Spin distribution and hydrogen-bond effect in ferromagnetic alpha-phase crystal of 2-(2',5'-dihydroxyphenyl)-4,4,5,5-tetramethyl-4,5-dihydro-1H-imidazolyl-1-oxy-3-oxide

The ferromagnetic phenyl nitronyl nitroxide derivate alpha-phase 2-(2('),5(')-dihydroxyphenyl)- 4,4,5,5-tetramethyl-4,5-dihydro-1H-imidazolyl-1-oxy-3-oxide has been studied by the electronic structure calculation based on the density functional theory. The result shows that the spin delocalization due to hyperconjugation effect plays an important role in the spin distribution and ferromagnetic coupling of the crystal.     

Trends in exchange coupling for trimethylenemethane-type Bis(semiquinone) biradicals and correlation of magnetic exchange with mixed valency for cross-conjugated systems

A magnetostructural correlation (conformational electron spin exchange modulation) within an isostructural series of biradical complexes is presented. X-ray crystal structures, variable-temperature electron paramagnetic resonance spectroscopy, zero-field splitting parameters, and variable-temperature magnetic susceptibility measurements were used to evaluate molecular conformation and electron spin exchange coupling in this series of molecules. Our combined results indicate that the ferromagnetic portion of the exchange couplings occurs via the cross-conjugated pi-systems, while the antiferromagnetic portion occurs through space and is equivalent to incipient bond formation. Thus, molecular conformation controls the relative amounts of ferro- and antiferromagnetic contributions to exchange coupling. In fact, the exchange parameter correlates with average semiquinone ring torsion angles via a Karplus-Conroy-type relation. Because of the natural connection between electron spin exchange coupling and electronic coupling related to electron transfer, we also correlate the exchange parameters in the biradical complexes to mixed valency in the corresponding quinone-semiquinone radical anions. Our results suggest that delocalization in the cross-conjugated, mixed-valent radical anions is proportional to the ferromagnetic contribution to the exchange coupling in the biradical oxidation states.     

Germanium,carbon‐germanium,and silicon‐germanium triangulenes

A series of germanium‐containing triangular molecules have been studied by density functional theory (DFT) calculations. The triangulene topology of the compounds provides for their high‐spin ground states and strong sign alternation of spin density and atomic charge distributions. High values of the exchange coupling constants witness ferromagnetic ordering of electronic structures of all studied triangulenes. The compounds bearing more electronegative atoms in a‐positions of the triangular networks possess higher aromatic character and stronger ferromagnetic ordering. © 2015 Wiley Periodicals, Inc.     

Hydrogen dangling bonds induce ferromagnetism in two-dimensional metal-free graphitic-C3N4 nanosheets

Ferromagnetic two-dimensional (2D) ultrathin nanosheets hold great promise for next generation electronics. Ferromagnetic metal-free materials that usually possess only an s/p electronic configuration with weak spin–orbit coupling and a large spin relaxation time, would play an important role in constructing future spintronic devices. However, the absence of an intrinsic spin ordering structure in most metal-free materials greatly hampers the widening scope of ferromagnetic 2D nanostructures as well as in-depth understanding of their ferromagnetic nature. Herein, the induction of intrinsic ferromagnetism in 2D metal-free g-C₃N₄ ultrathin nanosheets has been achieved through a new effective strategy whereby hydrogen dangling bonds are introduced. In our case, g-C₃N₄ ultrathin nanosheets with hydrogen dangling bonds showed obvious room temperature ferromagnetic behavior that could even be tuned by the concentration of hydrogen. This work will pave a new pathway to engineer the properties of 2D nanomaterial systems.     

Towards understanding of magnetic interactions within a series of tetrathiafulvalene-pi conjugated-verdazyl diradical cation system: a density functional theory study

The intramolecular magnetic exchange coupling constants (J) for a series of tetrathiafulvalene (TTF) and verdazyl diradical cations connected by a range of pi conjugated linkers have been investigated by means of methodology based on unrestricted density functional theory. The magnetic interaction between radicals is transmitted via pi-electron conjugation for all considered compounds. The calculation of J yields strong or medium ferromagnetic coupling interactions (in the range of 56 and 300 K) for diradical cations connected by linkers with an even number of carbon atoms that are able to provide a spin polarization pathway, while antiferromagnetic coupling is predicted when linkers with an odd number of carbon atoms are employed. The topological analysis of spin density distributions have been used to reveal the effects of the spin polarization on both linkers and spin carriers. The absence of heteroatoms that impede the spin polarization pathway, and the existence of a unique spin polarization path instead of several possible competitive routes are factors which contribute to large positive J values favoring ferromagnetic interactions between the two terminal pi-radicals. The magnitude of J depends strongly on the planarity of the molecular structure of the diradical cation since a more effective orbital overlap between the two pi-systems can be achieved. Hence, the dependence of J on the torsion angle (theta) of each spin carrier has been analyzed. In this respect, our findings show that this geometrical distortion reduces largely the calculated J values for ferromagnetic couplings, leading to weak antiferromagnetic interactions for a torsion angle of 90 degrees .     

Theoretical Design of High－spin Organic Molecules with Heterocycles as Ferromagnetic Coupling Units

IntroductionThe design and the syntheses of organicmolecules with very high- spin ground states havebeen a topic of great interest[1— 5] .One of rationalapproaches to designing high- spin molecules,which has been proposed and studied by severalgroups[6,7] ,consists in conceptually dividing themolecules into two components,i.e.,a spin- con-taining( SC) fragment which provides the unpairedelectron and a ferromagnetic coupling ( FC) unitwhich is connected with radical centers ferromag-netically…     

Comparative electronic band structure study of the intrachain ferromagnetic versus antiferromagnetic coupling in the magnetic oxides Ca3Co2O6 and Ca3FeRhO6

In the (MM'O6)infinity chains of the transition-metal magnetic oxides Ca3MM'O6 the MO6 trigonal prisms alternate with the M'O6 octahedra by sharing their triangular faces. In the (Co(2O6)infinity chains of Ca3Co2O6 (M = M' = Co) the spins are coupled ferromagnetically, but in the (FeRhO6)infinity chains of Ca3FeRhO6 (M = Fe, M' = Rh) they are coupled antiferromagnetically. The origin of this difference was probed by carrying out spin-polarized density functional theory electronic band structure calculations for ordered spin states of Ca3Co2O6 and Ca3FeRhO6. The spin state of a (MM'O6)infinity chain determines the occurrence of direct metal-metal bonding between the adjacent trigonal prism and octahedral site transition-metal atoms. The extent of direct metal-metal bonding in the (Co2O6)infinity chains of Ca3Co2O6 is stronger in the intrachain ferromagnetic state than in the intrachain antiferromagnetic state, so that the intrachain ferromagnetic state becomes more stable than the intrachain antiferromagnetic state. Such a metal-metal-bonding-induced ferromagnetism is expected to occur in magnetic insulators and magnetic metals of transition-metal elements in which direct metal-metal bonding can be enhanced by ferromagnetic ordering. In the (FeRhO6)infinity chains of Ca3FeRhO6 the ferromagnetic coupling does not lead to a strong metal-metal bonding and the adjacent spins interact by the Fe-O...O-Fe super-superexchange, hence leading to an antiferromagnetic coupling.     

A Rule to Design High Spin Molecules with Hetero spin Centers

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Peculiarity in the electronic structure of Cu(II) complex ferromagnetically coupled with bisimino nitroxides

By means of the electron spin resonance (ESR) technique, we have investigated the electronic structures of the tridentate imino nitroxyl diradical complex with copper(II) (Cu-bisimpy), which has a square planar structure and a ground quartet state with an extremely strong ferromagnetic exchange interaction, and its related compounds (bisimpy = 2,6-bis(1'-oxyl-4',4',5',5'-tetramethyl-4',5'-dihydro-1' H-imidazol-2'-yl)pyridine). It was clarified that Cu-bisimpy had unique magnetic orbitals, compared with the biradical ligand (bisimpy), a zinc(II) biradical complex (Zn-bisimpy) and a copper(II) terpyridine complex (Cu-tpy) (tpy = 2,2';6',2'-terpyridine). Multifrequency ESR spectroscopy provided a reliable set of magnetic parameters of Cu-bisimpy, which has a small g anisotropy ( g x = 2.02, g y = 2.01, g z = 2.08) and small hyperfine coupling with Cu (|A x| = 42.0 MHz, |A y|相似文献   

Magnetism of a free-standing W monoatomic sheet

The magnetism of a free-standing tungsten monoatomic sheet under different magnetization states has been studied by using the first-principles method based on the density functional theory. The results show that W atomic sheet at the equilibrium state is not magnetized. However, ferromagnetic or antiferromagnetic state can appear in the sheet when the lattice is elongated, moreover, the magnetism appears earlier and increases more stably and smoothly in the antiferromagnetic states than the ferromagnetic one. The antiferromagnetic states are always more stable than the ferromagnetic ones under the same lattice parameter. The most stable W atomic sheet among all the structures studied is a near-hexagonal plane structure, due to the Jahn–Teller effect. All the magnetic states studied are shown to be metallic. The electronic structure properties of various magnetic plane structures are presented, with the effect of spin–orbit coupling included.     

Intramolecular Spin Alignment within Mono‐Oxidized and Photoexcited Anthracene‐Based π Radicals as Prototypical Photomagnetic Molecular Devices: Relationships Between Electrochemical,Photophysical, and Photochemical Control Pathways

AnOV is a π‐conjugated radical built from an anthracene (An) unit linked by a p‐phenylene to an oxoverdazyl (OV) moiety. The mono‐oxidized (cationic) form of AnOV was generated both electrochemically and photochemically (in the presence of an electron acceptor). The triplet nature (S=1) of the electronic ground state of AnOV ⁺ was demonstrated by combining spectroelectrochemistry, electron‐spin resonance (ESR) experiments, and ab initio molecular orbital (MO) calculations. The intramolecular spin alignment (ISA) within AnOV ⁺ results from the ferromagnetic coupling (J_electrochem>0) of the two unpaired electrons located on the oxidized electron donor (An⁺) and on the pendant OV radical. The spin‐density distribution pattern of AnOV ⁺ is akin to that of AnOV when photopromoted ( AnOV *) to its high‐spin (HS) lowest excited quartet (S=3/2) state. This high‐spin state results from the ferromagnetic coupling (J_photophys>0) of the triplet locally excited state of An (³An*) with the doublet ground state of OV. As a shared salient feature, AnOV ⁺ and AnOV * (HS) show a spin delocalization within the domain of activated An in either An⁺ or ³An* (nexus states) forms. The present study essentially contributes to establish and clarify relationships between electrochemical, photophysical, and photochemical pathways to achieve ISA processes within AnOV . In particular, we discuss the impact of the spin polarization of the unpaired electron of OV on electronic features of the An electron‐donating subunit. Close analysis of this polarizing interplay allows one to derive a novel functional paradigm to manipulate electron spins at the intramolecular level with light and under an external magnetic field. Indeed, two original functional elements are identified: light‐triggered donors of spin‐polarized electrons and spin‐selective electron acceptors, which are of potential interest for molecular spintronics.     

Role of the coordination of the azido bridge in the magnetic coupling of copper(II) binuclear complexes

It is well-known that the azido bridge gives rise antiferromagnetic (AF) or ferromagnetic (F) coupling depending on its coordination mode, namely end-to-end or end-on, respectively. The aim of the present work is to analyse the factors contributing to this different magnetic behaviour. The difference dedicated configuration interaction (DDCI) method is applied to several binuclear Cu(II) azido-bridged models with both types of coordination. In end-on complexes, the direct exchange and the spin polarisation contributions are found to be responsible for the ferromagnetic coupling. In end-to-end complexes, both the direct exchange and the spin polarisation are small and the leading term is the antiferromagnetic dynamical polarisation contribution. The most relevant physical effects are included in the DDCI calculations so that good quantitative agreement is reached for the coupling constant as well as the spin densities.     

Electronic structure, chemical bonding, and spin polarization in ferromagnetic MnAl

The electronic structure of ferromagnetic τ-MnAl has been calculated using density-functional techniques (TB-LMTO-ASA, FLAPW) and quantum-chemically analyzed by means of the crystal orbital Hamilton population tool. While all observable quantities are in good agreement with experiment, the tetragonal structure of ferromagnetic MnAl is interpreted to arise from a nonmagnetic cubic structure by two subsequent steps, namely (a) an electronic distortion due to spin polarization followed by (b) a structural distortion into the tetragonal system. The various strengths of interatomic bonding have been calculated in order to elucidate the competition between electronic and structural distortion.     

Size-dependent alternation of magnetoresistive properties in atomic chains

Spin-polarized electronic and transport properties of carbon atomic chains are investigated when they are capped with magnetic transition-metal (TM) atoms like Cr or Co. The magnetic ground state of the TM-C(n)-TM chains alternates between the ferromagnetic (F) and antiferromagnetic (AF) spin configurations as a function of n. In view of the nanoscale spintronic device applications the desirable AF state is obtained for only even-n chains with Cr; conversely only odd-n chains with Co have AF ground states. When connected to appropriate metallic electrodes these atomic chains display a strong spin-valve effect. Analysis of structural, electronic, and magnetic properties of these atomic chains, as well as the indirect exchange coupling of the TM atoms through non-magnetic carbon atoms are presented.     

Magnetic coupling in dinuclear Gd complexes

A spin density functional (SDFT) study of carboxylate-bridged and diazenido-bridged dinuclear gadolinium compounds is presented. Calculated magnetic coupling constants for the carboxylate-bridged structures are in good agreement with experimental data, confirming the ability of the broken symmetry approach used in this work to predict magnetic behavior in such compounds. The systematic trend wherein symmetrically bridged complexes are antiferromagnetically coupled and asymmetrically bridged are ferromagnetically coupled is reproduced by the SDFT calculations. The mechanism underlying magnetic coupling in closed- and open-shell dinuclear complexes is described using a perturbative molecular orbital model that focuses the influence of the 4f(7)-5d exchange interaction on molecular orbitals with significant 5d-orbital character for the complex [[[(Me(3)Si)(2)N](2)(thf)Gd](2)(N(2))]. Open-shell electronic configurations facilitate strong ferromagnetic coupling, whereas in closed-shell systems antiferromagnetic coupling is usually preferred.     

The Orbital Origins of Magnetism: From Atoms to Molecules to Ferromagnetic Alloys

A chemical view of spin magnetic phenomena in finite (atoms and molecules) and infinite (transition metals and their alloys) systems using the concepts of bonding and electronic shielding is presented. The concept is intended to serve as a semiquantitative signpost for the synthesis of new ferromagnets. After a concise overview of the historic development of related theories developed within the physics community, the consequences of spin-spin coupling (made manifest in the exchange or Fermi hole) in atoms and molecules are explored. Upon moving to a paramagnetic state, the majority/minority spin species become more/less tightly bound to the nucleus, resulting in differences in the energies and spatial extents of the two sets of spin orbitals. By extrapolating well-known arguments from ligand-field theory, the paucity of ferromagnetic transition metals arises from quenching the paramagnetism of the free atoms due to strong interatomic interactions in the solid state. Critical valence electron concentrations in Fe, Co, and Ni, however, result in local electronic instabilities due to the population of antibonding states at the Fermi level varepsilon(F). Removal of these antibonding states from the vicinity of varepsilon(F) is the origin of ferromagnetism; in the pure metals this results in strengthening the chemical bonds. In the 4d and 5d transition metals, the valence d orbitals are too well shielded from the nucleus, so a transition to a ferromagnetic state does not result in sufficiently large changes to occur. Thus, the exceptional occurence of ferromagnetism only in the first transition series appears to parallel the special main-group chemistry of the first long period. A connection between ferromagnetism in the transition metals and Pearson's absolute hardness eta is easily established and shows that ferromagnetism appears only when eta<0.2 eV in the nonmagnetic calculation. As expected from the principle of maximum hardness, Fe, Co, and Ni all become harder upon moving to the more stable ferromagnetic state. Magnetism in intermetallic alloys follows the same path. Whether or not an alloy contains ferromagnetic elements, the presence of antibonding states at varepsilon(F) serves as a "fingerprint" to indicate a ferromagnetic instability. The differences in the sizes of the local magnetic moments on the constituent atoms of a ferromagnetic alloy can be understood in terms of the relative contributions to the density of states at varepsilon(F) in the nonmagnetic calculations. Appropriately parameterized, nonmagnetic, semi-empirical calculations can also be used to expose the ferromagnetic instability in elements and alloys. These techniques, which have become relatively commonplace, can be used to guide the synthetic chemist in search of new ferromagnetic materials.     

锰团簇Mn5和Mn6结构与磁性的分析

用密度泛函方法对过渡金属Mn₅, Mn₆的各种可能构型, 在PW91/ZoraTZ2P水平上进行了理论研究. 计算结果表明: 构型是自旋变化、磁性的敏感因素, Mn₅最稳定构型为弱铁磁性的三角双锥体(磁矩为3, D_3h). Mn₆的最稳定构型为铁磁性的畸变八面体(磁矩为16, C_4v). 各种异构体虽然多重度不同, 但每个原子的自旋极化度均在3以上. 构型稳定与否取决于原子间的交换耦合作用, 而原子间的这种作用又与自旋极化度的方向、大小息息相关.     

Two ferromagnetic azido-bridged copper(ll) complexes studied by first-principle electronic-structure calculation

The electronic structures of two ferromagnetic polynuclear copper(II) complexes, derived from end-to-end azido ligand and tridentate (NNN donor) Schiff base ligand, have been studied using the full-potential linearized augmented plane-wave method based on the density-functional theory. They are [Cu(L1)(micro-1,3-N3)]n(ClO4)n (1) and [Cu(L2)(micro-1,3-N3)]n(ClO4)n (2). The result shows that the spin populations in these two complexes are mainly distributed on the equatorial planes of a square pyramidal that surround the copper(II) ions. There are large and positive spin populations on copper(II) ions, small and positive spin populations on the three nitrogen atoms of tridentate Schiff base ligand, and the two terminal nitrogen atoms of asymmetrical end-to-end azido ligand, while weak and negative spin populations on the central nitrogen atoms of asymmetrical end-to-end azido ligand. Ferromagnetic coupling through the asymmetrical azido ligand in these two complexes has been mainly attributed to the spin delocalization, also with weak spin-polarization effect.     

Spintronics birefringence with an extended molecular loop-wire or spiral coupling

A ring with spin-orbit effects coupled to a conducting wire is shown to exhibit a phase delay which is spin dependent. The key is that the coupling of the ring to the wire is over an extended spatial range and not just along a single point; this breaks the symmetry and makes the ring states couple differently to forward and backward moving wire states. This results, for properly injected spin states, in a spin-flipping probability which is dependent on the energy of the injected electron and can therefore be easily controlled. Several systems are presented and shown to exhibit this effect including the basic ring which couples to a wire as well as a ring which mediates between two wires, and a spiral between two wires.     

Communication: A dramatic transition from nonferromagnet to ferromagnet in finite fused-azulene chain

One-dimensional fused-azulene oligomers (n = 2-6) are studied with the effective valence bond as well as density functional theory methods. A nonferromagnetic (closed-shell singlet) to ferromagnetic (triplet) ground state transformation is witnessed with increasing length of oligomers. The computational results are interpreted in terms of spin coupling between the unpaired electrons of two nonbonding molecular orbitals localized, respectively, on the top and bottom chains of the oligomers. The present study provides a theoretical suggestion for understanding the ferromagnetic spin polarizations that has been observed very recently in graphene nanoribbons.