Antiferromagnetic Exchange Constant Research Articles

Trinuclear nickel (II) string complexes and copper (II) coordination polymer with pyrazine modulated unsymmetrical dipyridylamino ligand: Synthesis, structure and bioactivity properties with molecular docking

The unsymmetrical pyrazine-modulated ligand, N-(pyridine-2-yl)pyrazin-2-amine (Hppza) (1), its 1D straight-chain copper (II) coordination polymer [Cu2(Hppza)2∙(NO3)4]n (2) and trinuclear metal string [Ni3(µ3-ppza)4Cl2] (3) and [Ni3(µ3-ppza)4(NCS)2] (4) complexes have been successfully synthesized and structurally characterized. The free ligand Hppza (1) exhibits anti-syn conformation. In an asymmetric unit of 2, there are two kinds of Cu(II) atoms, both of which have elongated octahedral coordination geometry, and the copper(II) centers are linked by oxygen atoms of one nitrate anion. There are only minor differences in Ni…Ni bond lengths between pyrazine-modulated ligand complexes of [Ni3(μ3-ppza)4×2] (X = Cl− (3), NCS− (4)) and their unmodulated analogs, while the distances between the terminal nickel atoms and the axial ligands 3 and 4 are shorter. The results of magnetic susceptibility measurements showed an antiferromagnetic exchange constant (J) of about J = –199.2 cm–1 for 3 and J = –178.0 cm–1 for 4. The electrochemical properties of nickel strings of Hppza differ significantly from their unmodulated counterparts. The activities of ligands and their metal complexes against various proteins that are AChE (PDB ID: 4M0E), BChE (PDB ID: 5NN0), α-Gly (PDB ID: 1UAS), hCA I (PDB ID: 2CAB), and hCA II (PDB ID: 3DC3) were compared, and then the ADME/T properties of molecules were examined. Afterward, the interactions occurring in the docking calculation were examined in detail with the Protein-Ligand Interaction Profiler (PLIP) service. These complexes were inhibitors of BChE, hCA I and II, α-glycosidase and AChE enzymes for complex 3 and 5 with Ki values of 12.65 and 42.95 μM for hCA I, 3.64 and 15.84 μM for hCA II, 5.44 and 50.07 μM for BChE, 15.85 and 4.77 μM for α-glycosidase and 8.58 and 86.24 μM for AChE, respectively.

Determination of the tricritical point, H–T phase diagram and exchange interactions in the antiferromagnet MnTa2O6

Using the analysis of the temperature and magnetic field dependence of the magnetization (M) measured in the temperature range of 1.5 K to 400 K in magnetic fields up to 250 kOe, the magnetic field-temperature (H–T) phase diagram, tricritical point and exchange constants of the antiferromagnetic MnTa2O6 are determined in this work. X-ray diffraction/Rietveld refinement and x-ray photoelectron spectroscopy of the polycrystalline MnTa2O6 sample verified its phase purity. Temperature dependence of the magnetic susceptibility χ (=M/H) yields the Néel temperature T N = 5.97 K determined from the peak in the computed ∂(χT)/∂T vs T plot, in agreement with the T N = 6.00 K determined from the peak in the C P vs T data. The experimental data of C P vs T near T N is fitted to C P = A|T − T N|−α yielding the critical exponent α = 0.10(0.13) for T > T N (T < T N). The χ vs T data for T > 25 K fits well with the modified Curie–Weiss law: χ = χ 0 + C/(T − θ) with χ 0 = −2.12 × 10−4 emu mol−1 Oe−1 yielding θ = −24 K, and C = 4.44 emu K mol−1 Oe−1, the later giving magnetic moment μ = 5.96 μ B per Mn2+ ion. This yields the effective spin S = 5/2 and g = 2.015 for Mn2+, in agreement with g = 2.0155 measured using electron spin resonance spectroscopy. Using the magnitudes of θ and T N and molecular field theory, the antiferromagnetic exchange constants J 0/k B = −1.5 ± 0.2 K and J ⊥/k B = −0.85 ± 0.05 K for Mn2+ ions along the chain c-axis and perpendicular to the c-axis respectively are determined. The χ vs T data when compared to the prediction of a Heisenberg linear chain model provides semiquantitative agreement with the observed variation. The H–T phase diagram is mapped using the M–H isotherms and M–T data at different H yielding the tricritical point T TP (H, T) = (17.0 kOe, 5.69 K) separating the paramagnetic, antiferromagnetic, and spin-flop phases. At 1.5 K, the experimental magnitudes of the exchange field H E = 206.4 kOe and spin-flop field H SF = 23.5 kOe yield the anisotropy field H A = 1.34 kOe. These results for MnTa2O6 are compared with those reported recently in the isostructural MnNb2O6.

Temperature dependence of dipole-induced exchange bias

A modified Monte Carlo method is used to study the dependence of exchange bias, induced by long-range ferromagnet/antiferromagnet interfacial dipolar interactions, on temperature after field cooling. Since sufficient nonzero surplus magnetization in the antiferromagnetic layer is preserved, a positive exchange bias field is yielded. Significantly, this exchange field increases with decreasing temperature and may level off at low temperatures. Then, the antiferromagnetic anisotropy constant, easy-axis direction with respect to the cooling-field direction, antiferromagnetic exchange constant, and antiferromagnetic layer thickness were modulated to study their roles in establishing the low-temperature plateau-like exchange bias field. A thick enough antiferromagnetic layer with the easy-axis direction aligning with the cooling field maximizes the plateau height with a large antiferromagnetic anisotropy constant, while a small antiferromagnetic exchange constant greatly widens the plateau, even from the exchange bias blocking temperature to the lowest temperature. On explicitly calculating the surplus magnetization values in the antiferromagnetic layer meanwhile the dipolar and Zeeman energies in the antiferromagnetic layer, it is found that the ferromagnet/antiferromagnet interfacial dipolar interactions are predominant at the descending branch of the loop to suppress the coercive field with decreasing temperature; in contrast, the magnetic field takes over the lead at the ascending branch and monotonically enhances the coercive field, at the same pace as the decrease in the coercive field at the descending branch. As a consequence, the loop retains a constant shift and becomes wider with decreasing temperature. The long-range noncontact exchange bias that is insensitive to temperature may be used to develop thermal-agitation-resistant spintronic devices with unidirectional anisotropy.

Distant exchange interactions in Cd1−xMnxS from magnetization steps method

Three new distant neighbor (DN) antiferromagnetic exchange constants have been measured in wurtzite Cd1–xMnxS (x = 0.0087 and x = 0.0278) using the magnetization steps method in the millikelvin regime. The second (J(2)), third (J(3)), and fourth (J(4)) largest exchange constants (after the two first neighbor exchange interactions J1 and J1′) have been measured. The results are J(2) = 300 ± 10 mK, J(3) = 173 ± 10 mK, and J(4) = 55 ± 10 mK. No significant change of the exchange constant values has been observed as a function of the Mn concentration. The mapping of the DN exchange interactions has been investigated within a sphere of radius 2 times the nearest neighbors distance. The only way to identify the J constants was using the number coordination Zn of the different DN classes: J(2) is associated to DN classes with Zn = 6, J(3) with Zn = 12, and J(4) is ascribed to a group of three DN classes with J values around J(4). The magnitude of the J constants is not in agreement with theoretical predictions of the monotonic decrease of the exchange constant with increasing distance. The present results are similar to those obtained for Cd1–xMnxSe but are different from the ones obtained for Zn1–xMnxO. The present work confirms the long-range character of the DN exchange interactions in the wurtzite type Mn-based II-VI diluted magnetic semiconductors.

Quantum chemical calculation of exchange interactions in supramolecularly arrangedN,N′-dioxy-2,6-diazaadamantane organic biradical

Quantum mechanical exchange effects in purely organic N,N′-dioxy-2,6-diazaadamantane biradical derivatives with promesogenic substituents have been studied. To determine intermolecular exchange energies, packing conditions of the radical core units in layered liquid crystalline phases are simulated using the Gaussian 09 program. The broken symmetry approach gives J ≈ 7 cm−1 for intramolecular ferromagnetic exchange interactions between nitroxyl radical centers in one molecule. Both ferromagnetic and antiferromagnetic intermolecular interactions are possible in this kind of systems according to the obtained calculation results. Depending on the mutual positioning and orientation of molecules, the intermolecular antiferromagnetic exchange constant can reach a value of −50 cm−1, and the intermolecular ferromagnetic constant a value of 10 cm−1. The simultaneous presence of intramolecular and intermolecular exchange between spin-carrying centers in this kind of supramolecularly ordered multispin systems is favorable for the formation of magnetically interacting chains and two-dimensional networks. © 2016 Wiley Periodicals, Inc.

Optical and magnetic resonance study of a-SiC x N y films obtained by magnetron sputtering

Amorphous silicon carbonitride (a-SiCxNy) thin films deposited on the SiO2 substrates by reactive magnetron sputtering was studied by Raman and electron paramagnetic resonance (EPR) spectroscopy. Raman analysis indicates the presence of C–N, Si–N, C–C bonds in a-SiCxNy films. Three EPR signals were revealed in a-SiCxNy/SiO2. One of them with g = 2.0033 was attributed to the carbon-dangling bonds (CDB). Based on the lineshape and linewidth, the EPR signal was attributed to the unpaired electron delocalized over sp2 carbon cluster. With the increase of nitrogen (N) content, the spin density of CDB significantly increases. From the temperature dependence of the linewidth and integral intensity of the CDB EPR signal, it was concluded that the antiferromagnetic ordering occurs in spin system. The antiferromagnetic exchange constant between CDBs was found to be J = −32 K. The second EPR signal having g = 2.009 was attributed to the interface defect representing threefold-coordinated Si dangling bond, which may appear due to the formation of the oxidized Si on the film surface. The third EPR signal with g = 2.05 was tentatively attributed to the trapped holes at Si atom near Si/SiO2 interface. The N incorporation in a-SiCx/SiO2 has no effect on the spin density of both interface defects.

Quantum critical behavior in three-dimensional one-band Hubbard model at half-filling

A one-band Hubbard model with hopping parameter t and Coulomb repulsion U is considered at half-filling. By means of the Schwinger bosons and slave fermions representation of the electron operators and integrating out the spin–singlet Fermi fields an effective Heisenberg model with antiferromagnetic exchange constant is obtained for vectors which identifies the local orientation of the spin of the itinerant electrons. The amplitude of the spin vectors is an effective spin of the itinerant electrons accounting for the fact that some sites, in the ground state, are doubly occupied or empty. Accounting adequately for the magnon–magnon interaction the Néel temperature is calculated. When the ratio tU is small enough (tU≤0.09) the effective model describes a system of localized electrons. Increasing the ratio increases the density of doubly occupied states which in turn decreases the effective spin and Néel temperature. The phase diagram in the plane of temperature TNU and parameter tU is presented. The quantum critical point (TN=0) is reached at tU=0.9. The magnons in the paramagnetic phase are studied and the contribution of the magnons’ fluctuations to the heat capacity is calculated. At the Néel temperature the heat capacity has a peak which is suppressed when the system approaches a quantum critical point. It is important to stress that, at half-filling, the ground state, determined by fermions, is antiferromagnetic. The magnon fluctuations drive the system to quantum criticality and when the effective spin is critically small these fluctuations suppress the magnetic order.

Infinite Copper(II) Coordination Architectures from a Resonative Aminotriazine-Derived Tripodal Ligand: Synthesis, Structures, and Magnetic Properties

The ligand 2,4,6-tris(2-picolylamino)-1,3,5-triazine (o-H3tpat) with essentially resonative structure and two copper(II)-based one-dimensional coordination chain structures, [Cu3Cl5(o-H2tpat)(H2O)]·MeOH·CH2Cl2 (1) and [Cu2(o-H2tpat)(H2O)(MeOH)(NO3)2](NO3)·3MeOH (2), with different structural patterns have been synthesized and characterized using single crystal X-ray diffraction analysis. For o-H3tpat, two crystalline forms showing different solid-state structural features are obtained from MeOH/Et2O (form I) and CH2Cl2/Et2O (form II), respectively. The o-H3tpat form I adopts an asymmetric-configured all-amino resonative tautomer with three cis-trans-trans-arranged pyridyl groups, whereas the o-H3tpat form II adopts also an identical resonative structure but where two of the three pyridyl groups are in a cis-manner and the third one is nearly coplanar with the central aminotriazine core. On the other hand, the designed tripodal ligand in both Cu(II)-complexes serves as a monoanion, o-H2tpat(-), which suits a propeller-configured all-imino resonative structure in 1 and a syn-anti-configured amino-imino-imino resonative structure in 2. These observations significantly indicate that the o-H3tpat ligand can self-adjust and interconvert its conformation via a possible structure transformation associated with proton-shift to adapt a change in the crystallization and self-assembly reaction systems. In the magnetic point of view, 1 is treated as repeated chains composed of infinite {Cu6Cl10} units wherein the hexanuclear unit is further decomposed to one {Cu(II)4Cl6} and two magnetically isolated {Cu(II)Cl2} subunits. Antiferromagnetic interactions are found for the Cu4 subunits (g = 2.33, 2J1 = -5.6 cm(-1), 2J2 = -8.6 cm(-1), 2J3 = -4.1 cm(-1), and J4 held to zero). For 2, it is considered as an infinite chain that composes of Cu2 units antiferromagnetically coupled (g = 2.03, 2J1 = -0.2 cm(-1)). The small antiferromagnetic exchange constants in both 1 and 2 suggest that the unpaired spins do not effectively interact through the tripodal o-H2tpat(-) ligands.

A Combined Experimental and Computational Study of the Magnetic Superexchange within a Triangular (μ3‐O)‐Pyrazolato‐FeIII3 Complex

AbstractThe triangular complex [FeIII3(μ3‐O)(μ‐4‐O2N‐C3H2N2)6Br3]2– has been characterized and studied by magnetic susceptibility measurements, 57Fe Mössbauer and electron paramagnetic resonance (EPR) spectroscopic methods. A combination of experimental and computational results has allowed the differentiation between two sets of acceptable isotropic exchange constants. The magnetic exchange within this complex has been modelled by a spin Hamiltonian involving antiferromagnetic exchange constants of average value Jav = –81.4 cm–1 and an antisymmetric exchange |dz| = 1.40 cm–1.

Phase diagrams of La1−xCaxMnO3 in double exchange model with added antiferromagnetic and Jahn-Teller interaction

The phase diagram of the multivalent manganites La1−x Ca x MnO3, in space of temperature and doping x, is a challenge for the theoretical physics. It is an important test for the model used to study these compounds and the method of calculation. To obtain theoretically this diagram for x < 0.5, we consider the two-band Double Exchange Model for manganites with added Jahn-Teller coupling and antiferromagnetic Heisenberg term. In order to calculate Curie and Néel temperatures we derive an effective Heisenberg model for a vector which describes the local orientation of the total magnetization of the system. The exchange constants of this model are different for different space directions and depend on the density of e g electrons, antiferromagnetic constants and the Jahn-Teller energy. To reproduce the well known phase transitions from A-type antiferromagnetism to ferromagnetism at low x and C-type antiferromagnetism to G-type antiferromagnetism at large x, we argue that the antiferromagnetic exchange constants should depend on the lattice direction. We show that ferromagnetic to A-type antiferromagnetic transition results from the Jahn-Teller distortion. Accounting adequately for the spin fluctuations, Curie and Néel temperatures are calculated. The results are in very good agreement with the experiment and provide values for the model parameters, which best describe the behavior of the critical temperature for x < 0.5.

Comparison of Semiempirical ZILSH and DFT Calculations of Exchange Constants in Fe4 Butterfly Complexes.

Magnetic interactions in a series of tetranuclear Fe(3+) complexes with the butterfly core structure have been studied with semiempirical ZILSH and density functional theory (DFT) calculations (B3LYP functional). A theoretical analysis of a previously used method of estimating exchange constants from a restricted number of spin configurations reveals systematic errors arising from asymmetry in the complexes, which cause large variations in results with different choices of spin configurations. Correction factors are derived that yield the correct results obtained from full configuration space (FCS) calculations. Exchange constants obtained from DFT FCS calculations for the "body-body" interaction were large and ferromagnetic, in disagreement with values obtained from empirical fits of magnetic susceptibility data for the complexes, established magnetostructural correlations in polynuclear Fe(3+) complexes, and ZILSH calculations. DFT calculations also gave unreasonably large antiferromagnetic exchange constants for interaction between "wingtip" ions that are not directly bridged, again in disagreement with ZILSH calculations. Estimates of exchange constants for interaction of body and wingtip ions obtained with ZILSH and DFT were similar, with the ZILSH values in slightly better agreement with empirical fits. Considering all interactions, the ZILSH method provides results in better accord with experiment than DFT for these complexes. Additional comparisons of exchange constants obtained with different spin coupling schemes showed that values appropriate for two-center spin eigenfunctions gave consistently better results than values calculated with the local spin operator. The effect of basis set was found to be very small. A brief analysis of these findings is given.

Anisotropic exchange interactions in [formula omitted] dinuclear systems (Ln 3+ = Dy, Tm, Yb): Magnetometry and Dual Mode X-band Electron Paramagnetic Resonance spectroscopic study

A recently developed experimental and theoretical procedure is used in order to calculate the magnitude and anisotropy of interaction between a lanthanide and a 3d-metal ion. The general formula of the molecular compounds is [Ln(H 2O) 3(dmf) 4(μ-CN)Fe–(CN) 5] · nH 2O where 1 ⩽ n ⩽ 1,5 and dmf = N, N′-dimethylformamide, abbreviated as [LnFe] from now on. The main parts of this procedure are (a) the evaluation of the effective g-parameters of the lanthanide ion with the help of EPR measurements. (b) The use of dual mode EPR spectroscopy to define the anisotropic exchange interactions with the help of an anisotropic Hamiltonian model. (c) Use of the same magnetic model to fit magnetization and susceptibility data in order to verify the EPR findings. It was possible to define some trends concerning the exchange components of the [DyFe] dimer according to which the antiferromagnetic isotropic exchange constant is smaller than 4 cm −1 and the anisotropic components are [ D exc, E exc] = [⩾6(1), 0.0] cm −1. Also for the case of [TmFe] and [YbFe] dimers the antiferromagnetic isotropic exchange constant is smaller than 0.3 cm −1 while the anisotropic components are [ D exc, E exc] = [⩾12.0, 0.0] cm −1 and [ D exc, E exc] = [0.4(1), 0.0] cm −1, respectively.

Anisotropic exchange interactions in hetero-one-dimensional Ln 3+–M 3+ systems (Ln 3+ = Er, Yb; M 3+ = Cr, Fe LS): Magnetometry and Dual Mode X-band Electron Paramagnetic Resonance spectroscopic studies

A recently introduced experimental and theoretical procedure is presented in order to calculate the magnitude and anisotropy of interaction between a lanthanide and a 3d-metal ion. The general formula of the hetero-one-dimensional molecular compounds is trans-[M(CN) 4(μ-CN) 2Ln(H 2O) 4(bpy)] n · nH 2O · 1.5 nbpy (M = Fe 3+, Cr 3+; Ln = Er 3+, Yb 3+), where bpy = 2,2′-bipyridine, abbreviated as [LnM] from now on. The main parts of this procedure are: (a) the evaluation of the effective g-parameters of the lanthanide ion with the help of EPR measurements; (b) the use of dual-mode EPR spectroscopy to define the anisotropic exchange interactions with the help of an anisotropic Hamiltonian model; (c) use of the same magnetic model to fit magnetization and susceptibility data in order to verify the EPR findings. It was possible to define some trends concerning the exchange components of the [ErCr] chain according to which the antiferromagnetic isotropic exchange constant is smaller than 0.7(1) cm −1 and the anisotropic components are [ D exc, E exc] = [−0.4(1), 0.0(1)] cm −1. Also for the case of [YbCr] and [YbFe] chains the antiferromagnetic isotropic exchange constant is smaller than 0.1(1) cm −1 and 0.25(1) cm −1, respectively while the anisotropic components are [ D exc, E exc] = [−0.3(1), 0.0(1)] cm −1 and [ D exc, E exc] = [−0.1(1), 0.0(1)] cm −1, respectively.

Magnetic behavior of manganese-doped ZnSe quantum dots

Magnetic properties of manganese-doped ZnSe quantum dots with the size of approximately 3.6 nm are investigated. The amount of Mn in the ZnSe quantum dots has been varied from 0.10% to 1.33%. The doping level in the quantum dots is much less than that used in the precursor. The co-ordination of Mn in the ZnSe lattice has been determined by electron paramagnetic resonance (EPR). Two different hyperfine couplings 67.3×10−4 and 60.9×10−4 cm−1 observed in the EPR spectrum imply that Mn atoms occupy two distinct sites; one uncoordinated (near the surface) and other having a cubic symmetric environment (nanocrystal core), respectively. Photoluminescence measurements also confirm the incorporation of Mn in ZnSe quantum dots. From the Curie-Weiss behavior of the susceptibility, the effective Mn-Mn antiferromagnetic exchange constant (J1) has been evaluated. The spin-glass behavior is observed in 1.33% Mn-doped ZnSe quantum dots, at low temperature. Magnetic behavior at a low temperature is discussed.

Magnetic properties of a Ni tetramer with a butterfly structure in high magnetic fields

We have performed magnetic susceptibility and low and high field magnetization measurements on single crystals of the Ni tetramer compound [Ni4(μ-CO3)2(aetpy)8] (ClO4)4 (aetpy = 2-aminoethyl-pyridine) with a butterfly (diamond) structure. We observed a two-step-like magnetization curve and a further increase of magnetization around 63 T in magnetic fields up to 68 T perpendicular to the Ni tetramer plane (c-axis). From the magnetic fields at steep jumps of the magnetization, we determine broadly the short diagonal (J1) and the side (J2) antiferromagnetic exchange constants between the Ni ions (J1/kB = 41.9 K and J2/kB = 9.2 K) in this compound. The long diagonal exchange interaction (J3) is ferromagnetic and the exchange constant is evaluated as J3/kB = −0.66± 0.04 K from the comparison between the magnetization measured in steady fields at 2 K and the calculated one. The origin of hysteresis of the magnetization in pulsed magnetic fields is discussed based on the calculated energy diagram.

Synthesis and characterization of polyureasilazane derived SiCN ceramics

Samples of SiCN ceramics were synthesized by thermal treatment of commercially available CERASET™ polyureasilazane, used as liquid-polymer precursor, at pyrolysis temperatures of 1000, 1050, 1100, 1150, and 1200°C. Electron paramagnetic resonance (EPR) signals due to sp2-hybridized carbon-related dangling bonds were recorded over the 4–300K temperature range at X band (9.6GHz), and the spectra showed the presence of an intense EPR line with g=2.0027 at room temperature for all samples; at liquid helium temperature an additional line was seen present as a shoulder to main line. These two signals are due to carbon-related dangling bonds present as (i) defects on the free-carbon phase and (ii) within the bulk of SiCN ceramic network. The value of the antiferromagnetic exchange constant between dangling bonds in the various samples, not hitherto available in the literature, was estimated from the temperature variation of the EPR linewidth to be anywhere from J=−12 to J=−15K in the samples synthesized at 1000, 1100, and 1150°C. The EPR linewidth of the samples decreased with increasing pyrolysis temperature of a sample, being 0.07mT at X band at room temperature for the samples synthesized at 1150 and 1200°C, which can be used potentially as stable g markers, due to their very narrow linewidth. The W-band (95GHz) room-temperature spectrum of the sample pyrolyzed at 1100°C exhibited only one line consistent with the X-band spectrum. At higher frequency of G band (170GHz) the EPR spectra are better resolved, clearly showing the presence of a strong (g=2.0027) and a weak (g=2.0032) EPR line at room temperature, and the latter corresponds to the line present at X band as shoulder at liquid-helium temperature. In addition, nuclear magnetic resonance measurements were carried out to confirm the existence of free-carbon phase and oxygen atoms in the samples.

Density functional theory of spin-coupled models for diiron-oxo proteins: Effects of oxo and hydroxo bridging on geometry, electronic structure, and magnetism

We have performed a comprehensive study of the electronic structure and magnetic properties of structurally characterized models for diiron-oxo proteins. Results from Kohn–Sham density functional theory show that two complexes, with formula Fe2(μ-O)(μ-O2CCH3)2(HBpz3)2 and [Fe2(μ-OH)(μ-O2CCH3)2(HBpz3)2]+, are strongly and weakly antiferromagnetically coupled, respectively, in agreement with experiment. The physical origin of the stronger and weaker exchange typically measured for oxo- and hydroxo-bridged diiron complexes, respectively, has been elucidated. The main superexchange pathways giving rise to molecular antiferromagnetism in both complexes have been identified. The dominant pathway in the oxo-bridged complex, Fe1(dxz):μ-O(px):Fe2(dxz), was formed by π interactions whereas that of the hydroxo-bridged, Fe1(dz2):μ-OH(p∥):Fe2(dz2), was formed by σ interactions. We also found a pathway mediated by the bridging acetates, Fe1(dx2−y2):bis(μ-acetato):Fe2(dx2−y2), which induces weak antiferromagnetism in the oxo-bridged complex but is significantly more important in the hydroxo-bridged complex. The antiferromagnetic exchange constants that parameterize the Heisenberg Hamiltonian H=JS1⋅S2 have been predicted for both, strongly and weakly, coupled complexes. Overall, the signs, trends, and magnitudes of the theoretical values (Jμ-Ocalc=+152.7 cm−1, Jμ-OHcalc=+23.3 cm−1) were in excellent agreement with experiment. The geometries of the complete molecular structures have been optimized in C2v symmetry and used to calculate molecular properties such as atomic charges and spin densities. The electronic configurations (Fe:4s0.293d5.93,μ-O:2s1.922p4.99;Fe:4s0.303d5.82,μ-OH:2s1.822p5.25,H:1s0.51) of the respective binuclear cores revealed relatively high occupancies for the nominally ferric ions, thus reflecting a donating character of their immediate N3O3 coordination. In addition, the diiron-oxo protein hemerythrin has been discussed. Theoretical and structural considerations indicated that the oxo-bridged diferric complex considered herein models extremely well the antiferromagnetic behavior of azidomet- and azidometmyo-hemerythrin. Finally, the magnetic behavior of closely related oxo-bridged diferric and hydroxo-bridged diferrous complexes containing Me3TACN capping ligands has been explained in light of the results presented in this work.

Effect of Fock exchange on the electronic structure and magnetic coupling in NiO

The effect of Fock exchange on the periodic description of the geometrical structure, elastic constants, and electronic and magnetic properties of NiO is analyzed. Hybrid density functionals which combine a portion of ``exact'' Fock exchange with conventional local density approximation (LDA) or generalized gradient approximation (GGA) functionals remedy a number of serious inconsistencies with the traditional LDA or GGA descriptions of this prototypical ``Mott'' insulator. For example, the hybrid B3LYP functional (which mixes \ensuremath{\sim}20% Fock exchange with GGA functionals) introduces a significant insulating gap and yields antiferromagnetic Heisenberg coupling constants between Ni sites ${(J}_{2})$ in semiquantitative agreement with experiment. Closer inspection shows that while the B3LYP orbital band gap is in excellent agreement with experiment, the magnitude of the antiferromagnetic coupling is overestimated by slightly more than 50%. This has led us to examine a simplified model which combines Fock exchange with the LDA exchange and correlation functionals. This combination allows us to study the magnitude and nature of the band gap, the magnitude of the unpaired spin densities in the different magnetic phases, and the two most important magnetic coupling constants as a function of the fraction of Fock exchange included. It is concluded that \ensuremath{\sim}35% Fock exchange gives a reasonably balanced description of all properties, including structural parameters, magnetic form factors, the antiferromagnetic Ni-Ni exchange constant, and the character and magnitude of the band gap.

Magnetization steps inMn0.084Zn0.916F2:Exchange constant and Mn distribution

The magnetization M of ${\mathrm{Mn}}_{0.084}{\mathrm{Zn}}_{0.916}{\mathrm{F}}_{2}$ was measured at 20 mK in magnetic fields H up to 115 kOe and at 0.65 K in fields up to 175 kOe. The magnetization was independent of magnetic field direction and showed no hysteresis. After a fast rise of M at low H, several magnetization steps (MS's) were observed. These MS's arose from ${\mathrm{Mn}}^{2+}$ pairs and triplets in which the spins are coupled by the dominant antiferromagnetic exchange constant, ${J}_{2}$ between next-nearest neighbors. The magnetic fields at the steps gave ${J}_{2}=\ensuremath{-}1.94\ifmmode\pm\else\textpm\fi{}0.01 \mathrm{K}$ in this diluted magnetic material. The observed magnetization curves are in very good agreement with computer simulations which assume a random Mn distribution. However, at 20 mK the widths of MS's are much larger than expected from thermal broadening alone. Possible mechanisms for nonthermal broadening are discussed. Theoretical results for cluster statistics, for ${J}_{2}$ clusters up to quintets, and for the fields at the MS's from the various quintet types are presented. These theoretical results also apply to nearest-neighbor ${(J}_{1})$ clusters in a body-centered-cubic cation lattice.

Distant-neighbors exchange constants in Cd 1− xMn xSe from magnetization steps

From magnetization measurements at T=20 mK on two bulk samples of Cd 1− x Mn x Se ( x=0.59% and x=1.15%), four distant-neighbors antiferromagnetic exchange constants have been measured. The results, in order of decreasing strength, after the two J's for nearest-neighbors pairs, are −0.35±0.01 K, −0.20±0.01 K, −0.10±0.01 K, and −0.08±0.01 K. These results confirm the long-range character of the distant-neighbor exchange interactions in the Mn-based II–VI diluted magnetic semiconductors.