Ferromagnetic Spin Coupling Research Articles

GROUND STATES OF A MIXTURE OF TWO SPECIES OF SPIN-1 BOSE GASES WITH INTERSPECIES SPIN EXCHANGE IN A MAGNETIC FIELD

We consider a mixture of two species of spin-1 atoms with both interspecies and intraspecies spin exchanges in a weak magnetic field. Under the usual single mode approximation, it can be reduced to a model of coupled giant spins. We find most of its ground states. This is a complicated problem of energy minimization, with three quantum variables under constraints, i.e., the total spin of each species and the total spin of the whole mixture, as well as four parameters, including intraspecies and interspecies spin coupling strengths and the magnetic field. The quantum phase diagram is very rich. Compared with the case without a magnetic field, the ground states are modified by a magnetic field, which also modifies the ground state boundaries or introduces new crossover regimes on the phase diagram. Without interspecies spin coupling, the quantum phase transitions existing in absence of a magnetic field disappear when a magnetic field is applied, which leads to crossover regimes in the phase diagram. Under ferromagnetic interspecies spin coupling, the ground states remain disentangled no matter whether there is a magnetic field. For antiferromagnetic interspecies spin coupling, a magnetic field entangles the ground states in some parameter regimes. When the intraspecies spin couplings are both ferromagnetic, the quantum phase transition between antiferromagnetic and zero interspecies spin couplings survives the magnetic field. When the intraspecies spin couplings are both antiferromagnetic, a magnetic field induces new quantum phase transitions between antiferromagnetic and zero interspecies spin couplings.

Effect of tunneling barrier as spacer on exchange coupling of CoFeB/AlO x/Co trilayer structures

Magnetic tunneling junctions (MTJs) have a sandwiched structure, which comprises a top ferromagnetic (FM1) layer, an insulating tunneling layer (spacer), and a bottom ferromagnetic (FM2) layer. Exchange coupling in MTJs has been extensively widely examined because the effect of spacer thickness on the ferromagnetic spin-coupling can be exploited in read-head sensors, spin-valve structures, and magntoresistance random access memories (MRAMs). In this investigation, MTJs were deposited in the sequence, glass/CoFeB(50 Å)/AlO x ( d)/Co(100 Å), where the thickness of the AlO x layer d = 12, 17, 22, 26 or 30 Å. Saturation magnetization ( M s) results demonstrate that the exchange coupling strength and coercivity ( H c) can be varied considerably by varying the tunneling barrier AlO x spacer. The X-ray diffraction patterns (XRD) include a main peak from hexagonal close-packed (HCP) Co with a highly (0 0 2) textured structure at 2 θ = 44.7°, and AlO x and CoFeB are amorphous phases. The full width at half maximum (FWHM) of the Co (0 0 2) peak declines as the AlO x thickness increases, revealing that the Co layer becomes more crystalline. The magnetic results reveal that the magnetic characteristics are related to the Co crystallinity. The exchange coupling strength increases with AlO x thickness. The coercivity ( H c) also increases, because the Co crystallinity is eliminated.

Ferromagnetic Spin Coupling between Endohedral Metallofullerene La@C82 and a Cyclodimeric Copper Porphyrin upon Inclusion

The cyclic host cyclo-[P(Cu)](2) carrying two covalently connected Cu(II) porphyrin units can accommodate La@C(82), a paramagnetic endohedral metallofullerene, in its cavity to form the inclusion complex cyclo-[P(Cu)](2)⊃La@C(82), which can be transformed into the caged complex cage-[P(Cu)](2)⊃La@C(82) by ring-closing olefin metathesis of its side-chain olefinic termini. On the basis of electron spin resonance (ESR) and electron spin transient nutation (ESTN) studies, cyclo-[P(Cu)](2)⊃La@C(82) is the first ferromagnetically coupled inclusion complex featuring La@C(82), whereas cage-[P(Cu)](2)⊃La@C(82) is ferrimagnetic.

Three-dimensional quantum phase diagram of the exact ground states of a mixture of two species of spin-1Bose gases with interspecies spin exchange

We find nearly all the exact ground states of a mixture of two species of spin-1 atoms with both interspecies and intraspecies spin exchanges in the absence of a magnetic field. The quantum phase diagram in the three-dimensional parameter space and its two-dimensional cross sections are described. The boundaries where the ground states are either continuous or discontinuous are determined, with the latter identified as where quantum phase transitions take place. The two species are always disentangled if the interspecies spin coupling is ferromagnetic or zero. Quantum phase transitions occur when the interspecies spin coupling varies between antiferromagnetic and zero or ferromagnetic while the two intraspecies spin couplings both remain ferromagnetic. On the other hand, by tuning the interspecies spin coupling from zero to antiferromagnetic and then back to zero, one can circumvent the quantum phase transition due to sign change of the intraspecies spin coupling of a single species, which is spin decoupled with the other species with ferromagnetic intraspecies spin coupling. Generally speaking, interplay among interspecies and two intraspecies spin exchanges significantly enriches quantum phases of spinor atomic gases.

Photoresponsive magnetization reversal in green fluorescent protein chromophore based diradicals

We present density functional theory based calculations on six diradicals in which imino nitroxide (IN) and IN or oxo- or phospha-verdazyl radical centres are linked by fragments of structures called cyan, blue and green fluorescent protein, respectively. In the latter two variants the indolyl fragment of cyan fluorescent protein (CFP) is replaced by phenyl and phenol moiety to obtain blue fluorescent protein Y66F GFP (BFPF) and green fluorescent protein (GFP). The photoinduced cis–trans isomerization of these diradical substituted chromophores is accompanied by changes from antiferro- to ferro-magnetic spin coupling on the radical centres. The calculations in this work provide predictions of the change of magnetic response to photo excitation. The estimated magnetic exchange coupling constants associated with cis–trans isomerization of the chromophores can be useful parameters in designing radical substituted isomeric fluorescent chromophores. Upon irradiation with light of appropriate wavelength, the dark trans diradicals turn into their fluorescent cis isomers. Therefore, photoinduced magnetic crossover from antiferromagnetic to ferromagnetic regime associated with the change in color would be noticed in all three cases. This is a novel observation in case of the systems with GFP chromophore and its variants. These diradicals are potentially very useful in different applications and biocompatibility of such systems makes them prospective in different in vivo treatments. Moreover, change in color associated with magnetic crossover for these diradicals increases their suitability as biological taggers.

Positive and negative exchange bias in IrMn/NiFe bilayers

We present the observation of double shifted hysteresis loops in IrMn/NiFe bilayer structures. The bilayer structures were fabricated using high vacuum DC magnetron sputtering system. The hysteresis loops of the as deposited samples show the double shifted loops at NiFe layer thicknesses 5 nm and 6 nm, whereas the IrMn layer thickness was kept constant at 15 nm. The results were interpreted as the contribution of both positive and negative exchange bias fields. We suppose that this phenomenon is occurring due to the ferromagnetic (FM) layer exchange coupled with the antiferromagnetic (AFM) layer in two different magnetization directions. The ferromagnetic coupling of the interface spins in some regions of the film generates the hysteresis loop shift toward negative fields and antiferromagnetic coupling toward positive fields in the other regions. The double shifted hysteresis loops disappeared after magnetic field annealing of the samples above Neel temperature of the AFM layer. The X-ray diffraction patterns of the sample show the IrMn (111) crystalline growth necessary for the development of exchange bias field in this system. The correlation between the Magnetic Force Microscopy (MFM) domain structures of the as deposited sample and the magnetization reversal process of the double shifted hysteresis loops were discussed. The results suggest that the larger multidomain formation in the AFM layer with different magnetization directions was responsible for the positive and negative exchange bias fields in IrMn/NiFe bilayer samples.

First-principles study of magnetic properties ofL10-ordered MnPt and FePt alloys

Using first-principles methods, we study the magnetic and electronic properties of three different spin configurations of the $\text{L}{1}_{0}$ phase of FePt and MnPt alloys. It is found that MnPt and FePt may be approximately considered as magnetic antipodes with opposite ferromagnetic (FM)---antiferromagnetic (AFM) and in-plane/out-of-plane magnetocrystalline anisotropy (MCA) relationships. In MnPt, the most stable phase is the AFM configuration with AFM chessboard spin coupling in the (001) plane, FM spin coupling between (001) planes, and all spin directions aligned in the (001) plane. Whereas in FePt, the most stable is the FM configuration with all spin directions aligned perpendicular to (001) plane. The out-of-plane MCA of MnPt is more than an order of magnitude less $(\ensuremath{\sim}0.1\text{ }\text{meV})$ than that of FePt $(\ensuremath{\sim}2.9\text{ }\text{meV})$ in their corresponding magnetic ground states. Our calculations indicate that an AFM state can be achieved in FePt by a small variation in tetragonality ratio (from 0.98 to 0.94). A pseudogap is observed at the Fermi energy for MnPt and just below the Fermi energy for FePt for the chessboard AFM model. This pseudogap may explain the ground-state magnetic configuration of MnPt.

Structural and Electronic Properties of Reduced Transition Metal Oxide Clusters, M3O8and M3O8−(M = Cr, W), from Photoelectron Spectroscopy and Quantum Chemical Calculations

We report a comparative study of reduced transition metal oxide clusters, M(3)O(8)(-) (M = Cr, W) anions and their neutrals, via anion photoelectron spectroscopy (PES) and density functional theory (DFT) and molecular orbital theory (CCSD(T)) calculations. Well-resolved PES spectra are obtained for M(3)O(8)(-) (M = Cr, W) at 193 and 157 nm photon energies. Different PES spectra are observed for M = Cr versus M = W. Extensive DFT and CCSD(T) calculations are performed to locate the ground and low-lying excited states for the neutrals and anions. The ground states of Cr(3)O(8) and Cr(3)O(8)(-) are predicted to be the (3)B(2) and (4)B(2) states of a C(2v) structure, respectively, revealing ferromagnetic spin coupling for Cr 3d electrons. In contrast, the ground states of W(3)O(8) and W(3)O(8)(-) are predicted to be the (1)A' state (C(s) symmetry) and the (2)A(1) state (C(2v) symmetry), respectively, showing metal-metal d-d bonding in the anion. The current cluster geometries are in qualitative agreement with prior DFT studies at the PBE level for M = Cr and the B3LYP level for M = W. The BP86 and PW91 functionals significantly outperform the B3LYP functional for the Cr species, in terms of relative energies, electron detachment energies, and electronic excitation energies, whereas the B3LYP functional is better for the W species. Accurate heats of formation for the ground states of M(3)O(8) are calculated from the clustering energies and the heats of formation of MO(2) and MO(3). The energetics have been used to predict redox reaction thermochemistry.

Syntheses, X-ray Structures, and Physicochemical Properties of Phenoxo-Bridged Dinuclear Nickel(II) Complexes: Kinetics of Transesterification of 2-Hydroxypropyl-p-nitrophenylphosphate

Four dinuclear nickel(II) complexes [Ni(II)(2)(L(1))(O(2)CMe)(2)(H(2)O)(2)][PF(6)].MeOH.3H(2)O (1), [Ni(II)(2)(L(1))(O(2)CMe)(2)(NCS)] (2), [Ni(II)(2)(L(2))(O(2)CMe)(2)(MeOH)(H(2)O)][ClO(4)] (3), and [Ni(II)(2)(L(2))(O(2)CMe)(2)(MeOH)(H(2)O)][BPh(4)].3MeOH.H(2)O (4) have been synthesized [HL(1): 2,6-bis[N-methyl-N-(2-pyridylethyl)amino]-4-methylphenol; HL(2): 2,6-bis[3-(pyridin-2-yl)pyrazol-1-ylmethyl]-4-methylphenol]. Complexes 1, 3, and 4 are new while complex 2 was reported previously by Fenton and co-workers (the structure of 2 was presented but no physicochemical properties of this complex were reported; in this work such studies have been completed). X-ray crystallographic analyses of 1 and 4 reveal that each nickel(II) center is six-coordinate, terminally coordinated by two nitrogen donors [(pyridin-2-yl)ethylamine unit in 1 and 3-(pyridin-2-yl)pyrazole moiety in 4], and bridged by an endogenous phenolate ion. Each of the acetate ions in 1 adopts a eta(2)-coordination mode (chelating) whereas in 4 each is coordinated in a mu-eta(1):eta(1) syn-syn bridging mode. In 1 each Ni(II) center has water coordination whereas in 4 one Ni(II) center has a methanol and the other has water coordination. The X-ray structure of 3 could not be determined. The physicochemical properties (electronic spectroscopy and cyclic voltammetry) of the cation of 3 are identical to that of 4. Magnetic susceptibility measurements have revealed the occurrence of ferromagnetic coupling of spins of the nickel(II) centers in 2 [J = +9.80 cm(-1)]. The nickel(II) centers in 1 and 3 are antiferromagnetically coupled, but to different extents [J = -48.4 cm(-1) (1); J = -1.24 cm(-1) (3)]. The magnetic properties are correlated with the nature of bridges between the nickel(II) ions. The two coordinated aqua ligands in 1 and the aqua and methanol ligands in 3 have enabled these dinuclear nickel(II) complexes to function as catalysts in the hydrolysis of 2-hydroxypropyl-p-nitrophenylphosphate (HPNP). Complex 1 is more effective in the conversion of substrate to product (p-nitrophenolate ion) than 3, under identical experimental conditions. Pseudo first-order kinetic treatment has been done for complexes 1 and 3. Temperature-dependent measurements were done to evaluate kinetic/thermodynamic parameters for the hydrolysis/transesterification reaction of HPNP and to propose a mechanistic pathway. The activation parameters are DeltaH(++) = 64 kJ mol(-1), DeltaS(++) = -104 J mol(-1) K(-1) for 1 and DeltaH(++) = 68 kJ mol(-1), DeltaS(++) = -109 J mol(-1) K(-1) for 3. A mechanism consistent with the kinetic data is presented.

Oxidation Unzipping of Stable Nanographenes into Joint Spin-Rich Fragments

When an all-benzenoid nanographene is linearly unzipped into oxygen-joined fragments, the oxidized benzenoid rings (aromatic sextets) selectively adopt the low-spin (DeltaS = 0) or high-spin conformation (DeltaS = 1) to yield the thermally most stable isomer. The selection of the conformation depends simply on the position of the aromatic sextets: the inner ones prefer the high-spin conformation, whereas the peripheral ones prefer the low-spin conformation. Therefore, the resulting most stable isomer has a total spin whose value equals the number of inner aromatic sextets (n(i)) along the oxidizing line. The nanographene fragments contained in this isomer have a ferromagnetic spin coupling. Due to the tautomerization between the high-spin and low-spin conformations, there also exist other possible isomers with higher energies and with spins at ground state ranging from 0 to (n(i) - 1). The rich geometrically correlated spins and the adjustable energy gaps indicate great potential of the graphene oxides in spintronic devices.

The magnetic structure of clinopyroxene-type LiFeGe 2O 6 and revised data on multiferroic LiFeSi 2O 6

The clinopyroxene compounds LiFeSi 2O 6 and LiFeGe 2O 6 have been investigated by constant wavelength neutron diffraction at low temperatures and by bulk magnetic measurements. Both compounds are monoclinic, space group P2 1/ c and do not exhibit a change in nuclear symmetry down to 1.4 and 5 K respective. However, they transform to a magnetically ordered state below 20 K. LiFeSi 2O 6 shows a simple magnetic structure with no indication of an incommensurate modulation. The magnetic space group is P2 1/ c′ and the structure is described by a ferromagnetic coupling of spins within the infinite M1 chains of edge-sharing octahedra, while the coupling between these M1 chains is antiferromagnetic. The magnetic phase transition is accompanied by magnetostriction of the lattice when passing through the magnetic phase transition. The magnetic structure of LiFeGe 2O 6 is different to the silicate: the space group is P 2 1 ′ / c and the magnetic unit cell doubled along the a-direction. Within the M1 chains spins are coupled antiferromagnetically, while the chain to chain coupling is antiferromagnetic when coupling goes via the Ge B tetrahedron and ferromagnetic when it goes via the Ge A tetrahedron.

Ferromagnetic Spin Coupling as the Origin of 0.7 Anomaly in Quantum Point Contacts

We study one-dimensional itinerant electron models with ferromagnetic coupling to investigate the origin of the 0.7 anomaly in quantum point contacts. Linear conductance calculations from the quantum Monte Carlo technique for spin interactions of different spatial range suggest that 0.7(2e;{2}/h) anomaly results from a strong interaction of low-density conduction electrons to ferromagnetic fluctuations formed across the potential barrier. The conductance plateau appears due to the strong incoherent scattering at high temperature when the electron traversal time matches the time scale of dynamic ferromagnetic excitations.

A cyclic tetranuclear Ni2Gd2 complex bridged by amino acidato ligands, with an S = 9 ground state, derived from ferromagnetic spin-coupling between nickel(ii) and gadolinium(iii) ions

Synthesis and structure of a cyclic tetranuclear Ni(2)Gd(2) complex bridged by amino acidato ligands, with an S = 9 spin ground state, derived from ferromagnetic spin-coupling between S(Gd) = 7/2 and S(Ni) = 1 are reported.

First Principles Study of NO and NNO Chemisorption on Silicon Carbide Nanotubes and Other Nanotubes.

Using methods based on first principles, we find that NO and NNO molecules can be chemisorbed on silicon carbide nanotubes (SiCNTs) with an appreciable binding energy and that this is not the case for either carbon nanotubes (CNTs) or boron nitride nanotubes (BNNTs). A detailed analysis of the energetics, geometry, and electronic structure of various isomers of the complexes was performed. The adsorption energy (∼-0.7 eV) is larger for the SiCNT-NO complex. The complex exhibits magnetism, and a ferromagnetic coupling of spins is observed when more than one NO molecule is adsorbed. This observation suggests that magnetic properties can be used to sense the amount of NO molecules adsorbed. The SiCNT-NNO complex is a nonmagnetic system in which five-membered rings form at the binding site.

Exchange interactions and critical temperature of bulk and thin films of MnSi: A density functional theory study

Recent theoretical work [H. Wu et al., Phys. Rev. Lett. 92, 237202 (2004); M. Hortamani et al., Phys. Rev. B 74, 205305 (2006); M. Hortamani, Ph.D. thesis, Freie Universit\at, Berlin, 2006] predicted ferromagnetism at zero temperature in thin MnSi films of B2-type crystal structure on Si(100). The relevance of this finding for finite-temperature experiments needs to be clarified by further investigations, since bulk MnSi is a weak ferromagnet with an experimentally measured Curie temperature of only ${T}_{c}=30\text{ }\text{K}$, and ${T}_{\text{c}}$ is generally expected to be lower in thin films than in bulk materials. Here, we estimate ${T}_{c}$ of such MnSi films using a multiple-sublattice Heisenberg model with first- and second-nearest-neighbor interactions determined from density functional theory calculations for various collinear spin configurations. The Curie temperature is calculated either in the mean-field approximation (MFA) or in the random-phase approximation (RPA). In the latter case we find a weak logarithmic dependence of ${T}_{\text{c}}$ on the magnetic anisotropy parameter, which was calculated to be 0.4 meV for this system. In stark contrast to the above mentioned rule, large Curie temperatures of above 200 K for a monolayer (ML) MnSi film and above 300 K for a two ML MnSi film with B2-type structure on Si(100) are obtained within the RPA, and even higher values in MFA. Complementary calculations of MnSi bulk structures and thin unsupported MnSi films are performed in order to analyze these findings. We find that bulk MnSi in the cubic B2 structure is paramagnetic, in contrast to MnSi in the B20 ground-state structure in agreement with the Stoner criterion. In a tetragonally distorted B2 structure, the Mn atoms gradually develop a spin magnetic moment, passing through a low-spin and a high-spin state. However, the ferromagnetism of the MnSi/Si(100) films cannot be explained by tetragonal distortions alone, since the distorted B2 bulk structure is found to order antiferromagnetically. Comparison of the calculations of supported and unsupported films suggests that the reduced coordination of Mn atoms near surfaces and interfaces is crucial for the ferromagnetic ground state of the films. The coordination number of the Mn atoms in B2-type MnSi films on Si(100) constitutes a borderline case, where the spin magnetic moments of Mn are still large despite their sixfold coordination to Si, but the $sp\text{\ensuremath{-}}d$ hybridization with Si states gives rise to a sizable ferromagnetic coupling of the Mn spins. We conclude that the Curie temperatures predicted from the Heisenberg Hamiltonian make thin MnSi films an interesting subject for further experimental investigation of spintronics materials.

Magnetic ordering and spin structure in Ca-bearing clinopyroxenes Ca M2+(Si, Ge) 2O 6, M=Fe, Ni, Co, Mn

The compounds CaFeSi 2O 6 (hedenbergite), CaNiGe 2O 6, CaCoGe 2O 6 and CaMnGe 2O 6 have been synthesized by hydrothermal or ceramic sintering techniques and were subsequently characterized by SQUID magnetometry and powder neutron diffraction in order to determine the magnetic properties and the spin structure at low temperature. All four compounds reveal the well-known clinopyroxene structure-type with monoclinic symmetry, space group C2/ c, Z=4 at all temperatures investigated. Below 35 K hedenbergite shows a ferromagnetic (FM) coupling of spins within the infinite M1 chains of edge-sharing octahedra. This FM coupling dominates an antiferromagnetic (AFM) coupling between neighbouring chains. The magnetic moments lie within the a– c plane and form an angle of 43° with the crystallographic a-axis. Magnetic ordering in CaFeSi 2O 6 causes significant discontinuities in lattice parameters, Fe–O bond lengths and interatomic Fe–Fe distances through the magnetic phase transition, which could be detected from the Rietveld refinements of powder neutron diffraction data. CaCoGe 2O 6 and CaNiGe 2O 6 show magnetic ordering below 18 K, the spin structures are similar to the one in hedenbergite with an FM coupling within and an AFM coupling of spins between the M1 chains. The moments lie within the a– c plane. The paramagnetic Curie temperature, however, decreases from CaFeSi 2O 6 (+40.2 K) to CaCoGe 2O 6 (+20.1 K) and CaNiGe 2O 6 (−13.4 K), suggesting an altered interplay between the concurring AFM and FM interaction in and between the M1 chains. CaMnGe 2O 6 finally shows an AFM ordering below 11 K. Here the magnetic moments are mainly oriented along the a-axis with a small tilt out from the a– c plane.

Fullerene complexes with divalent metal dithiocarbamates: structures, magnetic properties, and photoconductivity

A series of complexes of fullerenes C60 and C70 with metal dithiocarbamates {MII(R2dtc)2}·Cm (m = 60 or 70) and metal dithiocarbamates coordinated to nitrogen-containing ligands (L), {MII(R2dtc)2)x·L}·C60 (x = 1 or 2), where M = Cu, Zn, Cd, Hg, Mn, or Fe, R = Me, Et, Prn, Pri, or Bun, L is 1,4-diazabicyclo[2.2.2]octane (DABCO), N,N′-dimethylpiperazine, or hexamethylenetetramine, were synthesized. The shape of dithiocarbamate molecules is sterically compatible with the spherical shape of C60, resulting in an efficient interaction between their π systems. The resulting compounds are characterized by a layered or three-dimensional packing of the fullerene molecules. In the C60 complexes, iron(II) and manganese(II) dithiocarbamates exist in the high-spin states (S = 2 and 5/2). The magnetic susceptibility of {MII(Et2dtc)2}2·Cm (M = Fe or Mn, m = 60 or 70) in the temperature range of 200–300 K is described by the Curie-Weiss law with Θ = −250 and −96 K and with maxima at 110 and 46 K, respectively, which is indicative of a strong antiferromagnetic spin coupling between MII. The Weiss constants for the [{MII(Et2dtc)2}2·DABCO]·C60·(DABCO)2 complexes (M = Fe or Mn) are 1.7 and 0.3 K, respectively. The magnetic moments of the complexes containing Fe and Mn dithiocarbamates slightly increase at temperatures below 50 and 35 K, respectively, which is evidence of the ferromagnetic spin coupling between MII in {MII(Et2dtc)2}2·DABCO. Single crystals of the complexes exhibit low dark conductivity (10−10–10−11 S cm−1). The visible light irradiation of these crystals leads to an increase in the photocurrent by two–three orders of magnitude. The photogeneration of free charge carriers in the complexes occurs both due to the photoexcitation of metal dithiocarbamate (CuII(Et2dtc)2) and through the charge transfer from metal dithiocarbamate (MII(Et2dtc)2, M = Zn or Cd) to C60.

Mn- and Fe- doped GaN for spintronic applications

AbstractIn the context of ferromagnetic spin-coupling in dilute magnetic semiconductors, we present optical investigations on Mg co-doped GaMnN and Fe doped GaN. A complex luminescence feature occurring in Mg co-doped GaMnN around 1 eV was previously attributed to the internal 4T2(F)—4T1(F) transition of Mn4+ involved in different complexes. Selective excitation studies indicate the presence of at least three different complexes. Photoluminescence excitation spectra suggest that the internal Mn3+ transition may represent an excitation mechanism. Magneto photoluminescence spectra indicate equal g values for the ground and excited state. Low temperature infrared absorption spectra of Fe doped GaN allow to unambiguously establish the electronic structure of the Fe2+ center in GaN. Our results suggest that the Fe2+(5T2) state is stabilized against Jahn-Teller coupling by the reduced site-symmetry of the hexagonal lattice.

Magnetic Couplings in Vanadium Aromatic Sandwich Complexes and Their Crystals by Using DFT Methods

Using density functional theory, we have theoretically studied the origin of ferromagnetic coupling of spins in the long multiple deckers of vanadium sandwich complexes with benzene (Bz). This is done by calculating the band structure of their infinite one-dimensional (1D) crystals along the periodic (=Z) direction. We find that the magnetic coupling can be ascribed to spin polarization in the band (= those derived from d(z2)) whose k states are not involved in the formation of partial covalent bonds between vanadium atoms and benzene rings. We have also studied magnetic and electronic properties of multiple deckers of a naphthalene (Np)-vanadium complex. We find that there is a stronger covalent interaction as well as a stronger electrostatic interaction between V and Np rings than between V and Bz rings. This suggests that there is a possibility of longer multiple deckers of a Np-V complex. In addition, ferrimagnetic coupling of spins is expected, resulting in a magnetic moment 45% larger than that of the Bz-V complex at the same length. For their 1D crystals, band structure analysis also shows that the origin of magnetic coupling in long multiple deckers should be similar to that of Bz-V multiple deckers.

Dinuclear Copper(II) Complexes of Two Homologous Pyrazine‐Based Bis(terdentate) Diamide Ligands

AbstractThe 1:1 reactions of the new bis(terdentate) diamide ligandN,N'‐bis(2‐pyridylmethyl)pyrazine‐2,5‐dicarboxamide(H2L1) and its higher homologue N,N'‐bis[2‐(2‐pyridyl)ethyl]pyrazine‐2,5‐dicarboxamide (H2L2) with Cu(BF4)2·4 H2O in the absence of added base have consistently afforded dicopper(II) complexes of the doubly deprotonated ligands (L1)2– and (L2)2–. The complex [CuII2(L2)(H2O)2(MeCN)2](BF4)2 (2a) has been structurally characterised. Subsequently, reactions employing a correct stoichiometric 2:1 metal‐to‐ligand ratio in MeCN have afforded bulk samples of the dinuclear complexes. The compounds [CuII2(L1)(MeCN)2(H2O)2](BF4)2·H2O (1a·H2O) and [CuII2(L2)(H2O)4(BF4)2]·2 H2O (2b·2 H2O) have been structurally characterised. While complex 1a·H2O of the lower ligand homologue exhibits very weak antiferromagnetic spin coupling (J = –0.24 cm–1), complex 2b of the higher ligand homologue exhibits very weak ferromagnetic spin coupling (J = +0.67 cm–1). EPR studies have been carried out on polycrystalline powders and frozen DMF solutions of 1a·H2O and 2b·2 H2O. The EPR spectra of the polycrystalline powders indicate the presence of dipolar broadening and weak intermolecular exchange, while those of the frozen DMF solutions are characteristic of dipolar‐coupled CuII pairs within the dinuclear molecules, with no evidence of intra‐ or intermolecular exchange. The spectral simulations confirm that the binuclear structure and the Cu···Cu distances are retained in frozen solution. Dinuclear SiF62– containing compounds, [CuII2(L1)(H2O)4](SiF6) (1b) and {[CuII2(L1)(H2O)2(μ‐SiF6)]·4 H2O}∞ (1c·4 H2O), were obtained serendipitously, in nearly quantitative yield, by the 2:1 reaction of Cu(BF4)2·4 H2O with H2L1 in H2O. The unexpected SiF62– anions were generated in the course of the reaction by partial hydrolysis of the BF4– anions employed, thus forming traces of HF which reacted with the glassware. (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2005)