EPR investigation on exchange coupled Mn2+ pairs in (CH3)4NCdCl3

EPR experiments were performed at Q-band and 300 K on Mn²⁺ doped (CH₃)₄NCdCl₃. Beside the single ion resonance many weak lines could be observed which can be attributed to nearest neighbour pairs of Mn²⁺ lying along the crystallographic c-axis. The spectra can be described using a Hamiltonian for strongly exchange coupled pairs. The obtained data show that the ionic separation varies in the different spin states. So an additional interaction occurs on the Mn²⁺ dimer, which can be interpreted as an exchange striction effect.     

固相反应法制备Co掺杂ZnO的磁性和光学性能研究

研究了以固相反应法制备Co掺杂ZnO粉体的磁性和光学性能，测试结果表明对于均匀掺杂的Zn_0.95Co_0.05O粉体，Co²⁺随机取代Zn²⁺的位置进入ZnO晶格.Co²⁺之间的3d自旋电子耦合交换作用使得近邻的Co²⁺自旋反平行，Zn_0.95Co_0.05O粉体在3—300K表现为顺磁性，而非铁磁性. 关键词： ZnO 固相反应 稀磁半导体 顺磁性     

Structural, electronic, and magnetic properties of Co-doped ZnO

Density functional theory based calculations have been carried out to study structural, electronic, and magnetic properties of Zn1-xCoxO (x = 0, 0.25, 0.50, 0.75) in the zinc-blende phase, and the generalized gradient approximation proposed by Wu and Cohen has been used. Our calculated lattice constants decrease while the bulk moduli increase with the increase of Co 2+ concentration. The calculated spin polarized band structures show the metallic behavior of Co-doped ZnO for both the up and the down spin cases with various doping concentrations. Moreover, the electron population is found to shift from the Zn-O bond to the Co-O bond with the increase of Co 2+ concentration. The total magnetic moment, the interstitial magnetic moment, the valence and the conduction band edge spin splitting energies, and the exchange constants decrease, while the local magnetic moments of Zn, Co, O, the exchange spin splitting energies, and crystal field splitting energies increase with the increase of dopant concentration.     

The crystal field and exchange coupling in iron group metal carbonates

The wave functions of Co²⁺ and Fe²⁺ ions near the ground state in the CaCO_3? type lattice have been calculated from EPR data in the Abragam-Pryce approximation. The orbital angular momentum contributions to the anisotropic and antisymmetric parts of exchange coupling are determined assuming that this interaction between the magnetic ions occurring in nonequivalent positions is isotropic with respect to spin orientations. It is shown that, in the given approximation, the exchange coupling components in the basal plane for such Fe²⁺?Fe²⁺ and Co²⁺? Fe²⁺ ion pairs are missing. This fact explains the uniaxial antiferromagnetic ordering in FeCO₃ and the presence of a low-lying oscillation branch for Fe²⁺ impurity ions in antiferromagnetic CoCO₃. The EPR spectra of exchange-coupled Co²⁺?Co²⁺, Fe²⁺?Fe²⁺, and Co²⁺?Fe²⁺ pairs occupying nonequivalent positions have been calculated and their parameters have been numerically estimated.     

Magneto-optical study of ferromagnetically coupled cobalt (II) pairs in cadmium bromide

Low temperature polarized absorption and magnetic circular dichroism spectra are reported for crystals of CdBr₂ doped with varying concentrations of Co²⁺ in the region near 17 700 cm^-1. Sharp zero phonon lines are identified as the trigonal field components of U_g ′(² T _1g , ² H) in dilute crystals, and as states arising from ferromagnetically coupled in-plane pairs in the concentrated ones. Measurements of the temperature dependence of the lines, and Zeeman spectra recorded with applied fields up to 50 kG parallel and perpendicular to the crystal c-axis, lead to estimates of the anisotropy of the exchange interaction in the ground state, and to both isotropic and anisotropic exchange in the excited state. The preferred direction of spin alignment in both states is parallel to the c-axis.     

固相反应法制备的CoxZn1-xO磁性能的研究

采用固相反应烧结法制备了Co掺杂ZnO的粉末和压片样品. XRD衍射表明样品中Co²⁺离子取代Zn²⁺离子进入了ZnO晶格中. 在室温下样品均表现为顺磁性, 利用密度泛函理论(DFT+U)方法计算得到的Co₂Zn₁₄O₁₆体系反铁磁基态更稳定, 并通过Co, O原子电子迁移变化, 表明了CoZnO体系中磁性机理更倾向于Co²⁺—O^2-—Co²⁺成键的间接交换作用模型. 改进了安德森模型中的直接交换积分J_pd公式. 提出两个可能的途径来实现具有本征铁磁性氧化物半导体.     

Synthesis,structural, optical,and magnetic properties of Co doped,Sm doped and Co+Sm co-doped ZnS nanoparticles

The compositional, structural, optical and magnetic properties of ZnS, Zn_0.98Co_0.02S, Zn_0.98Sm_0.02S and Zn_0.96Co_0.02Sm_0.02S nanoparticles synthesized by a hydrothermal method are presented and discussed. X-ray diffraction (XRD), high resolution transmission electron microscopy (HRTEM) and selected area electron diffraction (SAED) studies revealed that all the samples exhibited cubic structure without any impurity phases. X-ray photoelectron spectroscopy (XPS) results revealed that the Co and Sm ions existed in +2 and +3 states in these samples. The photoluminescence (PL) spectra of all the samples exhibited a broad emission in the visible region. The room temperature magnetization versus applied magnetic field (M–H) curves demonstrated that the Sm+Co doped nanoparticles exhibited enhanced ferromagnetic behavior compare to Co and Sm individually doped ZnS nanoparticles, which is probably due to the exchange interaction between conductive electrons with local spin polarized electrons on the Co²⁺ or Sm³⁺ ions. This study intensifies the understanding of the novel performances of co-doped ZnS nanoparticles and also provides possibilities to fabricate future spintronic devices.     

Room‐Temperature Insulating Ferromagnetic (Ni,Co)1+2xTi1−xO3 Thin Films

Insulating uniaxial room‐temperature ferromagnets are a prerequisite for commonplace spin wave‐based devices, the obstacle in contemporary ferromagnets being the coupling of ferromagnetism with large conductivity. It is shown that the uniaxial A_{1 + 2x}Ti⁴⁺_{1 ? x}O₃ (ATO), A = Ni²⁺,Co²⁺, and 0.6 < x ≤ 1, thin films are electrically insulating ferromagnets already at room temperature. The octahedra network of the ATO and the corundum and ilmenite structures are the same yet different octahedra‐filling proved to be a route to switch from the antiferromagnetic to ferromagnetic regime. Octahedra can continuously be filled up to x = 1, or vacated (?0.24 < x < 0) in the ATO structure. TiO‐layers, which separate the ferromagnetic (Ni,Co)O‐layers and intermediate the antiferromagnetic coupling between the ferromagnetic layers in the NiTiO₃ and CoTiO₃ ilmenites, can continuously be replaced by (Ni,Co)O‐layers to convert the ATO‐films to ferromagnetic insulator with abundant direct cation interactions.     

ESR assessment of 3d7 transition metal impurity states in GaP,GaAs and InP

Photo-sensitive electron spin resonance of the 3d⁷-ions Fe⁺, Co²⁺, Ni³⁺ has been detected and analysed in GaP, GaAs and InP. For GaP : Ni³⁺, hyperfine interaction with the four nearest P³¹-ligands could be resolved.     

Tunable exchange bias in La1.5Sr0.5CoMnO6 double perovskite doped with nonmagnetic Ga ions

The crystal structure, electronic structure, and magnetic behaviors of nonmagnetic Ga ions doped double perovskite La_1.5Sr_0.5CoMnO₆ single phase crystals have been investigated. Different from the traditional magnetic dilution effect of nonmagnetic doping, Ga doping in La_1.5Sr_0.5CoMnO₆ enhances the ferromagnetic (FM) exchange interaction of Co³⁺-O-Mn³⁺. Moreover, both conventional and spontaneous exchange bias (EB) effects can be tuned by modulating the Ga doping content, which is accompanied by the variation of the Co^3+/4+ and Mn^3+/4+ and the effective magnetic moment. The EB field and magnetization can be improved by nonmagnetic Ga³⁺ doping with content lower than 0.2. The evolution of conventional and spontaneous EB effects in La_1.5Sr_0.5Co_1-xGa_xMnO₆ can be understood in terms of the unidirectional interface anisotropic coupling between FM/anti-FM, and/or FM/spin glass, which is affected by antisite disorder, spin glass, and the uncompensated coupling between Co and Mn.     

Spin wave relaxation and magnetic properties in [M/Cu] super-lattices; M=Fe, Co and Ni

In this work, we study the elementary excitations and magnetic properties of the [M/Cu] super-lattices with: M=Fe, Co and Ni, represented by a Heisenberg ferromagnetic system with N atomic planes. The nearest neighbour (NN), next nearest neighbour (NNN) exchange, dipolar interactions and surface anisotropy effects are taken into account and the Hamiltonian is studied in the framework of the linear spin wave theory. In the presence of the exchange alone, the excitation spectrum E(k) and the magnetization 〈S^z〉/S analytical expressions are obtained using the Green's function formalism. The obtained relaxation time of the magnon populations is nearly the same in the Fe and Co-based super-lattices, while these magnetic excitations would last much longer in the Ni-based super lattice. A numerical study of the surface anisotropy and long-ranged dipolar interaction combined effects are also reported. The exchange integral values deduced from a comparison with experience for the three super-lattices are coherent.     

Magnetic disorder induced enhanced magneto-resistance in La0.5Sr0.5Co1−xRuxO3

We observe a sharp increase in negative magneto-resistance ratio up to 40% for x=0.1, in La_0.5Sr_0.5Co_1−xRu_xO₃ which is due to the magnetic disorder induced by an anti-ferromagnetic interaction between Co and Ru ions. We also observe a metal to insulator and a ferromagnetic to anti-ferromagnetic transition for 0≤x≤0.3. Ruthenium (IV) ion disrupts an intermediate spin state of cobalt (Co³⁺:t_2g⁵e_g¹), forcing a double exchange mediated ferromagnetic state to an anti-ferromagnetic spin state for x≥0.2.     

The role of Co impurities and oxygen vacancies in the ferromagnetism of Co-doped SnO2: GGA and GGA+U studies

The electronic structure and ferromagnetic stability of Co-doped SnO₂ are studied using the first-principle density functional method within the generalized gradient approximation (GGA) and GGA+U schemes. The addition of effective U_Co transforms the ground state of Co-doped SnO₂ to insulating from half-metallic and the coupling between the nearest neighbor Co spins to weak antimagnetic from strong ferromagnetic. GGA+U_Co calculations show that the pure substitutional Co defects in SnO₂ cannot induce the ferromagnetism. Oxygen vacancies tend to locate near Co atoms. Their presence increases the magnetic moment of Co and induces the ferromagnetic coupling between two Co spins with large Co-Co distance. The calculated density of state and spin density distribution calculated by GGA+U_Co show that the long-range ferromagnetic coupling between two Co spins is mediated by spin-split impurity band induced by oxygen vacancies. More charge transfer from impurity to Co-3d states and larger spin split of Co-3d and impurity states induced by the addition of U_Co enhance the ferromagnetic stability of the system with oxygen vacancies. By applying a Coulomb U_O on O 2 s orbital, the band gap is corrected for all calculations and the conclusions derived from GGA+U_Co calculations are not changed by the correction of band gap.     

Aftereffects in zeolite‐encapsulated 57Co‐complexes

Tris(2,2'-bipyridyl)-⁵⁷Co^II and bis(2,2':6',2'-terpyridine)-⁵⁷Co^II complexes were synthesised in the supercages of zeolite‐Y in order to study the effect of molecular isolation on the aftereffects of the ⁵⁷Co(EC)⁵⁷Fe decay. As compared to the regular crystalline salts of the complex ions where, according to the emission Mössbauer spectra, the most abundant species is low‐spin Fe^II, the molecular isolation in the zeolite resulted in a larger fraction of low‐spin Fe^III and a varying amount of high‐spin Fe²⁺ species. In the investigated temperature range, 20 K to 295 K, the majority of the changes was observed above 80 K. In the case of tris (2,2'-bipyridyl)-⁵⁷Co^II-Y, the most characteristic change occurred in valence states, while for bis (2,2':6',2'-terpyridine)-⁵⁷Co^II‐Y, the temperature dependence of the spin states was more prominent. The change in the low spin valence states is explained partly by donor-acceptor properties of the zeolite lattice. The variation in the high spin fraction is explained by radiation damage of the ligand sphere and/or fragmentation of the complex ion followed by incomplete recombination in the supercage. Molecular isolation itself did not seem to increase the chance of fragmentation (as a consequence of charge neutralization following Auger ionization) of these highly conjugated complex molecules.     

The first and second nearest neighbour Cr3+ pairs in ruby

The optical spectrum of the exchange coupled first nearest neighbour Cr³⁺ pairs in ruby was studied by absorption, fluorescence and excitation spectroscopy. It was analysed by means of the group theoretical considerations given in part I [1]. The analysis could be verified by measuring the Zeeman effect in fields up to 150 KG. The Zeeman splitting deviates considerably from that of pure spin states even for the orbital singulet of the pair ground state. The exchange integrals of the ground state are found to beJ=–115 cm^–1 andj=–0.7 cm^–1.This work was supported by the Deutsche Forschungsgemeinschaft, SFB 65     

Ground state of BaCoS2 as a set of energy-degenerate orbital-ordered configurations of Co2+ ions

The linear muffin-tin orbital method in the local density approximation (LDA + U) explicitly considering Coulomb correlations has been applied to calculate the electronic structure, magnetic moments, and parameters of the Heisenberg exchange interaction for cobalt ions in BaCoS₂. Five solutions close in total energy with various orbital ordering of Co²⁺ ions and almost identical spin magnetic moments μ = 2.32μ_B of the Co²⁺ ion 3d-shell have been found. The BaCoS₂ ground state can be considered as a set of energy-degenerate orbital-ordered configurations of Co²⁺ ions in the high-spin state.     

Magnetic Properties and Spin State Transition of Gallium Doped Perovskite Cobaltite Oxide

A series of Ga doping perovskite cobaltite La2/3Sr1/3 （Co1-y Gay）03 （y = 0, 0.1, 0.2, 0.3 and 0.4） are prepared by the standard solid-state reaction method. Their magnetic properties and Co ions spin state transitions are studied. Upon doping, no appreciable structure changes can be found. However, the corresponding Curie temperature sharply decreases and the magnetization is greatly reduced, indicating that Ga doping destroys the ferromagnetic interaction in the system. In addition, the high temperature magnetization data follow the Curie-Weiss law. At least one kind of Co ions （Co^3＋ or Co^4＋） favours the mixed spin state, and most Co ions are at the lower spin state （low and intermediate state）. With increasing Ga content, more Co ions transit to the higher spin state.     

Mechanism for enhancing dispersion of Co3O4 nanoparticles in Co/SiO2 Fischer–Tropsch synthesis catalyst by adding glycol to impregnating solution: a quick‐XAFS study

In situ Co K‐edge quick‐EXAFS (QEXAFS) coupled with temperature‐programmed oxidation as well as ex situ XAFS was applied to investigating the mechanism for enhancing the dispersion of Co₃O₄ nanoparticles in a calcined Co/SiO₂ Fischer–Tropsch synthesis catalyst prepared by adding triethylene glycol (TEG) to a Co(NO₃)₂.6H₂O impregnating solution. Ex situ Co K‐edge XAFS indicated that, regardless of whether the catalysts were prepared with or without using TEG, the hexaaqua Co (II) complex was formed in impregnated samples which then underwent the dehydration process to some extent during the subsequent drying step at 393 K. In situ QEXAFS and ex situ EXAFS results also indicated that small oxide clusters were formed in the TEG‐modified catalyst calcined at ～400–470 K which interacted with polymer species derived from TEG. Since the Fischer–Tropsch synthesis activity of the TEG‐modified catalyst increased with an increase in the calcination temperature in a similar temperature range [Koizumi et al. (2011), Appl. Catal. A, 395 , 138–145], it was suggested that such an interaction enables the clusters to be distributed over the support surface uniformly, resulting in enhancing their dispersion. After combustion of polymer species, Co₃O₄‐like species were formed, and agglomeration of the Co₃O₄‐like species at high calcination temperatures was suppressed by the addition of TEG to the impregnating solution. It was speculated that the addition of TEG induced the formation of some surface silicate which worked as an anchoring site for Co₃O₄ and Co⁰ nanoparticles during calcination and H₂ reduction, respectively.     

Dynamical interaction of antiferromagnetic subsystems: a neutron scattering study of the spinwave spectrum of the garnet Fe2Ca3(GeO4)3

The Fe³⁺ ions in the garnet Ca₃Fe₂Ge₃O₁₂ form two identical antiferromagnetic subsystems. The interaction between the two subsystems is vanishing within molecular field approximation forq=0. A coupling appears due to the spin fluctuations. The dynamics of the system is described by the Hamiltonian for a Heisenberg antiferromagnet. Symmetry requirements impose two exchange parameters between the sublattices (nearest neighbours)J ₁ in the direction of the 3-fold axis andJ' ₁ in the other three space diagonals. The interaction within each sublattice (second nearest neighbours) is described by the exchange parameterJ ₂. The measured spin wave dispersion curves for the three principal symmetry directions are very well reproduced by a model calculation withJ ₁=-0.909(9) K,J' ₁=-0.307(8) K andJ ₂=-0.615(2)K. The observed intensities are in agreement with predictions from the model. Forq0 the model predicts two acoustic branches going towards zero frequency. A calculation beyond linear spin wave theory forq=0 predicts a quantum gap for the lower acoustic branch. This gap has been found at 0.033(4) THz. An anisotropy gap of 0.007 THz has been taken from the literature.     

Influence of Interface Structure of Co/Cu （100） Superlattices on Electronic Structure and Giant Magnetoresistance

Electronic structures and magnetoresistance （MR） of Co3 Cu5 and Co3 Cur models as well as their interface atom exchange models Co2 CuCoCu4 and Co2 CuCoCu6 are investigated by the tlrst-principles pseudopotential planewave method based on density functional theory. Charge transfer, magnetic moment, density of states, spin asymmetry factor, and MR ratio are discussed. The results show that the values of charge transfer and spin asymmetry factor at the Fermi level of Co layers are closely related to the neighbouring background of the Co layer. The Co layer with two sides adjacent to the Cu layer would transfer more charge to the Cu layer than other neighbouring background and have the largest spin asymmetry factor at the Fermi level. The Co layer with two neighbouring Co layers （interior Co） would gain a little charge and have the smallest spin asymmetry factor at the Fermi level. Two-current model is used to evaluate the MR ratio of Co2CuCoCu4 （Co2CuCoCu6） to be larger than that of Co3 Cu5 （Co3 CUT）, which can be explained by the charge transfer and spin asymmetry factor.