Interdimer Exchange Research Articles

Anomalous magnetic exchange in a dimerized quantum magnet composed of unlike spin species

We present here a study of the magnetic properties of the antiferromagnetic dimer material CuVOF$_4$(H$_2$O)$_6\cdot$H$_2$O, in which the dimer unit is composed of two different $S = 1/2$ species, Cu(II) and V(IV). An applied magnetic field of $\mu_0H_{\rm c1} = 13.1(1)~\rm T$ is found to close the singlet-triplet energy gap, the magnitude of which is governed by the antiferromagnetic intradimer, $J_0 \approx 21~\rm K$, and interdimer, $J' \approx 1~\rm K$, exchange energies, determined from magnetometry and electron-spin resonance measurements. The results of density functional theory (DFT) calculations are consistent with the experimental results and predicts antiferromagnetic coupling along all nearest-neighbor bonds, with the magnetic ground state comprising spins of different species aligning antiparallel to one another, while spins of the same species are aligned parallel. The magnetism in this system cannot be accurately described by the overlap between localized V orbitals and magnetic Cu orbitals lying in the Jahn-Teller (JT) plane, with a tight-binding model based on such a set of orbitals incorrectly predicting that interdimer exchange should be dominant. DFT calculations indicate significant spin density on the bridging oxide, suggesting instead an unusual mechanism in which intradimer exchange is mediated through the O atom on the Cu(II) JT axis.

Magneto-structural studies of a one dimensional Mn(II) coordination polymer derived from a flexible polydentate ligand

The polydentate ligand LHμ contains bipyridine, hydrazone and oxime functionalities and reacts with Mn(CH3COO)2·4H2O to afford an alternating one dimensional chain comprising Mn(II) ions, [{Mn2(LH)(CH3COO)3(CH3OH)2}·CH3OH]n(1). The structure of 1 contains two Mn(II) ions in the asymmetric unit; Mn1 adopts an unusual pentagonal bipyramidal coordination geometry comprising an [N3O4] donor set, while Mn2 adopts a more conventional octahedral geometry coordinated by an [NO5] donor set. The two manganese ions within the dinuclear unit are connected to each either via Ohydrazone and syn-syn Oacetate bridges. These dinuclear units are then linked by anti-anti Oacetate bridges, affording a magnetic 1-D alternating chain topology. Magnetic studies on 1 reveal the presence of weak antiferromagnetic interactions via Ohydrazone and acetate bridges. The magnetism of 1 was modelled using a dimer model [Ĥ = −2JŜ1Ŝ2], with inter-dimer exchange modelled using a mean field term (zJ’) to afford g = 2.046 ± 0.002, J/k = -1.03 ± 0.02 cm−1 and zJ’ = −0.06 ± 0.01 cm−1.

Revisiting the Role of Hydrogen Bonding in the Strong Dimer Superexchange of a 2D Copper(II) Halide Honeycomb-Like Lattice: Structural and Magnetic Study.

The title compound H2L(CuCl3H2O)Cl (H2L = 1-(4'-pyridinium)pyridin-4-ol-ium), 1) was synthesized and investigated structurally and magnetically as well as via a first-principles, bottom-up theoretical analysis of the potential magnetic superexchange pathways. Compound 1 can be described structurally as a well-isolated, distorted 2D-honeycomb lattice with two potential exchange pathways: a dimeric interaction via hydrogen-bonded pairs of (CuCl3H2O) ions and a chain structure via bridging chloride ions. Surprisingly, the experimental magnetic data are best fitted using both a simple dimer model with a Curie-Weiss correction for interdimer exchange (Jdimer = -107.4(1) K, θ = -1.22(4) K) and a strong-rung ladder model (Jrung = -105.8(7) K, Jrail = 2(7) K). Theoretical analysis at the UB3LYP/6-31+G(d) level supports the strong exchange observed through the [CuCl4(H2O)]2- dimer moiety superexchange pathway (-102 K = -71 cm-1). However, the apparent vanishingly small exchange through the single halide bridge is merely a brute average of competing ferromagnetic (FM) (+24.8 K = +17.0 cm-1) and antiferromagnetic (AFM) (-21.0 K = -14.6 cm-1) exchange interactions. Our computational study shows that these fitting parameters carry no physical meaning since a honeycomb plaquette must be taken as magnetic building block for 1. The competition between FM and AFM pair interactions leads to geometrical frustration in 1 and could induce interesting magnetic response at low temperatures, if the magnetic exchange is adequately tuned by modifying substituents in ligands and, in turn, interactions within the crystal packing.

Localized Magnetic Excitations in the Fully Frustrated Dimerized Magnet Ba_{2}CoSi_{2}O_{6}Cl_{2}.

Magnetic excitations of the effective spin S=1/2 dimerized magnet Ba_{2}CoSi_{2}O_{6}Cl_{2} have been probed directly via inelastic neutron scattering experiments at temperatures down to 4K. We observed five types of excitation at 4.8, 5.8, 6.6, 11.4, and 14.0meV, which are all dispersionless within the resolution limits. The scattering intensities of the three low-lying excitations were found to exhibit different Q dependencies. Detailed analysis has demonstrated that Ba_{2}CoSi_{2}O_{6}Cl_{2} is a two-dimensional spin dimer system described only by a single dimer site, where the triplet excitations are localized owing to the almost perfect frustration of the interdimer exchange interactions and the undimerized spins, even in small concentration, make an essential contribution to the excitation spectrum.

Triplon band splitting and topologically protected edge states in the dimerized antiferromagnet

Search for topological materials has been actively promoted in the field of condensed matter physics for their potential application in energy-efficient information transmission and processing. Recent studies have revealed that topologically invariant states, such as edge states in topological insulators, can emerge not only in a fermionic electron system but also in a bosonic system, enabling nondissipative propagation of quasiparticles. Here we report the topologically nontrivial triplon bands measured by inelastic neutron scattering on the spin-1/2 two-dimensional dimerized antiferromagnet Ba2CuSi2O6Cl2. The excitation spectrum exhibits two triplon bands that are clearly separated by a band gap due to a small alternation in interdimer exchange interaction, consistent with a refined crystal structure. By analytically modeling the triplon dispersion, we show that Ba2CuSi2O6Cl2 is the first bosonic realization of the coupled Su-Schrieffer-Heeger model, where the presence of topologically protected edge states is prompted by a bipartite nature of the lattice.

{CpFeII(CO)2SnII(Macrocycle•3-)} Radicals with Intrinsic Charge Transfer from CpFe(CO)2 to Macrocycles (Cp: Cp or Cp*); Effective Magnetic Coupling between Radical Trianionic Macrocycles•3.

Neutral {CpFeII(CO)2[SnII(Pc•3–)]} {Cp is cyclopentadienyl (1, 2) or Cp* is pentamethylcyclopentadienyl (3); Pc: phthalocyanine}, {Cp*FeII(CO)2[SnII(Nc•3–)]} (4, Nc: naphthalocyanine), and {CpFeII(CO)2[SnII(TPP•3–)]} (5, TPP: tetraphenylporphyrin) complexes in which CpFeII(CO)2 fragments (Cp: Cp or Cp*) are coordinated to SnII(macrocycle•3–) have been obtained. The product complexes were obtained at the reaction of charge transfer from CpFeI(CO)2 (Cp: Cp or Cp*) to [SnII(macrocycle2−)] to form the diamagnetic FeII and paramagnetic radical trianionic macrocycles. As a result, these formally neutral complexes contain S = 1/2 spins delocalized over the macrocycles. This provides alternation of the C–Nimine or C–Cmeso bonds in the macrocycles, the appearance of new bands in the near-infrared spectra of the complexes, and blue shift of both Soret and Q-bands. The {CpFeII(CO)2SnII(macrocycle•3–)} units (Cp: Cp or Cp*, macrocycle: Pc or Nc) form closely packed π-stacking dimers in 1 and 3 or one-dimensional chains in 2 and 4 with effective π–π interaction between the macrocycles. Such packing allows strong antiferromagnetic coupling between S = 1/2 spins. Magnetic interaction can be described well by the Heisenberg model for the isolated dimers in 1 and 3 with exchange interaction J/kB = −78 and −85 K, respectively. Magnetic behavior of 2 and 4 is described well by the model that includes contributions from an antiferromagnetically coupled S = 1/2 dimer (Jintra) and a Heisenberg S = 1/2 chain with alternating antiferromagnetic spin exchange between the neighbors (Jinter). Compound 2 demonstrates large intradimer interaction of Jintra/kB = −54 K and essentially weaker interdimer exchange interactions of Jinter/kB = −6 K, whereas compound 4 shows strong magnetic coupling of spins within the dimers (Jintra/kB = −170 K) as well as between the dimers (Jinter/kB = −40 K). Compound {CpFeII(CO)2[SnII(TPP•3–)]} (5) shows no π–π interactions between the porphyrin macrocycles, and magnetic coupling is weak in this case (Weiss temperature is −5 K). Preparation of a similar complex with indium(III) chloride phthalocyanine yields {CpFe(CO)2[In(Pc2–)]} (6). In this complex, indium(III) atoms are reduced instead of the phthalocyanine macrocycles that explains electron paramagnetic resonance silence of 6 in the 4–295 K range.

Spin nematics next to spin singlets

We provide a route to generate nematic order in a spin-1/2 system. Unlike the well-known magnon-binding mechanism, our spin nematics requires neigher the frustration effect nor a spin polarization in a high field or in the vicinity of a ferromagnet, but instead appears next to the spin singlet phase. We start from a state consisting of a quantum spin-1/2 singlet dimer placed on each site of a triangular lattice, and show that inter-dimer ring exchange interactions efficiently dope the SU(2) triplets that itinerate and interact, easily driving a stable singlet state to either Bose Einstein condensates or a triplet crystal, some hosting a spin nematic order. A variety of roles the ring exchange serves include the generation of a bilinear-biquadratic interaction between nearby triplets, which is responsible for the emergent nematic order separated from the singlet phase by a first order transition.

Single-crystal growth and study of the mixed spin-dimer system Ba3−xSrxCr2O8

The compounds Sr$_{3}$Cr$_{2}$O$_{8}$ and Ba$_{3}$Cr$_{2}$O$_{8}$ are insulating dimerized antiferromagnets with Cr$^{5+}$ magnetic ions. These spin-$\frac{1}{2}$ ions form hexagonal bilayers with a strong intradimer antiferromagnetic interaction, that leads to a singlet ground state and gapped triplet states. We report on the effect on the magnetic properties of Sr$_{3}$Cr$_{2}$O$_{8}$ by introducing chemical disorder upon replacing Sr by Ba. Two single crystals of Ba$_{3-x}$Sr$_{x}$Cr$_{2}$O$_{8}$ with $x=2.9$ (3.33\% of $mixing$) and $x=2.8$ (6.66\%) were grown in a four-mirror type optical floating-zone furnace. The magnetic properties on these compounds were studied by magnetization measurements. Inelastic neutron scattering measurements on Ba$_{0.1}$Sr$_{2.9}$Cr$_{2}$O$_{8}$ were performed in order to determine the interaction constants and the spin gap for $x=2.9$. The intradimer interaction constant is found to be $J_0$=5.332(2) meV, about 4\% smaller than that of pure Sr$_{3}$Cr$_{2}$O$_{8}$, while the interdimer exchange interaction $J_e$ is smaller by 6.9\%. These results indicate a noticeable change in the magnetic properties by a random substitution effect.

Temperature dependence of the effective interdimer exchange interaction in a weakly coupled antiferromagnetic dimer copper compound

We report a variation with temperature ($T$) of the effective interdimeric interaction $J^\prime_{\mathrm{eff}}$ in the antiferromagnetic (AFM) copper dimeric organic compound Cu$_2$[TzTs]$_4$ [N-thiazol-2-yl-toluenesulfonamidate Cu$^\mathrm{II}$]. This $T$ dependence was obtained from measurements of the effects in the electron paramagnetic resonance (EPR) spectra of the proposed quantum phase transition associated to the exchange narrowing processes. Cu$_2$[TzTs]$_4$ contains exchange coupled pairs of Cu$^\mathrm{II}$ spins $\mathbf{S_\mathrm{A}}$ and $\mathbf{S_\mathrm{B}}$ ($S$ = 1/2), with intradimeric AFM exchange coupling $J_0$ = (-115$\pm$1) cm$^{-1}$ ($\mathcal{H}_\mathrm{ex} = -J_\mathrm{0} \mathbf{S_\mathrm{A}}\cdot \mathbf{S_\mathrm{B}}$). The variation of the EPR line width of single crystals with field orientation around a "magic angle" where the transitions intersect, as well as the integrated signal intensity of the so-called "U-peak" of the powder spectrum were measured as a function of $T$. Modeling these data using arguments of exchange narrowing in the adiabatic regime considering the angular variation of the single crystal spectra and a geometric description, we find that $|J^\prime_{\mathrm{eff}}|$ associated with the exchange frequency $\omega_{ex}$ is negligible for $T<<|J_\mathrm{0}/k_\mathrm{B}|$, when the units are uncoupled, and $|J^\prime_{\mathrm{eff}}|$ = (0.080 $\pm$ 0.005) cm$^{-1}$ ($|J^\prime_{\mathrm{eff}}/J_0|$ = 7.0$\times$10$^{-4}$) at 298 K. Within this $T$-interval, two ranges of $J^\prime_{\mathrm{eff}}$ with linear temperature variation but different slopes, with a kink at $\sim$80 K, are observed and discussed. This $T$-dependence arises from the growing population of the triplet state and its relevance in the properties of various arrays of DUs is discussed. Our experimental procedures and results are compared with those of previous works.

Magnetostructural Characterization of Oxalamide Dihalo-Bridged Copper Dimers: Intra- and Interdimer Interactions Studied by Single-Crystal Electron Spin Resonance Spectroscopy.

Detailed single-crystal electron spin resonance (ESR) analysis of oxalamide complexes with halogen-bridged copper dimers, supported by X-ray, magnetic susceptibility, and powder ESR studies, is reported. Four complexes with two different ligands are synthesized: [CuLA (μ-X)]2 and [CuLV (μ-X)]2 , for which LA =N-(l-alanine methyl ester)-N'-[(2-pyridine-2-yl)methyl]oxalamide and LV =N-(l-valine methyl ester)-N'-[(2-pyridine-2-yl)methyl]oxalamide, for which X=Cl or Br. X-ray analysis shows that the geometry at each copper(II) ion is square pyramidal, whereas two pyramids share one base-to-apex edge with parallel basal planes. The complexes are linked by hydrogen bonds into infinite chains and are further linked into a 3D network. Susceptibility measurements show that the copper centers in the dimers are weakly antiferromagnetically coupled (|J|≈1-2 cm-1 ). From powder ESR spectroscopy, the g values and dx2-y2 orbital as the ground state of the unpaired electron are determined. The complexes show unusual anisotropic splitting and merging of the ESR lines if their single crystals rotate in a magnetic field. The observation of this partially resolved intradimer dipolar splitting enables estimation of the weak interdimer exchange interaction parameter |J'|≈0.001 cm-1 .

Frustrated spin- 12 molecular magnetism in the mixed-valence antiferromagnets Ba3MRu2O9(M=In , Y, Lu)

We have performed magnetic susceptibility, heat capacity, muon spin relaxation, and neutron scattering measurements on three members of the family Ba3MRu2O9, where M = In, Y and Lu. These systems consist of mixed-valence Ru dimers on a triangular lattice with antiferromagnetic interdimer exchange. Although previous work has argued that charge order within the dimers or intradimer double exchange plays an important role in determining the magnetic properties, our results suggest that the dimers are better described as molecular units due to significant orbital hybridization, resulting in one spin-1/2 moment distributed equally over the two Ru sites. These molecular building blocks form a frustrated, quasi-two-dimensional triangular lattice. Our zero and longitudinal field muSR results indicate that the molecular moments develop a collective, static magnetic ground state, with oscillations of the zero field muon spin polarization indicative of long-range magnetic order in the Lu sample. The static magnetism is much more disordered in the Y and In samples, but they do not appear to be conventional spin glasses.

Singlet ground state in the spin- 12 weakly coupled dimer compound NH4[(V2O3)2(4,4′−bpy)2(H2PO4)(PO4)2]·0.5H2O

We present the synthesis and a detailed investigation of structural and magnetic properties of polycrystalline NH$_4$[(V$_2$O$_3$)$_2$(4,4$^\prime$-$bpy$)$_2$(H$_2$PO$_4$)(PO$_4$)$_2$]$\cdot$0.5H$_2$O by means of x-ray diffraction, magnetic susceptibility, electron spin resonance, and $^{31}$P nuclear magnetic resonance measurements. Temperature dependent magnetic susceptibility could be described well using a weakly coupled spin-$1/2$ dimer model with an excitation gap $\Delta/k_{\rm B}\simeq 26.1$ K between the singlet ground state and triplet excited states and a weak inter-dimer exchange coupling $J^\prime/k_{\rm B} \simeq 4.6$ K. A gapped chain model also describes the data well with a gap of about 20 K. The ESR intensity as a function of temperature traces the bulk susceptibility nicely. The isotropic Land$\acute{\rm e}$ $g$-factor is estimated to be about $g \simeq 1.97$, at room temperature. We are able to resolve the $^{31}$P NMR signal as coming from two inequivalent P-sites in the crystal structure. The hyperfine coupling constant between $^{31}$P nucleus and V$^{4+}$ spins is calculated to be $A_{\rm hf}(1) \simeq 2963$ Oe/$\mu_{\rm B}$ and $A_{\rm hf}(2) \simeq 1466$ Oe/$\mu_{\rm B}$ for the P(1) and P(2) sites, respectively. Our NMR shift and spin-lattice relaxation rate for both the $^{31}$P sites show an activated behaviour at low temperatures, further confirming the singlet ground state. The estimated value of the spin gap from the NMR data measured in an applied field of $H = 9.394$ T is consistent with the gap obtained from the magnetic susceptibility analysis using the dimer model. Because of a relatively small spin gap, NH$_4$[(V$_2$O$_3$)$_2$(4,4$^\prime$-$bpy$)$_2$(H$_2$PO$_4$)(PO$_4$)$_2$]$\cdot$0.5H$_2$O is a promising compound for further experimental studies under high magnetic fields.

Experimental observation and computational study of the spin-gap excitation in Ba3BiRu2O9

${\mathrm{Ba}}_{3}{\mathrm{BiRu}}_{2}{\mathrm{O}}_{9}$ is a 6H-type perovskite compound containing face-sharing octahedral ${M}_{2}{\mathrm{O}}_{9}$ ($M=\mathrm{Ir}$, Ru) dimers, which are magnetically frustrated at low temperatures. On cooling through ${T}^{*}=176$ K, it undergoes a pronounced magnetostructural transition, which is not accompanied by any change in space group symmetry, long-range magnetic ordering, or charge ordering. Here, we report the first direct evidence from inelastic neutron scattering that this transition is due to an opening of a gap in the excitation spectra of dimers of low-spin ${\mathrm{Ru}}^{4+}$ ($S=1$) ions. X-ray absorption spectroscopy reveals a change in Ru-Ru orbital overlap at ${T}^{*}$, linking the emergence of this spin-gap excitation to the magnetostructural transition. Ab initio calculations point to a geometrically frustrated magnetic ground state due to antiferromagnetic interdimer exchange on a triangular ${\mathrm{Ru}}_{2}{\mathrm{O}}_{9}$ dimer lattice. X-ray total-scattering data rule out long-range magnetic ordering at low temperatures, consistent with this geometrically frustrated model.

Quasi-two-dimensional Bose-Einstein condensation of spin triplets in the dimerized quantum magnetBa2CuSi2O6Cl2

We synthesized single crystals of composition Ba$_2$CuSi$_2$O$_6$Cl$_2$ and investigated its quantum magnetic properties. The crystal structure is closely related to that of the quasi-two-dimensional (2D) dimerized magnet BaCuSi$_2$O$_6$ also known as Han purple. Ba$_2$CuSi$_2$O$_6$Cl$_2$ has a singlet ground state with an excitation gap of ${\Delta}/k_{\rm B}\,{=}\,20.8$ K. The magnetization curves for two different field directions almost perfectly coincide when normalized by the $g$-factor except for a small jump anomaly for a magnetic field perpendicular to the $c$ axis. The magnetization curve with a nonlinear slope above the critical field is in excellent agreement with exact-diagonalization calculations based on a 2D coupled spin-dimer model. Individual exchange constants are also evaluated using density functional theory (DFT). The DFT results demonstrate a 2D exchange network and weak frustration between interdimer exchange interactions, supported by weak spin-lattice coupling implied from our magnetostriction data. The magnetic-field-induced spin ordering in Ba$_2$CuSi$_2$O$_6$Cl$_2$ is described as the quasi-2D Bose-Einstein condensation of triplets.

Origin of the Spin-Orbital Liquid State in a Nearly J=0 Iridate Ba_{3}ZnIr_{2}O_{9}.

We show using detailed magnetic and thermodynamic studies and theoretical calculations that the ground state of Ba_{3}ZnIr_{2}O_{9} is a realization of a novel spin-orbital liquid state. Our results reveal that Ba_{3}ZnIr_{2}O_{9} with Ir^{5+} (5d^{4}) ions and strong spin-orbit coupling (SOC) arrives very close to the elusive J=0 state but each Ir ion still possesses a weak moment. Abinitio density functional calculations indicate that this moment is developed due to superexchange, mediated by a strong intradimer hopping mechanism. While the Ir spins within the structural Ir_{2}O_{9} dimer are expected to form a spin-orbit singlet state (SOS) with no resultant moment, substantial frustration arising from interdimer exchange interactions induce quantum fluctuations in these possible SOS states favoring a spin-orbital liquid phase down to at least 100mK.

Pseudo-isomeric zinc/copper coordination polymers based on 3-(2-pyridyl)pyrazole-5-carboxylic acid

Reaction of 3-(2-pyridyl)pyrazole-5-carboxylic acid (H2L) with Zn(II) or Cu(II) perchlorate yielded [ZnL]n (1) and [CuL]n (2) with similar composition. The ligand L2− adopts the same μ3κ5 coordination mode. The Zn(II) and Cu(II) atom takes square pyramid coordination polyhedron with some discrepancy. Bigger difference in the dimeric secondary building blocks leads to distinct two-dimensional metal–organic frameworks. The Zn(II) complex fluoresces stronger than the free ligand. The Cu(II) polymer displays strong antiferromagnetic intra-dimer exchange and ferromagnetic inter-dimer exchange.

Almost Perfect Frustration in the Dimer Magnet Ba2CoSi2O6Cl2

We determined the crystal structure of Ba$_2$CoSi$_2$O$_6$Cl$_2$, which was synthesized in this work, and investigated its quantum magnetic properties using single crystals. This compound should be described as a two-dimensionally coupled spin-1/2 XY-like spin dimer system. Ba$_2$CoSi$_2$O$_6$Cl$_2$ exhibits a stepwise magnetization process with a plateau at half of the saturation magnetization, irrespective of the field direction, although all the Co$^{2+}$ sites are equivalent. This indicates that spin triplets are localized owing to the almost perfect frustration of interdimer exchange interactions. Thus, the spin states for the zero and 1/2 magnetization-plateau states are almost exactly given by the simple product of singlet dimers and the alternate product of singlet and triplet dimers, respectively.

Evidence of weak ferromagnetism in doped plasticized polyaniline (PANI–DDoESSA)0.5 from electron spin resonance measurements

X-band electron spin resonance (ESR) measurements have been performed on a conducting free-standing film of polyaniline plasticized and protonated with di-n-dodecyl ester of sulfosuccinic acid (DDoESSA). The magnetic field was applied parallel and perpendicular to the plane of the film. At around 75 K a transition is observed from Pauli susceptibility to a localized state in which the spin 1/2 polarons behave as spin 1/2 dimers. A rough estimation of the intradimer and interdimer exchange constants is obtained. Below 5 K, ESR data reveal a weak ferromagnetism with the Dzyaloshinskii–Moriya vector mainly oriented in the plane of the film. The existence of a relatively well-defined n-fold axis along the chain direction in the crystalline regions confers a symmetry compatible with such analysis.

Crystal Structure and Magnetic Properties of Two New Antiferromagnetic Spin Dimer Compounds; FeTe3O7X (X = Cl, Br)

Two new isostructural layered oxohalides FeTe(3)O(7)X (X = Cl, Br) were synthesized by chemical vapor transport reactions, and their crystal structures and magnetic properties were characterized by single-crystal X-ray diffraction, Raman spectroscopy, magnetic susceptibility and magnetization measurements, and also by density functional theory (DFT) calculations of the electronic structure and the spin exchange parameters. FeTe(3)O(7)X crystallizes in the monoclinic space group P2(1)/c with the unit cell parameters a = 10.7938(5), b = 7.3586(4), c = 10.8714(6) Å, β = 111.041(5)°, Z = 4 for FeTe(3)O(7)Cl, and a = 11.0339(10), b = 7.3643(10), c = 10.8892(10) Å, β = 109.598(10)°, Z = 4 for FeTe(3)O(7)Br. Each compound has one unique Fe(3+) ion coordinating a distorted [FeO(5)] trigonal bipyramid. Two such groups share edges to form [Fe(2)O(8)] dimers that are isolated from each other by Te(4+) ions. The high-temperature magnetic properties of the compounds as well as spectroscopic investigations are consistent with an isolated antiferromagnetic spin dimer model with almost similar spin gaps of ~35 K for X = Cl and Br, respectively. However, deviations at low temperatures in the magnetic susceptibility and the magnetization data indicate that the dimers couple via an interdimer coupling. This interpretation is also supported by DFT calculations which indicate an interdimer exchange which amounts to 25% and 10% of the intradimer exchange for X = Cl and Br, respectively. The magnetic properties support the counterion character and a weak integration of halide ions into the covalent network similar to that in many other oxohalides.

Ferromagnetically coupled dimers on the distorted Shastry-Sutherland lattice: Application to (CuCl)LaNb2O7

A recent study [Tassel {\it et al.}, Phys. Rev. Lett. {\bf 105}, 167205 (2010)] has proposed a remarkable spin model for (CuCl)LaNb2O7, in which dimers are ferromagnetically coupled to each other on the distorted Shastry-Sutherland lattice. In this model, the intra-dimer exchange coupling J>0 is antiferromagnetic, while the inter-dimer exchange couplings are ferromagnetic and take different values, J_x,J_y<0, in the two bond directions. Anticipating that the highly frustrated character of this model may lead to a wide range of behaviors in (CuCl)LaNb2O7 and related compounds, we theoretically investigate the ground state phase diagram of this model in detail using the following three approaches: a strong-coupling expansion for small J_x and J_y, exact diagonalization for finite clusters, and a Schwinger boson mean field theory. When |J_x|, |J_y| <~ J, the system stays in a dimer singlet phase with a finite spin gap. This state is adiabatically connected to the decoupled-dimer limit J_x=J_y=0. We show that the magnetization process of this phase depends crucially on the spatial anisotropy of the inter-dimer couplings. The magnetization shows a jump or a smooth increase for weak and strong anisotropy, respectively, after the spin gap closes at a certain magnetic field. When |J_x| or |J_y| >~ J, quantum phase transitions to various magnetically ordered phases (ferromagnetic, collinear stripe, and spiral) occur. The Schwinger boson analysis demonstrates that quantum fluctuations split the classical degeneracy of different spiral ground states. Implications for (CuCl)LaNb2O7 and related compounds are discussed in light of our theoretical results and existing experimental data.