Theory of the electronic structure and spin susceptibility of La2−x SrxCuO4

We solve the problem of the effect of strong electron correlations on the homogeneous spin susceptibility of current carriers in CuO₂ planes. We show that the dependence of the spin susceptibility χ(T) of high-T _c superconductors of the La_2−x Sr_xCuO₄ type on temperature and the doping index x can be explained fairly well by the two-band model suggested earlier (the singlet-correlated oxygen band plus the lower Hubbard band of copper). The model has features in common with the phenomenological t-J model but cannot be reduced to the latter completely. In contrast to the t-J model, the density of states of the oxygen holes has a peak near the bottom of the band. It is the presence of this peak together with the non-Fermi-liquid properties that explain the unusual behavior of the spin susceptibility of La_2−x Sr_xCuO₄. Zh. éksp. Teor. Fiz. 112, 1763–1777 (November 1997)     

Au1-xNix合金(x=0.30-0.42)低温电阻率极小值的机理探讨

为了解释Au_1-xNi_x合金(x=0.30—0.42)低温出现电阻率极小值的实验结果,本文提出一个低浓度自旋集团顺磁态合金的模型,得到自旋集团孤立近似下和自旋集团耦合作用下的电阻率公式,并和晶格散射的贡献(由Au₈₀Ni₂₀合金或Au-Cu合金的ρ_i(T)实验数据代替)联合起来,得到ρ(T)的计算曲线,和实验结果符合得很好。当Tmin,电阻率随温度增高而下降,主要是自旋集团孤立近似下的Kondo效应引起的。自旋集团之间的RKKY耦合作用对电阻率的贡献在低温时大,随着温度增高按1/T规律迅速减小,所以ρ(T)-ρ_i(T)实验值在相当宽温度范围出现logT关系。随着温度增高,晶格散射对电阻率的贡献将变得重要,当T=T_min,电阻率出现极小值。 关键词：     

Dynamical solutions of a quantum Heisenberg spin glass model

We consider quantum-dynamical phenomena in the SU(2), S = 1/2 infinite-range quantum Heisenberg spin glass. For a fermionic generalization of the model we formulate generic dynamical self-consistency equations. Using the Popov-Fedotov trick to eliminate contributions of the non-magnetic fermionic states we study in particular the isotropic model variant on the spin space. Two complementary approximation schemes are applied: one restricts the quantum spin dynamics to a manageable number of Matsubara frequencies while the other employs an expansion in terms of the dynamical local spin susceptibility. We accurately determine the critical temperature T _c of the spin glass to paramagnet transition. We find that the dynamical correlations cause an increase of T _c by compared to the result obtained in the spin-static approximation. The specific heat C(T) exhibits a pronounced cusp at T _c . Contradictory to other reports we do not observe a maximum in the C(T)-curve above T _c .Received: 13 July 2004, Published online: 5 November 2004PACS: 75.10.Nr Spin glass and other random models - 75.10.Jm Quantized spin models     

Magnetic properties of LiNi<Subscript>0.5</Subscript>Mn<Subscript>0.47</Subscript>Al<Subscript>0.03</Subscript>O<Subscript>2</Subscript> as positive electrode for Li-ion batteries

The layered LiNi_0.5Mn_0.47Al_0.03O₂ was synthesized by wet chemical method and characterized by X-ray diffraction and analysis of magnetic measurements. The powders adopted the α-NaFeO₂ structure. This substitution of Al for Mn promotes the formation of Li(Ni_0.47²⁺Ni_0.03³⁺Mn_0.47⁴⁺Al_0.03³⁺)O₂ structures and induces an increase in the average oxidation state of Ni, thereby leading to the shrinkage of the lattice unit cell. The concentration of antisite defects in which Ni²⁺ occupies the (3a) Li lattice sites in the Wyckoff notation has been estimated from the ferromagnetic Ni²⁺(3a)–Mn⁴⁺(3b) pairing observed below 140 K. The substitution of 3% Al for Mn reduces the amount of antisite defects from 7% to 6.4–6.5%. The analysis of the magnetic properties in the paramagnetic phase in the framework of the Curie–Weiss law agrees well with the combination of Ni²⁺ (S = 1), Ni³⁺ (S = 1/2) and Mn⁴⁺ (S = 3/2) spin-only values. Delithiation has been made by the use of K₂S₂O₈. According to this process, known to be softer than the electrochemical one, the nickel ions in the (3b) sites are converted into Ni⁴⁺ in the high spin configuration, while Ni²⁺(3a)–Mn⁴⁺(3b) ferromagnetic pairs remain, as the Li⁺(3b) ions linked to the Ni²⁺(3a) ions in the antisite defects are not removed. The results show that the antisite defect is surrounded by Mn⁴⁺ ions, implying the nonuniform distribution of the cations in agreement with previous NMR and neutron experiments.     

A μSR Study of Er Spin Dynamics in (Y1−x Er x )Ni2B2C Superconductors

Zero field μSR has been used to probe rare earth spin dynamics in the magnetic superconductors, Y_1−x Er _x Ni₂B₂C. The muon spin relaxation function is stretched exponential, exp (−(λt)^β), in form, as usually found for spin glass systems above the glass temperature. However, the Y_1−x Er _x Ni₂B₂C compounds show no evidence of coexisting superconducting and static spin glass ground states even at concentrations below the critical value (x=0.6) for long range antiferromagnetic order. The temperature dependence of both the muon spin relaxation rate λ and the exponent β suggests that Er spin dynamics change significantly at the superconducting transition temperature. This revised version was published online in September 2006 with corrections to the Cover Date.     

Effect on the magnetic properties of small substitutions of iron and copper for nickel in YNi3 and Y2Ni7 intermetallics

Measurements of magnetization in static fields up to 60 kOe and magnetic susceptibility in weak ac fields of the Y(Ni_1−x M_x)₃ and Y₂(Ni_1−x M_x)₇ intermetallics (M=Fe and Cu, x _max=0.2), as well of the specific heat of these systems for some compositions are reported. It was found that these intermetallics are ferromagnetic at low temperature, and that their spontaneous magnetizations M _s at 4.2 K and Curie temperatures T _C decrease monotonically with increasing copper concentration. The magnetic susceptibility of the Y(Ni_1−x Fe_x)₃ system with 0.06≲x≲0.2 was observed to decrease rapidly with the temperature lowered below a characteristic temperature T _s. One of possible reasons for such an anomaly is shown to be the onset in this concentration region of the reentrant spin-glass state at low temperatures. The results obtained demonstrate that the tight-binding d-band model fails to explain the evolution of the magnetic properties of YNi₃ and Y₂Ni₇ in the case of small substitutions of iron and copper for nickel. Fiz. Tverd. Tela (St. Petersburg) 39, 1828–1830 (October 1997)     

Inelastic neutron scattering study and Hubbard model description of the antiferromagnetic tetrahedral molecule Ni<Subscript>4</Subscript>Mo<Subscript>12</Subscript>

The tetrameric Ni(II) spin cluster Ni₄Mo₁₂ has been studied by INS. The data were analyzed extensively in terms of a very general spin Hamiltonian, which includes antiferromagnetic Heisenberg interactions, biquadratic 2-spin and 3-spin interactions, a single-ion magnetic anisotropy, and Dzyaloshinsky-Moriya interactions. Some of the experimentally observed features in the INS spectra could be reproduced, however, one feature at 1.65 meV resisted all efforts. This supports the conclusion that the spin Hamiltonian approach is not adequate to describe the magnetism in Ni₄Mo₁₂. The isotropic terms in the spin Hamiltonian can be obtained in a strong-coupling expansion of the Hubbard model at half-filling. Therefore detailed theoretical studies of the Hubbard model were undertaken, using analytical as well as numerical techniques. We carefully analyzed its abilities and restrictions in applications to molecular spin clusters. As a main result it was found that the Hubbard model is also unable to appropriately explain the magnetism in Ni₄Mo₁₂. Extensions of the model are also discussed.     

Spin fluctuations in metals: Application to the actinides

We present here a review of the spin fluctuation theory and of its applications to transition and actinide systems, with a particular emphasis to the latter where some very anomalous properties find an explanation in terms of spin fluctuation effects. Firstly, we summarize the development of the spin fluctuation model which had been initially applied to transition metals and alloys such as palladium or Pd–Ni alloys. Then, we present the extension of the paramagnon model to nearly magnetic actinide systems by taking into account explicitly the temperature dependence of the Stoner susceptibility, because the 5f-band of actinides is much narrower than the d-band of transition metals. As a result the paramagnon contribution to the resistivity departs from the usual T ² and T power laws at temperatures higher than the spin fluctuation one and saturates at high temperatures, with eventually the presence of a maximum at intermediate temperatures. We present also the calculation of the other properties of actinide systems, namely the thermal resistivity, the thermoelectric power, the magnetic susceptibility, the specific heat capacity and the NMR relaxation rate, which are generally enhanced by the presence of paramagnons. Finally, we have introduced the concept of ‘antiferromagnetic-like’ spin fluctuations which have a maximum of the q-dependent susceptibility _χ(q) at a q value different from q =0, in contrast to the regular ferromagnetic spin fluctuations; both types of spin fluctuation give the same resistivity behaviour, while they yield a markedly different behaviour of the magnetic susceptibility, in agreement with experiment. The spin fluctuation theory is applied successfully to the different properties of neptunium and plutonium metals and of many nearly magnetic compounds such as UAl₂.     

Molecular magnetism in closo-azadodecaborane supericosahedrons

ABSTRACT

The connection of 12 s = ½ closo-azadodecaborane radical units (NB₁₁H₁₁?), where a hydrogen atom is removed from the nitrogen atom, produces a supericosahedron [(NB₁₁H₆?)₁₂]^(S), S being the total spin of the system. This work describes the study of the low-lying energy spin-projected states of this supericosahedron with two different geometrical arrangements, each nitrogen atom pointing (1) inwards or (2) outwards with respect to radial axes. These spin-projected states are mapped into a Heisenberg spin Hamiltonian, thus allowing the determination of coupling constants between magnetic sites. The eigenvalues of this model Hamiltonian then predict the ground spin state and the corresponding combinations of spin orientations of the magnetic centres. We show that the energy minimum in the [(N_in/outB₁₁H₆?)₁₂]^(S) systems corresponds to a high-spin S = 6 state.     

Spin state and exchange in the quasi-one-dimensional antiferromagnet KFeS2

We report the optical spectra and single crystal magnetic susceptibility of the one-dimensional antiferromagnet KFeS₂. Measurements have been carried out to ascertain the spin state of Fe³⁺ and the nature of the magnetic interactions in this compound. The optical spectra and magnetic susceptibility could be consistently interpreted using aS=1/2 spin ground state for the Fe³⁺ ion. The features in the optical spectra have been assigned to transitions within thed-electron manifold of the Fe³⁺ ion, and analysed in the strong field limit of the ligand field theory. The high temperature isotropic magnetic susceptibility is typical of a low-dimensional system and exhibits a broad maximum at ∼565K. The susceptibility shows a well defined transition to a three dimensionally ordered antiferromagnetic state atT _N=250 K. The intra and interchain exchange constants,J andJ′, have been evaluated from the experimental susceptibilities using the relationship between these quantities, andχ _max,T _max, andT _N for a spin 1/2 one-dimensional chain. The values areJ=−440.71 K, andJ′=53.94 K. Using these values ofJ andJ′, the susceptibility of a spin 1/2 Heisenberg chain was calculated. A non-interacting spin wave model was used belowT _N. The susceptibility in the paramagnetic region was calculated from the theoretical curves for an infiniteS=1/2 chain. The calculated susceptibility compares well with the experimental data of KFeS₂. Further support for a one-dimensional spin 1/2 model comes from the fact that the calculated perpendicular susceptibility at 0K (2.75×10⁻⁴ emu/mol) evaluated considering the zero point reduction in magnetization from spin wave theory is close to the projected value (2.7×10⁻⁴ emu/mol) obtained from the experimental data.     

Theoretical studies of the Spin Hamiltonian parameters and local structures for Cu2+ centers in MNB ternary glasses

ABSTRACT

The spin Hamiltonian parameters (g factors g_|| and g_⊥ and the hyperfine structure constants A_|| and A_⊥) for the doped Cu²⁺ ion (in the form of CuO) in ternary glasses (i.e. xMgO·(30-x)Na₂O·69B₂O₃·CuO, with 5?<?x < 17?mol%) are theoretically investigated based on the high-order perturbation formulas for a tetragonally elongated octahedral 3d⁹ complex. In these formulas, the required crystal-field parameters are estimated from the superposition model which enables correlation of the crystal-field parameters and hence the spin Hamiltonian parameters with the tetragonal distortion (characterized by relative tetragonal elongation δ along the C₄ axis due to the Jahn–Teller effect) of [CuO₆]^10? cluster. The concentration dependences of the spin Hamiltonian parameters are illustrated by the approximately linear increases of the cubic field parameter D_q and the covalency factor N as well as the relative elongation δ with increasing the MgO concentration x. Based on the calculation, the [CuO₆]^10? clusters in the MNB glasses are found to suffer the relative elongations of about δ (≈ 0.125?Å) along the tetragonal axis due to the Jahn–Teller effect. The theoretical results show good agreement with the experimental data. And the improvement is also achieved in present work with respect to the previous theoretical analysis based on the conventional crystal-field model formulas by including the ligand orbital and spin–orbit coupling contributions.     

Strongly correlated quantum spin liquid in herbertsmithite

Strongly correlated Fermi systems are among the most intriguing and fundamental systems in physics. We show that the herbertsmithite ZnCu₃(OH)₆Cl₂ can be regarded as a new type of strongly correlated electrical insulator that possesses properties of heavy-fermion metals with one exception: it resists the flow of electric charge. We demonstrate that herbertsmithite’s low-temperature properties are defined by a strongly correlated quantum spin liquid made with hypothetic particles such as fermionic spinons that carry spin 1/2 and no charge. Our calculations of its thermodynamic and relaxation properties are in good agreement with recent experimental facts and allow us to reveal their scaling behavior, which strongly resembles that observed in heavy-fermion metals. Analysis of the dynamic magnetic susceptibility of strongly correlated Fermi systems suggests that there exist at least two types of its scaling.     

Perturbative study of thermodynamic properties for the two-leg spin ladder

We apply finite-temperature perturbation theory to study thermodynamic properties of the two-leg antiferromagnetic spin ladder in the strong interchain coupling limit. The internal energy, specific heat and uniform susceptibility are calculated analytically by third-order perturbation expansions. At zero temperature, the present method results in the same ground state energy as that obtained by the strong coupling expansion without temperature. At finite-temperature, we obtain a peak in the specific heat and a broad maximum in the uniform susceptibility. The results agree quite well with experimental data for the material Cu₂(C₅H₁₂N₂)₂Cl₄ and the numerical data of 8-order series expansion theory.     

Substituent effects on indirect carbon–carbon couplings,J(CC), in substituted thiophenes,pyrroles, and furans studied by experiment and theory

An excellent linear correlation is found between a large set of experimental spin–spin carbon–carbon coupling constants, J(CC), in thiophene, pyrrole, and furan systems and the corresponding B3PW91/6‐311++G(2d,p)//B3PW91/6‐311++G(2d,p) calculated estimates. The correlation does not differ significantly from the simplest relationship possible, J(CC)_exp. = J(CC)_calcd., within a small and random spread of about 1 Hz. There are 285 experimental values considered, and 202 out of these are new and come from the present work. The character of the correlation indicates that rovibronic effects on aromatic J(CC)'s, and those of nuclear motions on aromatic J(CC)'s are practically negligible. All of this is in a perfect agreement with our recent extensive studies on aromatic J(CC)'s in pyridine and benzene ring systems. As has been shown by computations, not only large one‐bond couplings but also almost all long‐range ones occurring between the carbons of the heteroaromatic rings are, with a few exceptions, positive. Significant substituent effects experimentally observed in the one‐bond as well as long‐range couplings are very accurately reproduced by the computation. The experimental coupling magnitudes vary from ca. 1 to 98 Hz. The J(CC)'s computed for the model variously substituted trimethylsilyl and fluoro derivatives, which are not easily accessible experimentally, span a range of about 130 Hz, from ca. ?2 in up to ca. +125 Hz . Copyright © 2012 John Wiley & Sons, Ltd.     

Theoretical explanation of optical absorption spectra and covalent effect of Ni2+ ions in octahedral sites of Ca3Sc2Ge3O12 crystal

The optical absorption spectra (d-d transition bands) and covalent effect of Ni²⁺ ions in octahedral sites of Ca₃Sc₂Ge₃O₁₂ crystal have been investigated by the full energy matrix based on the two spin–orbit coupling parameters model. The bond length of octahedral site is R_i?=?2.19 Å, which can be determined by the cubic crystal-field parameter and optical spectral data. The lattice distortion of the Ni²⁺ center in Ca₃Sc₂Ge₃O₁₂ crystal is also obtained from the calculations. In addition, the result has shown that the covalent effect of Ni²⁺ ion in the octahedral site of Ga₃Sc₂Ge₃O₁₂ is obvious and cannot be ignored. The calculated d-d transition bands agree well with that of the experimental findings, suggesting that the present methods can explain reasonably the optical spectral data and covalent effect of 3d⁸ ions in octahedral lattices.     

Modeling Spectroscopic Properties of Ni2+ Ions in the Haldane Gap System Y2BaNiO5

Modeling of spin Hamiltonian parameters enables correlation of crystallographic, spectroscopic, and magnetic data for transition ions in crystals. In this paper, based on the crystallographic data and utilizing the point-charge model and superposition model, the crystal field parameters (CFPs) are estimated for Ni²⁺(3d ⁸) ions in the Haldane gap system Y₂BaNiO₅. The CFPs serve as input for the perturbation theory expressions and the crystal field analysis package for microscopic spin Hamiltonian modeling of the zero-field splitting parameters (ZFSPs) D and E. Results of an extensive literature search of the pertinent crystallographic data, experimental ZFSPs, and model parameters are briefly outlined. The modeling aims at verification of the experimental ‘single ion anisotropy’ parameters and explanation of the controversy concerning the maximal rhombic distortion |E/D| ≈1/3 reported for Ni²⁺ ions in Y₂BaNiO₅. The preliminary results call for reanalysis of some magnetic studies of the Haldane gap systems.     

Impact of iron composition on the calculated electronic and magnetic properties of Fe3−xNixSi

ABSTRACT

The structural, electronic and magnetic properties of Fe_3?xNi_xSi alloys with variable iron composition (0?≤?x?≤?1) have been investigated within by using Projector augmented wave (PAW) method. The exchange–correlation potential was treated with the generalised gradient approximation (GGA) for the calculation of the structural properties and for the computation of the electronic and magnetic properties in order to treat the d states. These alloys crystallize in cubic Heusler structures; The Fe₃Si and Fe₂NiSi have a regular structure DO₃ and L2₁ respectively. To describe the experimental proprieties we use the on-site Coulomb interactions of U_eff(Ni)?=?3.1?eV and U_eff(Fe)?=?3.4?eV. A good agreement between calculated and experimental magnetic moments is found for the cubic Heusler phases without the addition of Hubbard-model. The obtained results of the density of states and the spin-polarized band structure show that the Fe₂NiSi alloy has half-metallic property. Through the obtained values of the total spin magnetic moment, we conclude that in general, the Fe₂NiSi alloy is half-metallic ferromagnet material whereas the Fe₃Si alloy has a metallic nature.     

Antiferromagnetic phase transition in garnet-type AgCa2Co2V3O12 and AgCa2Ni2V3O12

Antiferromagnetic phase transition in two vanadium garnets AgCa₂Co₂V₃O₁₂ and AgCa₂Ni₂V₃O₁₂ has been found and investigated extensively. The heat capacity exhibits sharp peak due to the antiferromagnetic order with the Néel temperature T_N=6.39 K for AgCa₂Co₂V₃O₁₂ and 7.21 K for AgCa₂Ni₂V₃O₁₂, respectively. The magnetic susceptibilities exhibit broad maximum, and these T_N correspond to the inflection points of the magnetic susceptibility χ a little lower than T(χ_max). The magnetic entropy changes from zero to 20 K per mol Co²⁺ and Ni²⁺ ions are 5.31 J K⁻¹ mol-Co²⁺-ion⁻¹ and 6.85 J K⁻¹ mol-Ni²⁺-ion⁻¹, indicating S=1/2 for Co²⁺ ion and S=1 for Ni²⁺ ion. The magnetic susceptibility of AgCa₂Ni₂V₃O₁₂ shows the Curie-Weiss behavior between 20 and 350 K with the effective magnetic moment μ_eff=3.23 μ_B Ni²⁺-ion⁻¹ and the Weiss constant θ=−16.4 K (antiferromagnetic sign). Nevertheless, the simple Curie-Weiss law cannot be applicable for AgCa₂Co₂V₃O₁₂. The complex temperature dependence of magnetic susceptibility has been interpreted within the framework of Tanabe-Sugano energy diagram, which is analyzed on the basis of crystalline electric field. The ground state is the spin doublet state ²E(t₂⁶e) and the first excited state is spin quartet state ⁴T₁(t₂⁵e²) which locates extremely close to the ground state. The low spin state S=1/2 for Co²⁺ ion is verified experimentally at least below 20 K which is in agreement with the result of the heat capacity.     

Theoretical studies of absorption spectra and microstructure of Ni2+ at the octahedral site of the NiFe2O4 nanoparticle

The optical absorption spectra, microstructure and electronic spin resonance parameters (electronic spin resonance (ESR) g factor) for Ni²⁺ ions at octahedral centers of nickel ferrite nanoparticles are calculated from the two-spin–orbit-parameter model. The effect of spin–orbital coupling of the central metal 3d⁸ ions and ligand oxygen ions has been taken into account in the full energy matrix and ESR g formula. The calculated results are in good agreement with the observed values. In addition, the microstructures of Ni²⁺ ions at octahedral centers in NiFe₂O₄ are reasonably determined from the calculations.     

Ab initio cluster calculations of Co3+ spin states in RBaCo2O5.5 (R=Ho, Gd)

Two types of oxygen-deficient perovskites RBaCo₂O_5.5(R=Ho,Gd) related to the “122” type structure (a _p × 2a _p × 2a _p ) have been studied on the basis of ab initio cluster calculations. We consider the peculiar behavior of the trivalent ions Co³⁺(3d ⁶) in either octahedral or pyramidal oxygen coordinations, which is related to a structural first-order phase transition in both compounds. Relative energy positions of low spin (LS, S = 0), intermediate spin (IS, S = 1) and high spin (HS, S = 2) electron configurations are calculated for the low-and high-temperature lattice structures of RBaCo₂O_5.5. A combined analysis of the calculated results and experimental structural data leads to a simple model that captures the most prominent features of the phase transition common to both compounds.