Isotropic Exchange Parameters Research Articles

Theoretical investigation of the magnetic properties of ferromagnetic CrI3 monolayer

The CrI3 monolayer is the first ferromagnetic two-dimensional material experimentally discovered and the study of its properties is a hot topic. First-principles calculations and experimental measurements show that the CrI3 monolayer is a typical S=3/2 localized magnetic system. In the present work, we investigated the intrinsic magnetic properties of CrI3 monolayer based on the anisotropic Heisenberg model and using the double-time Green’s function method. The Hamiltonian parameter values of CrI3 monolayer are estimated theoretically based on the experimental Curie temperature of CrI3 monolayer and the experimental magnetic parameters of CrI3 bilayers. The spontaneous magnetization and zero-field susceptibility are calculated, and the critical behaviors of the system are analyzed. We observe that the maximum magnetic susceptibility value is sensitive to the isotropic exchange parameter and external magnetic field, but almost independent of anisotropic strengths.

Two Types of Hexanuclear Partial Tetracubane [Ni4Ln2] (Ln = Dy, Tb, Ho) Complexes of Thioether-Based Schiff Base Ligands: Synthesis, Structure, and Comparison of Magnetic Properties.

New Schiff base ligand H2L containing N2O4S donor atoms has been explored for its ability to provide complexes of selected 3d and 4f metal ions. Room temperature reaction of H2L with NiCl2·6H2O and Ln(NO3)3·5H2O in the presence of Et3N in MeCN-MeOH (2:1) medium resulted in [Ni4Ln2(L)2(μ-Cl)2(μ3-OH)4(H2O)6]Cl4·2H2O (where Ln = Dy3+ (1), Tb3+ (2), and Ho3+ (3) and H2L = 2-((2-(2-(2-hydroxy-3-methoxybenzylideneamino)ethylthio)ethylimino)methyl)-6-methoxyphenol). Use of Ni(SCN)2·4H2O during synthesis provided SCN- ions for bridging and terminal coordination in [Ni4Ln2(L)2(μ-NCS)2(μ3-OH)4(NCS)4(H2O)2]·xMeOH·yH2O (where Ln = Dy3+ (4), x = 2, y = 4; Tb3+ (5) and Ho3+ (6), x = 0, y = 14.1). All six complexes possess a hexanuclear defective tetracubane topology having exchangeable bridging groups. The study of direct current magnetic susceptibility measurements revealed that the Ni(II) ions are engaged in ferromagnetic interaction with the DyIII, TbIII, and HoIII ions and have significant magnetic anisotropy in all six complexes. Alternating current susceptibility measurements confirmed that both of the two types of compounds qualify as zero-field single-molecule magnets (SMMs), and the effective barrier for the reversal of the magnetic moment was found to be in the range Ueff = 23-31 K for 1-2 and 4-5, respectively. Detailed insight into the electronic structure and magnetic properties was calculated using DFT- and CASSCF-based analyses. The found isotropic exchange parameter (J) values are JNi-Ni = -4.7 cm-1 for 1 and JNi-Ni = +29.2 cm-1 for 4 and clearly indicate that the μ-NCS-bridge is a better candidate than μ-Cl for ferromagnetic exchange interactions. Out of the six complexes, only complex 5 displays TbIII centered emission peaks at 451 and 480 nm.

Ytterbium and Europium Complexes of Redox-Active Ligands: Searching for Redox Isomerism.

The reaction of (dpp-Bian)EuII(dme)2 (3) (dpp-Bian is dianion of 1,2-bis[(2,6-diisopropylphenyl)imino]acenaphthene; dme is 1,2-dimethoxyethane) with 2,2'-bipyridine (bipy) in toluene proceeds with replacement of the coordinated solvent molecules with neutral bipy ligands and affords europium(II) complex (dpp-Bian)EuII(bipy)2 (9). In contrast the reaction of related ytterbium complex (dpp-Bian)YbII(dme)2 (4) with bipy in dme proceeds with the electron transfer from the metal to bipy and results in (dpp-Bian)YbIII(bipy)(bipy-̇) (10) - ytterbium(III) derivative containing both neutral and radical-anionic bipy ligands. Noteworthy, in both cases dianionic dpp-Bian ligands retain its reduction state. The ligand-centered redox-process occurs when complex 3 reacts with N,N'-bis[2,4,6-trimethylphenyl]-1,4-diaza-1,3-butadiene (mes-dad). The reaction product (dpp-Bian)EuII(mes-dad)(dme) (11) consists of two different redox-active ligands both in the radical-anionic state. The reduction of 3,6-di-tert-butyl-4-(3,6-di-tert-butyl-2-ethoxyphenoxy)-2-ethoxycyclohexa-2,5-dienone (the dimer of 2-ethoxy-3,6-di-tert-butylphenoxy radical) with (dpp-Bian)EuII(dme)2 (3) caused oxidation of the dpp-Bian ligand to radical-anion to afford (dpp-Bian)(ArO)EuII(dme) (ArO = OC6H2-3,6-tBu2-2-OEt) (12). The molecular structures of complexes 9-12 have been established by the single crystal X-ray analysis. The magnetic behavior of newly prepared compounds has been investigated by the SQUID technique in the range 2-310 K. The isotropic exchange model has been adopted to describe quantitatively the magnetic properties of the exchange-coupled europium(II) complexes (11 and 12). The best-fit isotropic exchange parameters are in good agreement with their density functional theory-computed counterparts.

Experimental and Theoretical Investigations of Magnetic Exchange Pathways in Structurally Diverse Iron(III) Schiff-Base Complexes.

The synthesis, and the structural and magnetic properties, of the following new iron(III) Schiff base complexes with the {O',N,O″}-chelating ligand H2L (2-hydroxyphenylsalicylaldimine) are reported: K[FeL2]·H2O (1), (Pr3NH)[FeL2]·2CH3OH (2), [FeL(bpyO2) (CH3OH)][FeL2]·CH3OH (3), [Fe2L3(CH3OH)]·2CH3OH·H2O (4), and [{Fe2L2}(μ-OH)2{FeL(bpyO2)}2][BPh4]2·2H2O (5), where Pr3NH(+) represents the tripropylammonium cation and bpyO2 stands for 2,2'-bipyridine-N-dioxide. A thorough density functional theory (DFT) study of magnetic interactions (the isotropic exchange) at the B3LYP/def-TZVP level of theory was employed, and calculations have revealed superexchange pathways through intramolecular/intermolecular noncovalent contacts (π-π stacking, C-H···O and O-H···O hydrogen bonds, diamagnetic metal cations) and/or covalent bonds ((μ-O(Ph), μ-OH) or bis(μ-O(Ph)) bridging modes), which helped us to postulate trustworthy spin Hamiltonians for magnetic analysis of experimental data. Within the reported family of compounds 1-5, the mediators of the antiferromagnetic exchange can be sorted by their increasing strength as follows: π-π stacking (J(DFT) = -0.022 cm(-1)/J(mag) = -0.025(4) cm(-1) in 2) < C-H···O contacts and π-π stacking (J(DFT) = -0.19 cm(-1)/J(mag) = -0.347(9)cm(-1) in 1) < O-H···O hydrogen bonds (J(DFT) = -0.53 cm(-1)/J(mag) = -0.41(1) cm(-1) in 3) < bis(μ-O(Ph)) bridge (J(DFT) = -13.8 cm(-1)/J(mag) = -12.3(9) cm(-1) in 4) < (μ-O(Ph), μ-OH) bridge (J(DFT) = -18.0 cm(-1)/J(mag) = -17.1(2) cm(-1) in 5), where J(DFT) and J(mag) are the isotropic exchange parameters derived from DFT calculations, and analysis of the experimental magnetic data, respectively. The good agreement between theoretically calculated and experimentally derived isotropic exchange parameters suggests that this procedure is applicable also for other chemical and structural systems to interpret magnetic data properly.

Single Crystal EPR of the Mixed-Ligand Complex of Copper(II) with l-Glutamic Acid and 1,10-Phenanthroline: A Study on the Narrowing of the Hyperfine Structure by Exchange

We report an EPR study at X- and Q-bands of polycrystalline and single crystal samples of the mixed copper(II) complex with L-glutamic acid (glu) and 1,10-phenantroline (phen), [Cu(glu)(phen)(H(2)O)](+) NO(3)(-)·2(H(2)O). The polycrystalline sample spectrum at Q-band showed well resolved g(∥ )and g(⊥) features and partially solved hyperfine structure at g(∥), typical for weakly exchange coupled systems. This is confirmed from the angular variation of the EPR spectra which shows for certain magnetic field orientations a partially solved hyperfine structure characteristic of weak exchange, whereas a single Lorentzian line corresponding to strong exchange is observed for others. Analysis and simulation of the single crystal EPR spectra were performed using the random frequency modulation model of Anderson. Numerical simulations of the angular variation of the EPR spectra showed that the narrowing of the hyperfine structure is due to an exchange-mediated mechanism in which transitions between any pair of lines are equally likely. The exchange interaction responsible for this process is mediated by hydrophobic interactions between two phen molecules and a mixed chemical path of the type CuA-O(ap)H···O-C-O(eq)-CuB, for which we evaluated an average isotropic exchange parameter |J| ≈ 25 × 10(-4) cm(-1).

Antisymmetric Exchange in Triangular Tricopper(II) Complexes: Correlation among Structural, Magnetic, and Electron Paramagnetic Resonance Parameters

Two new trinuclear copper(II) complexes, [Cu(3)(μ(3)-OH)(daat)(Hdat)(2)(ClO(4))(2)(H(2)O)(3)](ClO(4))(2)·2H(2)O (1) and [Cu(3)(μ(3)-OH)(aaat)(3)(H(2)O)(3)](ClO(4))(2)·3H(2)O (2) (daat = 3,5-diacetylamino-1,2,4-triazolate, Hdat = 3,5-diamino-1,2,4-triazole, and aaat = 3-acetylamino-5-amino-1,2,4-triazolate), have been prepared from 1,2,4-triazole derivatives and structurally characterized by X-ray crystallography. The structures of 1 and 2 consist of cationic trinuclear copper(II) complexes with a Cu(3)OH core held by three N,N-triazole bridges between each pair of copper(II) atoms. The copper atoms are five-coordinate with distorted square-pyramidal geometries. The magnetic properties of 1 and 2 and those of five other related 1,2,4-triazolato tricopper(II) complexes with the same triangular structure (3-7) (whose crystal structures were already reported) have been investigated in the temperature range of 1.9-300 K. The formulas of 3-7 are [Cu(3)(μ(3)-OH)(aaat)(3)(H(2)O)(3)](NO(3))(2)·H(2)O (3), {[Cu(3)(μ(3)-OH)(aat)(3)(μ(3)-SO(4))]·6H(2)O}(n) (4), and [Cu(3)(μ(3)-OH)(aat)(3)A(H(2)O)(2)]A·xH(2)O [A = NO(3)(-) (5), CF(3)SO(3)(-) (6), or ClO(4)(-) (7); x = 0 or 2] (aat =3-acetylamino-1,2,4-triazolate). The magnetic and electron paramagnetic resonance (EPR) data have been analyzed by using the following isotropic and antisymmetric exchange Hamiltonian: H = -J[S(1)S(2) + S(2)S(3)] - j[S(1)S(3)] + G[S(1) × S(2) + S(2) × S(3) + S(1) × S(3)]. 1-7 exhibit strong antiferromagnetic coupling (values for both -J and -j in the range of 210-142 cm(-1)) and antisymmetric exchange (G varying from to 27 to 36 cm(-1)). At low temperatures, their EPR spectra display high-field (g < 2.0) signals indicating that the triangles present symmetry lower than equilateral and that the antisymmetric exchange is operative. A magneto-structural study showing a lineal correlation between the Cu-O-Cu angle of the Cu(3)OH core and the isotropic exchange parameters (J and j) has been conducted. Moreover, a model based on Moriya's theory that allows the prediction of the occurrence of antisymmetric exchange in the tricopper(II) triangles, via analysis of the overlap between the ground and excited states of the local Cu(II) ions, has been proposed. In addition, analytical expressions for evaluating both the isotropic and antisymmetric exchange parameters from the experimental magnetic susceptibility data of triangular complexes with local spins (S) of (1)/(2), (3)/(2), or (5)/(2) have been purposely derived. Finally, the magnetic and EPR results of this work are discussed and compared with those of other tricopper(II) triangles reported in the literature.

Exchange coupling in polynuclear nickel(II) complexes with pivalate and hexafluoroacetylacetonate ligands: a quantum chemical analysis

The electronic structures of polynuclear different-ligand nickel(ii) complexes [K2Ni6Piv7(hfac)3(OH)4(HPiv)2(Me2CO)2], [Ni6Piv4(hfac)4(OH)4(Me2CO)4], and [Ni8Piv10(hfac)2(OH)2(MeO)2(MeOH)2(HPiv)2] were studied within the framework of the DFT approach. For each complex, the isotropic exchange parameters for all pairs of paramagnetic centers were calculated. Based on the results obtained, models for Heisenberg-Dirac-van Vleck exchange interaction (with minimal number of parameters) were proposed, which can be used to describe the magnetic properties of the compounds under study.

Slow magnetic relaxation and electron delocalization in an S = 9/2 iron(II/III) complex with two crystallographically inequivalent iron sites

The magnetic, electronic, and Mössbauer spectral properties of [Fe(2)L(μ-OAc)(2)]ClO(4), 1, where L is the dianion of the tetraimino-diphenolate macrocyclic ligand, H(2)L, indicate that 1 is a class III mixed valence iron(II∕III) complex with an electron that is fully delocalized between two crystallographically inequivalent iron sites to yield a [Fe(2)](V) cationic configuration with a S(t) = 9∕2 ground state. Fits of the dc magnetic susceptibility between 2 and 300 K and of the isofield variable-temperature magnetization of 1 yield an isotropic magnetic exchange parameter, J, of -32(2) cm(-1) for an electron transfer parameter, B, of 950 cm(-1), a zero-field uniaxial D(9∕2) parameter of -0.9(1) cm(-1), and g = 1.95(5). In agreement with the presence of uniaxial magnetic anisotropy, ac susceptibility measurements reveal that 1 is a single-molecule magnet at low temperature with a single molecule magnetic effective relaxation barrier, U(eff), of 9.8 cm(-1). At 5.25 K the Mössbauer spectra of 1 exhibit two spectral components, assigned to the two crystallographically inequivalent iron sites with a static effective hyperfine field; as the temperature increases from 7 to 310 K, the spectra exhibit increasingly rapid relaxation of the hyperfine field on the iron-57 Larmor precession time of 5 × 10(-8) s. A fit of the temperature dependence of the average effective hyperfine field yields |D(9∕2)| = 0.9 cm(-1). An Arrhenius plot of the logarithm of the relaxation frequency between 5 and 85 K yields a relaxation barrier of 17 cm(-1).

Interaction of Radical Pairs Through-Bond and Through-Space: Scope and Limitations of the Point−Dipole Approximation in Electron Paramagnetic Resonance Spectroscopy

The validity of the popular point-dipole approximation for interpretation of the zero-field splitting (ZFS) parameter (D-value) in EPR spectroscopy is studied. This approximation is of central importance for the determination of distances by analysis of EPR data. In this work, a detailed experimental (EPR spectroscopy and X-ray crystallography) and theoretical study for a model system (2,2',5,5'-tetra(tert-butyl)-4,4'-bis(ethoxy-carbonyl)-3,3'-bipyrrolyl-1,1'-dioxyl) was performed to understand the scope and limitations of the point-dipole model in EPR spectroscopy. For this diradical, the radical-radical distance derived with the point-dipole approximation deviates significantly (by approximately 40%) from the results derived from the X-ray analysis. Explicit quantum chemical calculation of the D-value on the basis of B3LYP density functional calculations leads to excellent quantitative agreement with the measured D-value. The quantitative accuracy of the employed methodology was confirmed for two additional systems that have previously been experimentally characterized. We therefore analyzed the contributions to the D-value of the target system in detail. This analysis leads to insight into the reasons for the failure of the point-dipole approximation. The analysis was then extended to an in silico study of five classes of model systems. Linkers of varying length and bond saturation were introduced between the radical-carrying groups. This allows for the analysis of the distance dependence of the D-parameter as well as the through-bond and through-space spin-spin interaction. From these results we established the limitations of the point-dipole approximation. The results of this analysis demonstrate that even very modest amounts of spin delocalization can cause significant deviations from pure point-dipole behavior and consequently cause the EPR derived distances to deviate from the N-O midpoint distance by up to several angstroms. If unsaturated linkers are used, the distance dependence of D does not follow the inverse cubic behavior predicted by the point-dipole model. However, for commonly used nonaromatic nitroxide rings connected by a saturated linker, the point-dipole approximation works well. Among the various point-dipole variants tested in this work for delocalized spins, the most successful one is based on distributed point-dipoles with spin populations derived from quantum chemical calculations. The distance dependence of the isotropic Heisenberg exchange parameter J has also been studied theoretically. The decay was found to be monoexponential with a decay constant of approximately 1 A(-1). Thus at linker lengths between 6-8 carbon atoms between a nitroxide radical pair, a switch from the strong to the weak exchange limit is predicted.

Ferromagnetic coupled μ-phenoxo-μ-carboxylato heterodinuclear complexes based on the Cr(salen) moiety: structural and magnetic characterization

The synthesis, crystal structure, and magneto-chemical characterization of two new unprecedented -phenoxo--carboxylato heterodinuclear complexes based on the Cr(salen) moiety (salen = N,N-bis(salicylidene)ethylenediamine), [MII(O2C(CH3)3)(OH2)2(mu-O2C(CH3)3)(-salen)CrIII(O2C(CH3)3)], M = Ni (2), Co(3) are reported. The dinuclear complexes were obtained starting from the mononuclear trans-[Cr(salen)(CN)2]PPh4 (1), whose crystal structure is also reported. They show a trans arrangement of the Cr(salen) unit, bridging through the phenolate O atoms to a second metal center. An additional 2-O2-carboxylato bridge and a further monodentating carboxylate ligand complete the roughly octahedral Cr(III) coordination sphere. The highly distorted octahedral M(II) coordination environment is completed by two coordinated water molecules and an additional monodentating carboxylate. Variable-temperature solid-state DC magnetization studies were carried out in the 2.0-300 K range. Ferromagnetic isotropic pairwise exchange parameters were found with values of J = 4.1 cm-1 (2) and J = 2.1 cm-1 (3). Additionally, for complex 3, a ZFS parameter, D, was employed to properly fit the experimental data. Magnetization (M) vs. field (H) and temperature (T) data further support the presence of this anisotropic component and confirm ground states S = 5/2 and S = 3 for 2 and 3, respectively. Broken symmetry DFT calculations properly reproduce the experimental J values supporting the ferromagnetic exchange interaction experimentally observed. No out of phase susceptibility signal was observed in 0 DC magnetic field for both complexes. However, in the case of complex 3 a non-zero is observed when a small external field is applied below 3 K, suggesting slow relaxation of the magnetization which at 0 DC field is suppressed, probably due to efficient tunnelling relaxation pathways. The low symmetry of the Co(II) site in complex 3 may lead to the presence of transversal anisotropic components which could be responsible for the enhanced tunnelling pathway.

A ligand-field study of the ground spin-state magnetic anisotropy in a family of hexanuclear Mn(iii) single-molecule magnets

A ligand field analysis of two structurally related hexanuclear Mn(iii) coordination complexes reveals that the observed difference in their ground spin-state anisotropy originates from the difference in projection coefficients of the single-ion anisotropy to spin states of different total spin quantum-number, S, rather than the geometrical distortions of the metal ions. Furthermore we show that the single-ion second order anisotropy induces fourth and higher order anisotropy terms to the ground spin states of the studied systems, as a consequence of spin-state mixing effects due to the comparable magnitude of the single-ion second order anisotropy and the isotropic exchange parameters.

Exchange Interactions and Electron Delocalization in the Mixed-Valence Cluster V4IVV2VO7(OC2H5)12

The mixed-valence cluster compound V4IVV2VO7(OC2H5)12 was studied by X-band electron paramagnetic resonance (EPR) in the temperature range of 4.2-293 K. According to X-ray diffraction study, the crystal structure of the compound was described by a R3m space group at 295 K (four d1 electrons are equally delocalized on all vanadium ions) and changed to a P21/n space group on cooling the crystals to 173 K (the electrons are preferably localized on the four equatorial vanadium ions). The EPR spectra originate from the S = 1 total spin states with the fine structure averaged to a single Lorentzian line and from the S = 2 total spin states with fine structure partly averaged in the temperature range of 295-200 K and well averaged below 45-50 K. The states of S = 1 and S = 2 of comparable energy (DeltaE approximately 2 cm(-1); ES=1 < ES=2) were shown to be the lowest ones. The VIV <--> VV unpaired electron transfers together with isotropic Heisenberg exchange were shown to determine the total spin states composition and the intracluster dynamics of the compound. Two types of electron transfers were assumed: the single-jump transfer leading to the averaged configurations of the V4IVV2V <--> V3IVVV VIVVV type and to the splitting of the total spin states by intervals comparable in magnitude with the isotropic exchange parameter J approximately 100 cm-1 and the double-jump transfer resulting in dynamics. Temperature dependence of the transition rates nutr was observed. In the range of 295-210 K, the value of nutr = (0.5-0.6) x 10(10) s(-1) is sufficient for averaging the fine structure of the S = 1 states, and below 45 K the value of nutr approximately 1.5 x 10(10) s(-1) also averages the fine structure of the S = 2 state. A change in the localization plane of the VIV ions in the temperature range of 40-50 K was discovered.

Structure and properties of the dinuclear complex [Co 2(μ-OAc) 2(OAc) 2(μ-H 2O)(phen) 2

The dinuclear complex [Co 2(μ-OAc) 2(OAc) 2(μ-H 2O)(phen) 2] has been prepared and its structure was determined. The compound crystallizes in the monoclinic space group P2(1)/ c. The Co–Co distance is 3.574 Å and is similar to the Fe–Fe distance in the reduced methane monooxygenase hydroxylase. The electronic and IR spectra of the complex confirm octahedral coordination of the cobalt atoms and formation of strong O–H⋯O hydrogen bonds in the solid state. The dependence of the magnetic susceptibility of the complex on temperature indicates an antiferromagnetic interaction, the value of the isotropic exchange parameter J was estimated to be −2.1 cm −1. The 1H NMR spectra show that in organic solvents the structure of compound is the same as in the solid state, however, in water solution the complex dissociates giving compounds with different Co:phen ratios.

Density-functional study of theCr8antiferromagnetic ring

We present the results of an ab initio characterization of the electronic and magnetic properties of the ${\mathrm{Cr}}_{8}$ molecular ring. The original molecule with formula $[{\mathrm{Cr}}_{8}{\mathrm{F}}_{8}{\mathrm{Piv}}_{16}]$($\mathrm{HPiv}$=pivalic acid, trimethyl acetic acid) has been approximated by replacing the disordered methyl groups by H atoms, obtaining de facto the formula $[{\mathrm{Cr}}_{8}{\mathrm{F}}_{8}({\mathrm{O}}_{2}\mathrm{CH}{)}_{16}]$. We find in agreement with experiments that the Cr atoms attain a local spin moment of $\frac{3}{2}$, and that the preferred arrangement of Cr atoms in the ground state is antiferromagnetic. By the total energy difference we calculate the value of the isotropic Heisenberg exchange parameter and we show that only after the inclusion of on-site correlation effects in the Cr $d$ orbitals the value deduced by specific heat and torque measurements is recovered.

Single crystal EPR studies of water bridged one‐dimensional paramagnet diacetato‐copper (II)‐bis‐(p‐toluidine) trihydrate

AbstractX‐band EPR measurements of g‐factor, lineshape, and linewidth in three mutually orthogonal planes of diacetato‐copper(II)‐bis‐(p‐toluidine) trihydrate at 77 and 300 K are reported. The lineshape is found to be Lorentzian in all directions including the chain and the magic angle. Excellent fitting of the linewidth anisotropy is obtained on the basis of the anisotropic exchange (AE) theories of McGregor and Soos and Ritter et al. The z‐direction of the AE tensor is found to be coincident with the chain direction. Values of isotropic and anisotropic exchange parameters derived from the above fitting are found to be in good agreement with those obtained from magnetic susceptibility measurements and the well‐known Moriya relation, respectively.

Magnetic excitations in modulated PrNi 2Si 2

The magnetic excitations in a single crystal of PrNi 2Si 2 have been studied by inelastic neutron scattering. Dispersion curves have been followed through the centered tetragonal Brillouin zone. In the paramagnetic phase, at 30 K, the lowest energy magnetic branch exhibits strong dispersion ranging from 1.9 to 3.5 meV, the minimum energy occurring at the wave vector of the ordered phase; from the observed dispersion the inter-ionic isotropic bilinear exchange parameters are deduced. At 4.2 K, in the modulated phase, the same branch presents slightly larger, but not significantly different, dispersion.

EPR spectrum of the ion-pair [Fe(CO) −4] 2 in irradiated crystals of PPN +HFe(CO) −4

UV- or γ-irradiated crystals of PPN +HFe(CO) − 4 contain two paramagnetic species which are stable for days at ambient temperature and which exhibit EPR spectra characteristic of electronic triplets ( S=1). No electronic doublet ( S= 1 2 ) species were detected. Measurements of the spectral intensities as a function of temperature show that both species have singlet ground states and small isotropic exchange parameters, J≈30 cm −1. The values and location of the g - and fine-structure tensors in the crystal structure are consistent with the presence of exchange-coupled isolated pairs of Fe(CO) − 4 ions formed between neighbouring damaged anions.

EPR spectrum of the ion pair [CR(CO)5−]2 in irradiated crystals of PPh4+ HCr(CO)5−

AbstractUV‐irradiated crystals of PPh4+HCr(CO)5− exhibit anisotropic EPR spectra at room temperature and below which are characteristic of the presence of an electronic triplet (S = 1). From measurements of the angular dependence of the spectrum it is established that the carrier has axial symmetry with g‖ = 2.0019, g⟂ = 2.0202, |D| = 163.0 MHz, and with the unique direction parallel to the c axis of the tetragonal (P4/n) crystal. The magnitudes of the zero‐field parameter D and the g‐tensor components show that the carrier is a biradical consisting of a pair of Cr(CO)5− radicals separated approximately by one unit cell length along c. A weak single‐line spectrum which accompanies the triplet spectrum has the identical g‐tensor, and is assigned to the Cr(CO)5− radical itself. A third spectrum with the same g‐tensor, which is present in γ‐irradiated crystals only, is ascribed to another electronic triplet with |D| = 84 and E ≈︁ 0 MHz. It is associated with a second pair of exchange‐coupled Cr(CO)5− radicals whose interionic axis lies skew in the crystal axis system. Measurements of the signal intensities as a function of temperature show that both biradicals have singlet ground states and small isotropic exchange parameters, J ≈︁ 35 and 11 cm−1, respectively.

Excitations of isolated clusters ofCo2+magnetic ions in KZn0.92Co0.08F3

Inelastic neutron scattering from clusters of ${\mathrm{Co}}^{2+}$ ions in cubic KZn${\mathrm{F}}_{3}$ is reported. From measurements of the lowest pair mode at (1.676\ifmmode\pm\else\textpm\fi{}0.008) THz, we find that the exchange between ${\mathrm{Co}}^{2+}$ ions is surprisingly isotropic, with any bond anisotropy being less than 2.4%. An accurate value of the isotropic exchange parameter has been obtained [$J=(0.330\ifmmode\pm\else\textpm\fi{}0.005)$ THz] free of the many-body effects that complicate the interpretation of exchange coupling in concentrated crystals. Neutron scattering was also observed from triplet and quartet states at (2.47\ifmmode\pm\else\textpm\fi{}0.02) and (1.09\ifmmode\pm\else\textpm\fi{}0.05) THz, respectively, and a higher-frequency pair state at 8.8 THz. The results are interpreted in terms of an elementary theory of cluster spectra for ions with nonzero angular momentum.

Magnetic excitations in TmCu

The magnetic excitations have been studied in the rare-earth intermetallic compound TmCu by means of inelastic neutron scattering. Dispersion curves have been followed through the cubic Brillouin zone, in particular an intense and dispersive transverse branch: this latter presents a lifting of the degeneracy along particular directions. Within a generalized susceptibility formalism, this is related to the coupling of the excitations at q and q + Q , where Q = (π/ a, π/ a, 0) is the antiferromagn etic propagation vector in TmCu. The inter-ionic isotropic bilinear exchange parameters are deduced and compared with those of isomorphous compounds.