Ferromagnetic Spin Coupling Research Articles

Electronic structure of Mn 2O −: ferromagnetic spin coupling stabilized by oxidation

The electronic structure of Mn 2O − was investigated by photoelectron spectroscopy and the density-functional theory (DFT). The cluster anion was found to possess a spin magnetic moment as large as 11 μ B (Bohr magneton) due to ferromagnetic coupling between the spins localized on each manganese site. The ferromagnetic state gains substantial stability via superexchange-type interaction through an oxygen atom at the bridge site. This mechanism is manifested in its electronic structure, which exhibits significant hybridization among the Mn 3d, Mn 4s, and O 2p orbitals.

Ferromagnetism in a dilute magnetic semiconductor: Generalized RKKY interaction and spin-wave excitations

Carrier-mediated ferromagnetism in a dilute magnetic semiconductor has been studied using i) a single-impurity based generalized RKKY approach which goes beyond linear response theory, and ii) a mean-field-plus-spin-fluctuation (MF+SF) approach within a (purely fermionic) Hubbard-model representation of the magnetic impurities, which incorporates dynamical effects associated with finite frequency spin correlations in the ordered state. Due to a competition between the magnitude of the carrier spin polarization and its oscillation length scale, the ferromagnetic spin coupling is found to be optimized with respect to both hole doping concentration and impurity-carrier spin coupling energy $J$ (or equivalently $U$). The ferromagnetic transition temperature $T_c$, deteremined within the spin-fluctuation theory, corresponds closely with the observed $T_c$ values. Positional disorder of magnetic impurities causes significant stiffening of the high-energy magnon modes. We also explicitly study the stability/instability of the mean-field ferromagnetic state, which highlights the role of competing AF interactions causing spin twisting and noncollinear ferromagnetic ordering.

Chemically induced ferromagnetic spin coupling: Electronic and geometric structures of chromium–oxide cluster anions, Cr2On− (n=1–3), studied by photoelectron spectroscopy

Dichromium oxide cluster anions, Cr2On− (n=1–3), were found to possess highly spin-polarized electronic structures, which were revealed by the measurements of photoelectron spectra and the analyses by the density-functional calculations. Their spin magnetic moments were as large as 9, 9, and 7 μB for n=1, 2, and 3, respectively, due to a ferromagnetic coupling between local spins on the chromium atoms. The ferromagnetic spin couplings were caused predominantly by a superexchange-type Cr–Cr interaction through an oxygen atom at the bridge site, where a significant mixing of Cr 3d with O 2p orbitals stabilized the ferromagnetic states. The high-spin characters of Cr2On− are in striking contrast to that of a pure chromium dimer, which is known to exhibit an antiferromagnetic spin coupling due to the strong Cr–Cr covalent bond. The present ferromagnetic spin couplings should, therefore, be induced by oxidation. These findings support a concept that a chemical reaction controls magnetic properties of molecules and clusters.

Fully localized mixed-valence oxidation products of molecules containing two linked dimolybdenum units: an effective structural criterion.

Two previously reported compounds [Mo(2)](CH(3)O)(2)M(CH(3)O)(2)[Mo(2)] (Cotton, F. A.; Liu, C. Y.; Murillo, C. A.; Wang, X. Inorg. Chem. 2003, 42, 4619), in which [Mo(2)] is an abbreviation for the quadruply bonded Mo(2)(4+) unit embraced by three (p-anisyl)NC(H)N(p-anisyl) anions and M = Zn (1) or Co (2), have been chemically oxidized. One-electron oxidation products [Mo(2)](CH(3)O)(2)M(CH(3)O)(2)[Mo(2)](PF(6)) (3, M = Zn; 4, M = Co) and the two-electron oxidation product [Mo(2)](CH(3)O)(2)Zn(CH(3)O)(OH)[Mo(2)](PF(6))(2) (5) have been isolated and structurally characterized. As expected, oxidations occur at the dimolybdenum units. The mono-charged cations in 3 and 4 have asymmetric molecular structures with two distinct [Mo(2)] units. In each case, one of the [Mo(2)] units has a lengthened Mo-Mo bond distance of 2.151[1] A, as expected for one-electron oxidation, whereas the other remains unchanged at 2.115[1] A. These correspond to bond orders of 3.5 (sigma(2)pi(4)delta(1)) and 4.0 (sigma(2)pi(4)delta(2)), respectively. The crystallographic results thus show unambiguously that in the crystalline state, the mixed-valence compounds (3 and 4) are electronically localized and the unpaired electron is trapped on one [Mo(2)] unit. These results are supported by the EPR spectra. The doubly oxidized compound 5 has two equivalent [Mo(2)] units, both with a Mo-Mo bond distance of 2.149[1] A. EPR and magnetic susceptibility measurements for 5 indicate that there is no significant ferromagnetic or antiferromagnetic spin coupling and the species is valence-trapped.

Chemical control of magnetism: oxidation-induced ferromagnetic spin coupling in the chromium dimer evidenced by photoelectron spectroscopy.

The photoelectron spectrum of the dichromium oxide cluster anion, Cr2O-, and the analysis by the density-functional theory revealed that the spins of the two Cr atoms in Cr2O- are ferromagnetically coupled, and that its total spin magnetic moment is as large as 9 mu(B). This ferromagnetic spin coupling is induced by oxidation; the mixing of Cr 3d with O 2p orbitals plays an important role in a spin coupling between the localized electrons at the two Cr sites bridged by the O atom. The present finding is in marked contrast to the pure chromium dimer, which is known to be antiferromagnetic due to the strong sextuple Cr-Cr bond.

Ladderlike ferromagnetic spin coupling network on a pi-conjugated pendant polyradical.

A poly(9,10-anthryleneethynylene)-based polyradical with two pendant stable phenoxyls in one anthracene skeleton was newly synthesized via polymerization of the corresponding bromoethynylanthracene monomer using a Pd(0) catalyst. The average molecular weight of the polymer reached M(n) = 5 x 10(3) and was soluble in common organic solvents. The polyradical was prepared from the corresponding hydroxyl precursor polymer and was appropriately stable at room temperature. The ESR spectrum of the corresponding monomeric radical suggested an effectively delocalized spin density distribution on the backbone anthracene. The magnetization and the static magnetic susceptibility of the polyradical were measured using a SQUID magnetometer. The large average spin quantum number (S = (5)/(2)) of the polyradical indicated that the ferromagnetic spin coupling network of the polyradical had spread throughout the pi-conjugated chain and that it was considerably insensitive to spin defects.

Ferromagnetic spin coupling in the manganese trimer ion evidenced by photodissociation spectroscopy

The optical spectrum of the manganese trimer ion, Mn3+, was obtained by measurement of the photodissociation cross section in the photon-energy range between 1.43 and 4.13 eV. Analysis of the spectrum by quantum-chemical calculations derived its electronic and geometric structures. The geometric structure was found to be an isosceles triangle (C2v) with bond lengths of 3.03 Å and an apex angle of 144°. The ground electronic state was found to be B217. The electronic structure of the valence orbitals indicates that the chemical bond is formed weakly by the 4s electrons. The 3d electrons are localized on the atomic sites, as is suggested by the nonbonding nature of the nearly degenerate occupied orbitals. All of the local spins are in the majority-spin state, and give rise to a total spin magnetic moment as large as 16μB. The ferromagnetic nature is due to the weak binding among constituent atoms and to the strong 3d-4s exchange interaction in the manganese atom. This finding is in marked contrast to the antiferromagnetism of bulk manganese.

Nature of copper(II)-lanthanide(III) magnetic interactions and generation of a large magnetic moment with magnetic anisotropy of 3d-4f cyclic cylindrical tetranuclear complexes [CuIILLnIII(hfac)2]2, (H3L = 1-(2-hydroxybenzamido)-2-(2-hydroxy-3-methoxybenzylideneamino)ethane and Hhfac = hexafluoroacetylacetone, LnIII = Eu, Gd, Tb, Dy).

A cyclic cylindrical 3d-4f tetranuclear structure, in which the 3d and 4f magnetic ions are arrayed alternately, has been found to be a suitable molecular design to produce a large magnetic moment and large magnetic anisotropy. Complexes 3-10 with the chemical formula [MLLn(hfac)2]2 ((MII, LnIII) = (Cu, Eu) (3), (Cu, Gd) (4), (Cu, Tb) (5), (Cu, Dy) (6), (Ni, Eu) (7), (Ni, Gd) (8), (Ni, Tb) (9), (Ni, Dy) (10)) have been synthesized, where H3L = 1-(2-hydroxybenzamido)-2-(2-hydroxy-3-methoxybenzylideneamino)ethane and Hhfac = hexafluoroacetylacetone. The powder X-ray diffractions and FAB-mass spectra demonstrated that these complexes assume a similar tetranuclear structure. The crystal structures of 4 and 5 showed that each complex has a cyclic cylindrical tetranuclear CuII2LnIII2 structure, in which the CuII complex functions as a "bridging ligand-complex" to two adjacent LnIII ions. The temperature-dependent magnetic susceptibilities from 2 to 300 K and the field-dependent magnetizations at 2 K from 0 to 5 T have been measured for four pairs of CuII2LnIII2 and NiII2LnIII2, in which compound NiII2LnIII2 containing diamagnetic NiII ion was used as the reference complex to evaluate the CuII-LnIII magnetic interaction. Comparison of the magnetic properties of the CuII2LnIII2 complex with those of the corresponding NiII2LnIII2 complex showed that the magnetic interaction between CuII and EuIII ions is weakly ferromagnetic and that between CuII and either of GdIII, TbIII, and DyIII ions is ferromagnetic. Complex CuII2GdIII2, 4, has an S = 8 spin ground state, due to the ferromagnetic spin coupling between SGd = 7/2 and SCu = 1/2 with coupling constants of J1 = +3.1 cm-1 and J2 = +1.2 cm-1. The magnetic measurements showed that compounds 5 and 6, CuII2LnIII2 (LnIII = Tb, Dy), exhibit large magnetic moments and large magnetic anisotropy due to the LnIII ion.

Synthesis of a Pendant Polyradical with a New π-Conjugated Polymer Backbone Containing an Anthracene Skeleton and Its Ferromagnetic Spin Coupling

A poly(9,10-anthryleneethynylene)-based polyradical with pendant stable phenoxyls was newly synthesized via polymerization of the corresponding bromoethynylanthracene monomer using Pd(0) catalysts. The average molecular weight of the polymer reached Mn = 5 × 103 by the polymerization reaction for a few hours, and the polymer was soluble in common organic solvents. The polyradical was prepared from the corresponding hydroxyl precursor polymer and was appropriately stable at room temperature. The ESR spectrum of the polyradical suggested an effectively delocalized spin density distribution on the backbone anthracene. The magnetization and the static magnetic susceptibility of the polyradical were measured using a SQUID magnetometer, and the results revealed moderately strong ferromagnetic spin coupling (2J = 39 ± 3 cm-1) through the π-conjugated chain, depending on the degree of spin density distribution on the anthracene backbone.

Magnetic behavior of a ferromagnetic semiconductor system

Single crystals of the CdGa 2−2 x Cr 2 x S 4 magnetic semiconductor system with x=1, 0.8 and 0.5 were grown by chemical transport method. X-ray crystal structure determination showed that the spinel structure is preserved in all the samples. Magnetic measurements suggest that the ferromagnetic Curie temperature decreases as the Cr concentration decreases. Admixtures of Ga ions affected the ferromagnetic spin coupling between the chromium ions. Above the Curie temperature the data was well fitted with the Curie–Weiss law given a positive paramagnetic Curie temperature which decreases with the Cr concentration.

Density functional studies of oxidized and reduced methane monooxygenase. Optimized geometries and exchange coupling of active site clusters.

The conflicting protein crystallography data for the oxidized form (MMOH(ox)) of methane monooxygenase present a dilemma regarding the identity of the solvent-derived bridging ligands within the active site: do they comprise a diiron unit bridged by 1H2O and 1OH(-) as postulated for Methylococcus capsulatus or 2OH(-) ligands as suggested for Methylosinus trichosporium? Using models derived explicitly from the M. capsulatus and M. trichosporium protein data, spin-unrestricted density functional methods have been used to study two structurally characterized forms of the hydroxylase component of methane monooxygenase. The active site geometries of the oxidized (MMOH(ox)) and two-electron-reduced (MMOH(red)) states have been geometry optimized using several quantum cluster models which take into account the antiferromagnetic (AF) and ferromagnetic (F) coupling of electron spins. Trends in cluster geometries, energetics, and Heisenberg J values have been evaluated. For the majority of models, calculated geometries are in good agreement with the X-ray analyses and appear relatively insensitive to the F or AF alignment of electron spins on adjacent Fe sites. Discrepancies between calculation and experiment appear in the orientation of the coordinated His and Glu amino acid side chains for both MMOH(ox) and MMOH(red) and also in unexpected intramolecular proton transfer in the MMOH(ox) cluster models. There is additional dispersion between (and among) calculated and experimental Fe(3+)-OH(-) distances with relevance to the correct protonation state of the solvent-derived ligands. In an accompanying paper (Lovell, T.; Li, J.; Noodleman, L. Inorg. Chem. 2001, 40, 5267), a comparison of the related energetics of the active site models examined herein is further evaluated in the full protein and solvent environment.

Polaron-Type Magnetic Conjugated Polymer with Liquid Crystallinity

We synthesized poly(para- and meta-phenylenevinylene) derivative with liquid crystalline group. The polymer is composed of p-phenylenevinylene as a spin generating unit and m-phenylene as a ferromagnetic spin coupling unit. The polymer showed nematic LC phase. The polymer was doped by iodine to generate the polaron, after the polymer was cooled down through the LC phase. ESR measurements showed that the spin concentration increased by means of external magnetic force alignment of the polymer.

Poly(9,10-anthryleneethynylene)-based polyradicals with pendant phenoxyls

We synthesized monomers of poly(9,10-anthryleneethynylene)s which were anthracene derivatives substituted with one or two phenol residues at the β-position which would receive less steric hindrance from the poly(9,10-anthryleneethynylene) backbone. The monomers were polymerized by Pd(0) catalyst to give the corresponding precursor polymer with M ¯ w ∼ 10 4 . The polyradicals were synthesized by oxidation of the hydroxyl group. The clearer hyperfine structure of the corresponding monomeric radicals suggested that the polyradicals probably also involved an effectively delocalized spin density distribution into the anthryleneethynylene skeleton. Diradical models for the poly(9,10-anthryleneethynylene)-based polyradicals were also synthesized, and the magnetic properties of diradicals suggested defect insensitive strong ferromagnetic spin coupling of the corresponding polyradicals.

Magnetic and electrical properties of poly(3-radical-substituted thiophene)s

Radical groups, tert-butylnitroxide, nitronyl nitroxide, and 2,6-di- tert-butylphenoxyl, were introduced at the 3-position of thiophene, and the thiophene derivatives were then polymerized to yield poly(3-radical-substituted thiophene)s. The acetoxyphenyl-substituted thiophene gave regioregular poly(3-phenol-substituted thiophene) via oxidative polymerization with ferric chloride at low temperature, while the polymerization of the amine-substituted thiophenes did not proceed with any oxidizing agents under any reaction conditions. The phenoxyl-substituted polyradicals, 7 and 8, with spin concentrations of 0.30 and 0.25, respectively, displayed S values of 2/2 to 3/2 and 2/2, respectively, indicating an intramolecular ferromagnetic spin-coupling through the polythiophene backbone. The polyradicals, 7 and 8, themselves and the polyradical doped with iodine showed an electrical conductivity of 10 −5 and 10 −2∼−1 S cm −1, respectively.

Regioregular Polythiophene with Pendant Phenoxyl Radicals: A New High-Spin Organic Polymer

Poly[3-(3‘,5‘-di-tert-butyl-4‘-acetoxyphenyl)thiophene] (1a) was regioselectively synthesized via simple oxidative polymerization of 3-(3‘,5‘-di-tert-butyl-4‘-acetoxyphenyl)thiophene (2a) with ferric chloride. Head-to-tail content of 1a with a molecular weight of >104 reached 96% by modulating both the polymerization conditions and the purification procedure of the polymer. Visible absorption and fluorescence maxima were bathochromically shifted for both 1a and poly[3-(3‘,5‘-di-tert-butyl-4‘-hydroxyphenyl)thiophene] (1b), which suggests an extended π-conjugation, i.e., a regioregular structure. Redox of the phenolate and of the thiophene residue of 1b was observed reversibly and independently under alkaline conditions. The polyphenoxyl 1 derived from the oxidation of 1b showed an electrical conductivity of 10-5 S cm-1. The polyradical 1 satisfies both an alternant but non-Kekulé-type π-conjugation and ferromagnetic connectivity of the unpaired electrons of the pendant phenoxyls. 1 with a spin concentration of 0.3 spin/unit displayed a spin quantum number (S) value of 2/2 to 3/2, indicating a high-spin ground state and an intramolecular ferromagnetic spin coupling through the polythiophene backbone.

A cyclic tetranuclear Cu2Gd2 complex with an S = 8 ground state derived from ferromagnetic spin-coupling between copper(ii) and gadolinium(iii) ions arrayed alternately

A cyclic tetranuclear CuII2GdIII2 complex, in which CuII and GdIII ions are arrayed alternately, has an S = 8 spin ground state, due to the ferromagnetic spin coupling between SGd = 7/2 and SCu = 1/2.

Density functional investigation on the ferromagnetic coupling of spins in phenylenevinylene-bridged nitroxide radicals: Monomer and polymer cases

Magnetic interactions in phenylenevinylene-bridged organic radicals are investigated as realistic models of pendant-type ferromagnetic polymers, since several molecules having some types of phenylenevinylene skeletons with phenoxy or nitroxide radicals were recently synthesized and their ferromagnetic characters were also confirmed by experimental works. In the present study, a spin source of NOH nitroxide radical is examined as a model of stable tertiary-butyl nitroxide in this system. According to NOH orientations, it is possible to form hydrogen bond between radical O and vinylene H atoms which may work as additional effects on the ferromagnetic spin coupling. To elucidate this point, density-functional calculations are carried out for both monomers and polymers by using Becke’s exchange with Lee–Yang–Parr correlation functional. It is detected that the influence of hydrogen bond is emphasized in polymers by the cooperative interaction arising from one-dimensional network along chain. Implications of the hydrogen-bonding effects are briefly discussed in relation to hydrogen abstraction reactions by nitroxide groups and stabilizations of ferromagnetic polymers by selections of substitution positions of radical groups. Computational results were also compared with available experimental results by Nishide, Tsuchida, and others. Finally, a new proposal for synthesis of thermal and photo-induced organic ferrimagnets is presented.

Observation of ferromagnetic correlation in phase-separated Cu-rich La 2CuO 4.003

Ferromagnetic correlation stemmed from hole doping was directly observed by magnetization vs magnetic field (hysteresis loops) measurements in phase-separated Cu-rich La 2CuO 4.003 crystal from 33 K up to 300 K. It shows that the remanent magnetism increases monotonically with the decrease of temperature. This ferromagnetic coupling of spins, together with the observed charge ordering, is suggested to be the intrinsic features of phase separation in lightly oxygen-doped Cu-rich La 2CuO 4.003 system.

On the Interplay of Magnetic and Molecular Forces in Curie–Weiss Ferrofluid Models

We consider a mean-field continuum model of classical particles in Rd with Ising or Heisenberg spins. The interaction has two ingredients, a ferromagnetic spin coupling and a spin-independent molecular force. We show that a feedback between these forces gives rise to a first-order phase transition with simultaneous jumps of particle density and magnetization per particle, either at the threshold of ferromagnetic order or within the ferromagnetic region. If the direct particle interaction alone already implies a phase transition, then the additional spin coupling leads to an even richer phase diagram containing triple (or higher order) points.

Higher-spin pi multiradical sites in doped polyarylamine polymers

P-Doping of conjugated polymers based on an extended m-phenylenediamine motif produces a disperse population of ferromagnetically coupled spin clusters. To increase the average size of these spin clusters variations have been explored both in the structure of the polymer and in the method of doping. Introduction of ferromagnetic spin coupling ‘2,7 through the naphthalene nucleus’ rather than ‘1,3 through the benzene nucleus’ has been investigated as have variations in the aryl core/alkyl side chain ratio but the biggest improvements were achieved by adopting a doping procedure in which thin films of the polymer were exposed to gaseous antimony pentachloride. A thin film (ca. 1 µm thick) of the polymer obtained by the Pd0-mediated coupling of 1,3-dibromo-5-tetradecylbenzene with tris[2-butoxy-4-(dihydroxyboranyl)phenyl]amine was exposed to gaseous antimony pentachloride. Under these conditions 40–60% of the potentially dopable sites were oxidised to the amminium ion (N˙+) level. A Brillouin function fit to the field dependence of the magnetisation of this doped polymer at 2 K corresponds to behaviour equivalent to a nonet (S = 4) pi multiradical.