain state and the rung dimer plus Haldane state. The research on the origin of the spin gap of the model discloses that there are three different types of spin excitations at different parameter regimes due to the competition among the interactions J₁, J₂ and J₃.     

EPR studies of weakly coupled oxomolybdenum (V) and low-spin iron (III) porphyrin centers

Novel compounds containing twoS=1/2 coupled spin centers (Mo(V) and low spin Fe(III) have been investigated in detail by X- and Q-band EPR spectroscopy, spectral simulation and molecular modelling calculations. For one system with a Mo?Fe distance of ≈9.4 Å the dominant dipolar coupling allows distinction among structures that are consistent with molecular modelling calculations. For the second system with a Mo?Fe of ≈ 7.9 Å the exchange interaction is dominant (0.5 <J < 3.0 GHz). These coupled systems are preliminary benchmarks for using EPR to investigate the Mo?Fe interaction in sulfite oxidase.     

Theoretical investigation of magnetic parameters in two-dimensional sheets of pure organic BEDT-TTF and BETS molecules by using ab initio MO and DFT methods

The effective exchange integrals (J _ab(M)) between two cation radicals of title compounds in the Heisenberg model were calculated by ab initio molecular orbital (MO) and density functional theory (DFT) methods, together with hybrid DFT methods. The J _ab(D) values between two dimer mono-cation radicals were also estimated assuming J _ab(D) = J _ab(M)/2. It is found that the spin lattice obtained by the ab initio method is square planar and linear in BEDT-TTF and BETS planes, respectively, although other previous calculations show that spin lattice in the BEDT-TTF plane is a triangular one. The J _ab and overlap integrals (s_ab ) values by the ab initio methods were used to determine transfer integral (t_ab ) and Coulomb repulsion (U_cff ) parameters of the Hubbard model, which were compared with those of the previous results. Implications of the calculated results are discussed in relation to the spin-mediated mechanism for superconductivity.     

Exchange constants for a V<Subscript>15</Subscript> magnetic molecular nanocluster

The spin structure and ground-state energies of a V₁₅ magnetic molecular nanocluster are calculated using a modified Lanczos method. The exchange interaction constants for the V₁₅ magnetic nanocluster (J=290 K, J′=60 K, J₁=30 K, J′′=200 K, and J₂=68 K) are determined from the comparison of the calculated and experimental data. The exchange constants obtained differ significantly from those predicated earlier and permit one to describe quantitatively the magnetization of the V₁₅ nanocluster in weak and strong magnetic fields.     

Strong short-range magnetic order in a frustrated FCC lattice and its possible role in the iron structural transformation

The magnetic properties of a frustrated Heisenberg antiferromagnet with the fcc lattice and exchange interaction between the nearest (J ₁) and next-to-nearest (J ₂) neighbors are studied in this work. For the collinear phase with the wave vector Q= (π,π,π), the equations of the self-consistent spin-wave theory are obtained and solved numerically for the sublattice magnetization and the averaged short-range order parameter. The dependence of the Néel temperatureT _N on the ratio J ₂/J ₁ is found. It is shown that, in the case of a sufficiently strong frustration, strong short-range magnetic order persists over a wide temperature range above T _N. The possible application of this result to the mechanism of structural phase transition from α-Fe to γ-Fe is considered.     

The first principle study of magnetic properties of Mn2WSn,Fe2YSn (Y=Ti,V), Co2YSn (Y=Ti,Zr, Hf,V, Mn) and Ni2YSn (Y=Ti,Zr, Hf,V, Mn) heusler alloys

The spin polarized electronic band structures, density of states (DOS) and magnetic properties of Mn₂WSn, Fe₂YSn (Y=Ti, V), Co₂YSn (Y=Ti, Zr, Hf, V, Mn) and Ni₂YSn (Y=Ti, Zr, Hf, V, Mn) huesler compounds are reported. The calculations are performed by using full-potential linearized augmented plane wave method (FP-LAPW) within density functional theory. The magnetic trend in these compounds is studied using values of magnetic moments, exchange interaction and calculated band gap. The results reveal that Mn₂WSn and Ni₂VSn show 100% spin polarization, Co₂YSn (Y=Ti, Zr, Hf, Mn), Fe₂YSn (Y=Ti, V), and Ni₂MnSn exhibit metallic nature and Ni₂YSn (Y=Ti, Zr, Hf) and Co₂VSn show semi-conducting behavior.     

Hyperfine coupling and large amplitude motions interaction in the water dimer

Experimental and theoretical analyses of the hyperfine structure of the non-rigid water dimer (H₂O)₂ were carried out. Measurements were performed with an FT-MW spectrometer allowing us to resolve most of the hyperfine components. Eight hyperfine patterns were recorded involving rotational-tunneling sublevels with J ? 2. Hyperfine patterns were analyzed accounting for the magnetic spin-rotation and spin-spin hyperfine couplings. Symmetry adapted nuclear spin wavefunctions were built to account for the interaction of the hyperfine coupling with the large amplitude motions displayed by the water dimer and to build total rotational-tunneling-hyperfine wavefunctions obeying the Pauli exclusion principle. This accounts for the strong dependence of the hyperfine patterns on the symmetry species of the rotational-tunneling sublevels. These theoretical results were used to perform individual and global analyses of the hyperfine patterns. The latter analysis yields values for some of the components of the spin-rotation coupling tensors.     

LiV2O4: electronic,magnetic structure and heavy-fermion behaviour from first principles

First principles studies based on density functional theory (DFT) calculations within the generalized gradient approximations (GGA) and GGA + U approach using the full-potential, augmented plane wave + local orbitals (APW + lo) method, as implemented in the WIEN2k code, have been used to investigate the structural, electronic and magnetic properties of spinel-structure LiV₂O₄, in particular regarding the heavy fermion (HF) behaviour. The calculations were performed for ferromagnetic, anti-ferromagnetic, and ferrimagnetic configurations using two kinds of magnetic structures (tetragonal and rhombohedral). The GGA results showed that the Fermi energy lies in the V 3d (t_2g) bands with 1.5 electrons per V atom occupying this band, and the V 3d bands are separated by a ～1.9 eV energy gap from the O 2p bands and further split into t_2g and e_g bands with a ～1.0 eV energy gap, which are in good agreement with the photoelectron spectra. The GGA + U method indicates that the ground state of LiV₂O₄ is the tetragonal anti-ferromagnetic configuration with metallic character, and ferromagnetic order character at slightly higher energy, which is consistent with experimental result. The geometric frustration and hybridization between 3d (V) and 2p (O) could induce spin fluctuation and help to explain the instability of specific heat, susceptibility and HF behaviour.     

Low-temperature specific heat study of SrCu2(BO3)2 with an exactly solvable ground state

The specific heat of a two-dimensional spin gap system SrCu₂(BO₃)₂ realizing the Shastry-Suther-land model was measured between 1.3 and 25 K under various magnetic fields up to 12 T. The analysis based on an isolated dimer model in a low temperature region revealed that the value of the spin gap at zero field is Δ = 34.4 K. It turned out that Δ decreases in proportion to H due to the Zeeman splitting of the excited triplet levels. This simplest model, however, fails to reproduce the result in a high-temperature region, suggesting rather strong spin-spin correlation of the system.     

Quantum Monte Carlo Investigation of the magnetic properties of weakly interacting antiferromagnetic chains with an alternating exchange interaction with spin S = 1/2

An approximation dependence of the spontaneous magnetic moment at a site, σ/σ(0) ? 1 = and the antiferromagnet-singlet state phase boundary, J ₂/J ₁ = 0.52(3)δ, are determined by the quantum Monte Carlo method in the self-consistent sublattice molecular field approximation for weakly inter-acting (J ₂) antiferromagnetic chains with spin S = 1/2 and alternating exchange interaction (J ₁ ± δ). The Néél temperature and a number of critical temperatures which could be related with the filling energy of two singlets (ΔS ^z = 0) and one triplet (ΔS ^z = 1) spin bands, each of which is split by the sublattice field (h ^{x, y} ≠ h ^z into two subbands, are determined on the basis of the computed correlation radii of the two-and four-spin correlation function, the squared total spin 〈 (S ^z)²〉 with respect to the longitudinal components, the dimerization parameter, and the correlation functions between the nearest neighbors with respect to longitudinal and transverse spin components. On the basis of the Monte Carlo calculations, the critical temperatures and possible energy gaps at the band center are determined for the antiferromagnets CuWO₄ and Bi₂CuO₄ and for the singlet compounds (VO)₂P₂O₇ and CuGeO₃, agreeing satisfactorily with existing results, and new effects are also predicted.