Synthesis, Structure, and Magnetic Properties of an Antiferromagnetic Spin-Ladder Complex: Bis(2,3-dimethylpyridinium) Tetrabromocuprate

LT Scaling in Depleted Quantum Spin Ladders

Using a combination of neutron scattering, calorimetry, quantum Monte Carlo simulations, and analytic results we uncover confinement effects in depleted, partially magnetized quantum spin ladders. We show that introducing nonmagnetic impurities into magnetized spin ladders leads to the emergence of a new characteristic length L in the otherwise scale-free Tomonaga-Luttinger liquid (serving as the effective low-energy model). This results in universal LT scaling of staggered susceptibilities. Comparison of simulation results with experimental phase diagrams of prototypical spin ladder compounds bis(2,3-dimethylpyridinium)tetrabromocuprate(II) (DIMPY) and bis(piperidinium)tetrabromocuprate(II) (BPCB) yields excellent agreement.

Quantum Spin‐1/2 Dimers in a Low‐Dimensional Tetrabromocuprate Magnet

This work describes a homometallic spin‐1/2 tetrabromocuprate adopting a bilayer structure. Magnetic‐susceptibility measurements show a broad maximum centred near 70 K, with fits to this data using a Heisenberg model consistent with strong antiferromagnetic coupling between neighbouring copper atoms in different layers of the bilayer. There are further weak intralayer ferromagnetic interactions between copper cations in neighbouring dimers. First‐principles calculations are consistent with this, but suggest there is only significant magnetic coupling within one direction of a layer; this would suggest the presence of a spin ladder within the bilayer with antiferromagnetic rung and weaker ferromagnetic rail couplings.

Disentangling the magnetic dimensionality of an alleged magnetically isolated cuprate spin-ladder CuHpCl system: a long-lasting issue

The Cu2(1,4-diazacycloheptane)2Cl4 (CuHpCl) crystal is a molecular transition metal antiferromagnetic complex, whose magnetism has been a long-lasting issue. The outcome of a variety of experimental studies (on magnetic susceptibility, heat capacity, magnetization, spin gap and INS) reported many different J values depending on the fitting ladder model used. From all available experimental data, one can infer that CuHpCl is a very complex system with many competing microscopic magnetic JAB interactions that lead to its overall antiferromagnetic behavior. A first-principles bottom-up study of CuHpCl is thus necessary in order to fully disentangle its magnetism. Here we incorporate data from ab initio computations providing the magnitude of the JAB interactions to investigate the microscopic magnetic couplings in CuHpCl and, ultimately, to understand the macroscopic magnetic behavior of this crystal. Strikingly, the resulting magnetic topology can be pictured as a 3D network of interacting squared plaquette magnetic building blocks, which does not agree with the suggested ladder motif (with uniform rails) that arises from direct observation of the crystal packing. The computed magnetic susceptibility, heat capacity and magnetization data show good agreement with the experimental data. In spite of this agreement, only the calculated magnetization data are used to discriminate between the different spin regimes in CuHpCl, namely gapped singlet, partially polarized and fully polarized phases. Additional analysis of the magnetic wavefunction enables the conclusion that long-range spin correlation can be discarded as being responsible for the partially polarized phase, whose magnetic response is in fact due to the complex interplay of the magnetic moments in the 3D magnetic topology.

A spin-1/2 gapped compound CdCu2(SeO3)2Cl2 with a ladder structure

The exploration of two-leg spin-ladder materials is a great challenge to the chemical community since it is one of the most ideal models for the study of low-dimensional magnetism and high-Tc superconductivity. Herein, we report on a successful synthesis of a new Cu2+-based two-leg ladder compound constructed by CuO4Cl2 octahedra along the [101] direction. The magnetic results exhibit a broad peak at Tmax ∼ 265 K, and suggest that CdCu2(SeO3)2Cl2 has a spin singlet ground state. The fitting of the isolated two-leg spin-ladder model shows J⊥/kB = 429 K and J‖/kB = 21 K, leading to a large spin gap of ∼409 K.

Spectroscopic and structural studies, thermal characterization, optical proprieties and theoretical investigation of 2-aminobenzimidazolium tetrachlorocobaltate(II)

In this study we present the crystal structure, spectroscopic and thermal behavior, Hirshfeld surface analysis, and DFT calculations of a new organic-inorganic hybrid compound (C₇H₈N₃)₂[CoCl₄]. This compound crystallizes in the centrosymmetric space group P1¯. Single-crystal X-ray diffraction analysis indicates that structure consists of a succession of mixed layers formed by organic cations and inorganic anions parallel to the (001) plane and propagate according to the c-axis. Layers further are assembled into a 3D supramolecular architecture through N-H^…Cl hydrogen bonds and π^…π interactions. The peak positions of the experimental PXRD pattern are in agreement with the simulated ones from the crystal structure, indicating phase purity of the title compound. The presence of the different functional groups and the nature of their vibrations were identified by ATR-FTIR and FT-Raman spectroscopies. The tetrahedral environment of Co²⁺ was confirmed by UV-visible spectroscopy, where the spectrum shows three weak absorption bands in the visible range due to d-d electronic transitions ⁴A₂(F) → ⁴T₂(F), ⁴A₂(F) → ⁴T₁(F) and ⁴A₂(F) → ⁴T₁(P) typical of Co(II) coordination compounds. The direct and indirect optical band gap values were determined by Tauc method. The optimized structure and calculated vibrational frequencies were obtained by density functional theory (DFT) using B3LYP functional. TGA and DSC coupled to mass spectrometry (MS) experiments under argon atmosphere in the temperature range (25-950 °C) were carried out in order to determine the thermal stability of the title compound.

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 (J_dimer = -107.4(1) K, θ = -1.22(4) K) and a strong-rung ladder model (J_rung = -105.8(7) K, J_rail = 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.

Revising the common understanding of metamagnetism in the molecule-based bisdithiazolyl BDTMe compound

The BDTMe molecule-based material is the first example of a thiazyl radical to exhibit metamagnetic behavior. Contrary to the common idea that metamagnetism occurs in low-dimensional systems, it is found that BDTMe magnetic topology consists of a complex 3D network of almost isotropic ferromagnetic spin-ladders that are coupled ferromagnetically and further connected by some weaker antiferromagnetic interactions. Calculated magnetic susceptibility χT(T) data is in agreement with experiment. Calculated M(H) data clearly show the typical sigmoidal shape of a metamagnet at temperatures below 2 K. The calculated critical field becomes more apparent in the dM/dH(H) plot, being in very good agreement with experiment. Our computational study concludes that the magnetic topology of BDTMe is preserved throughout the entire experimental range of temperatures (0-100 K). Accordingly, the ground state is the same irrespective of the temperature at which we study the BDTMe crystal. Revising the commonly accepted understanding of a metamagnet explained as ground state changing from antiferromagnetic to ferromagnetic, the Boltzmann population of the different states is here suggested to be the key concept: at 2 K the ground singlet state has more weight (24%) than at 10 K (1.5%), where excited states have an important role. Changes in the antiferromagnetic interactions that couple the ferromagnetic skeleton of BDTMe will directly affect the population of the distinct states that belong to a given magnetic topology and thus its magnetic response. Accordingly, this strategy could be valid for a wide range of bisdithiazolyl BDT-compounds whose magnetism can be tuned by means of weak antiferromagnetic interactions.

Low dimensional and frustrated antiferromagnetic interactions in transition metal chloride complexes with simple amine ligands

New transition metal chloride complexes with hydrazinium and methylhydroxylamine ligands are reported featuring low dimensional and frustrated magnetic interactions.

Crystal structure and Hirshfeld surface analysis of bis-(2,6-di-amino-pyridinium) tetra-chlorido-cobaltate(II)

In the title mol-ecular salt, (C5H8N3)2[CoCl4], the cations are protonated at their pyridine N atoms and the anion is an almost regular tetra-hedron. The crystal structure consists of alternating inorganic layers, built from tetra-chlorido-cobaltate anions, and organic layers formed by protonated cations of 2,6-di-amino-pyridinium. The crystal packing is governed by C/N-H⋯Cl hydrogen-bonding inter-actions between the organic and the inorganic ions and Cl⋯Cl inter-actions. Moreover, the cations show a π-π stacking inter-action [inter-centroid distance = 3.763 (2) Å]. The prevalence of these inter-actions is illustrated by an analysis of the three-dimensional Hirshfeld surface and by two-dimensional fingerprint plots.

Spin-singlet Quantum Ground State in Zigzag Spin Ladder Cu(CF3 COO)2

The copper salt of trifluoroacetic acid, Cu(CF₃ COO)₂ , offers a new platform to investigate the quantum ground states of low-dimensional magnets. In practice, it realizes the ideal case of a solid hosting essentially isolated magnetic monolayers. These entities are constituted by well-separated two-leg half-integer spin ladders organized in a zigzag fashion. The ladders are comprised of dimeric units of edge-sharing tetragonal pyramids coupled through carbon ions. The spin-gap state in this compound was revealed by static and dynamic magnetic measurements. No indications of long range magnetic ordering down to liquid helium temperature were obtained in specific heat measurements. First principles calculations allow estimation of the main exchange interaction parameters, J_⊥ =176 K and J_∥ =12 K, consistent with the weakly interacting dimers model.

Bound States and Field-Polarized Haldane Modes in a Quantum Spin Ladder

The challenge of one-dimensional systems is to understand their physics beyond the level of known elementary excitations. By high-resolution neutron spectroscopy in a quantum spin-ladder material, we probe the leading multiparticle excitation by characterizing the two-magnon bound state at zero field. By applying high magnetic fields, we create and select the singlet (longitudinal) and triplet (transverse) excitations of the fully spin-polarized ladder, which have not been observed previously and are close analogs of the modes anticipated in a polarized Haldane chain. Theoretical modeling of the dynamical response demonstrates our complete quantitative understanding of these states.

Magnetic-Order Crossover in Coupled Spin Ladders

We report a novel crossover behavior in the long-range-ordered phase of a prototypical spin-1/2 Heisenberg antiferromagnetic ladder compound (C_{7}H_{10}N)_{2}CuBr_{4}. The staggered order was previously evidenced from a continuous and symmetric splitting of ^{14}N NMR spectral lines on lowering the temperature below T_{c}≃330  mK, with a saturation towards ≃150  mK. Unexpectedly, the split lines begin to further separate away below T^{*}∼100  mK, while the linewidth and the line shape remain completely invariable. This crossover behavior is further corroborated by the NMR relaxation rate T_{1}^{-1} measurements. A very strong suppression reflecting the ordering, T_{1}^{-1}∼T^{5.5}, observed above T^{*}, is replaced by T_{1}^{-1}∼T below T^{*}. These original NMR features are indicative of the unconventional nature of the crossover, which may arise from a unique arrangement of the ladders into a spatially anisotropic and frustrated coupling network.

Dichotomy between Attractive and Repulsive Tomonaga-Luttinger Liquids in Spin Ladders

We present a direct NMR method to determine whether the interactions in a Tomonaga-Luttinger liquid (TLL) state of a spin-1/2 Heisenberg antiferromagnetic ladder are attractive or repulsive. For the strong-leg spin ladder compound (C_{7}H_{10}N)_{2}CuBr_{4} we find that the isothermal magnetic field dependence of the NMR relaxation rate T_{1}^{-1}(H) displays a concave curve between the two critical fields bounding the TLL regime. This is in sharp contrast to the convex curve previously reported for a strong-rung ladder, (C_{5}H_{12}N)_{2}CuBr_{4}. We show that the concavity and the convexity of T_{1}^{-1}(H), which is a fingerprint of spin fluctuations, directly reflect the attractive and repulsive fermionic interactions in the TLL, respectively. The interaction sign is alternatively determined from an indirect method combining bulk magnetization and specific heat data.

Emergent Interacting Spin Islands in a Depleted Strong-Leg Heisenberg Ladder

Properties of the depleted Heisenberg spin ladder material series (C_{7}H_{10}N)_{2}Cu_{1-z}Zn_{z}Br_{4} have been studied by the combination of magnetic measurements and neutron spectroscopy. Disorder-induced degrees of freedom lead to a specific magnetic response, described in terms of emergent strongly interacting "spin island" objects. The structure and dynamics of the spin islands is studied by high-resolution inelastic neutron scattering. This allows us to determine their spatial shape and to observe their mutual interactions, manifested by strong spectral in-gap contributions.

Magnetic Structure and Exchange Interactions in Quasi-One-Dimensional MnCl2(urea)2

MnCl2(urea)2 is a new linear chain coordination polymer that exhibits slightly counter-rotated Mn2Cl2 rhomboids along the chain-axis. The material crystallizes in the noncentrosymmetric orthorhombic space group Iba2, with each Mn(II) ion equatorially surrounded by four Cl(-) that lead to bibridged ribbons. Urea ligands coordinate via O atoms in the axial positions. Hydrogen bonds of the Cl···H-N and O···H-N type link the chains into a quasi-3D network. Magnetic susceptibility data reveal a broad maximum at 9 K that is consistent with short-range magnetic order. Pulsed-field magnetization measurements conducted at 0.6 K show that a fully polarized magnetic state is achieved at Bsat = 19.6 T with another field-induced phase transition occurring at 2.8 T. Zero-field neutron diffraction studies made on a powdered sample of MnCl2(urea)2 reveal that long-range magnetic order occurs below TN = 3.2(1) K. Additional Bragg peaks due to antiferromagnetic (AFM) ordering can be indexed according to the Ib'a2' magnetic space group and propagation vector τ = [0, 0, 0]. Rietveld profile analysis of these data revealed a Néel-type collinear ordering of Mn(II) ions with an ordered magnetic moment of 4.06(6) μB (5 μB is expected for isotropic S = (5)/2) oriented along the b-axis, i.e., perpendicular to the chain-axis that runs along the c-direction. Owing to the potential for spatial exchange anisotropy and the pitfalls in modeling bulk magnetic data, we analyzed inelastic neutron scattering data to retrieve the exchange constants: Jc = 2.22 K (intrachain), Ja = -0.10 K (interchain), and D = -0.14 K with J > 0 assigned to AFM coupling. This J configuration is most unusual and contrasts the more commonly observed AFM interchain coupling of 1D chains.

Syntheses, crystal structures, weak interaction, magnetic and photoluminescent properties of two new organic-inorganic molecular solids with disubstituted benzyl triphenylphosphinium and tetra(isothiocyanate)cobalt(II) anion

Two new organic-inorganic molecular solids of tetra(isothiocyanate)cobalt(II) dianion and disubstituted benzyl triphenylphosphinium, [2Cl4FBzTPP]2[Co(NCS)4] (1) and [2Cl4ClBzTPP]2[Co(NCS)4] (2) ([2Cl4FBzTPP](+)=1-(2'-chloro-4'-fluorobenzyltriphenylphosphonium) and [2Cl4ClBzTPP](+)=1-(2',4'-dichlorobenzyltriphenylphosphonium), were synthesized and characterized by elemental analysis, FT-IR, UV-Vis spectra, ESI-MS and single crystal X-ray diffraction method. Compounds 1 and 2 crystallize in the monoclinic Pc and triclinic P-1, respectively. The Co(II) ion of the [Co(NCS)4](2-) anion shows a distorted tetrahedral coordination geometry. The [2Cl4FBzTPP](+) cations containing P(2) atoms in 1 form a column by the Cl⋯π interactions, while the [2Cl4ClBzTPP](+) cations in 2 form two columns by the C-H⋯π and π⋯π interactions. The anion and the cation are linked by C-H⋯S hydrogen bonds and Cl⋯S interactions. Magnetic susceptibility measurement in the temperature range 2-300K shows that both 1 and 2 exhibit a weak antiferromagnetic exchange interaction as the temperature falls, and ultraviolet fluorescence emission in the solid state at room temperature.

1-(2,3-Di-methyl-phen-yl)piperazine-1,4-diium tetra-chlorido-cuprate(II)

In the title salt, (C12H20N2)[CuCl4], the Cu(II) atom occupies a general position in a flattened tetra-hedral environment by Cl ligands, characterized by Cl-Cu-Cl angles of 134.04 (3) and 137.18 (4)°. The six-membered piperazinediium ring adopts a chair conformation. The organic cation and inorganic anion inter-act through N-H⋯Cl and C-H⋯Cl hydrogen bonds, forming a three-dimensional network.

A magnetically isolated cuprate spin-ladder system: synthesis, structures, and magnetic properties

Two magnetically isolated cuprate spin ladders have been synthesized. Their crystal structures and preliminary investigation into the magnetic properties were reported.

Attractive Tomonaga-Luttinger liquid in a quantum spin ladder

We present NMR measurements of a strong-leg spin-1/2 Heisenberg antiferromagnetic ladder compound (C7H10N)2CuBr4 under magnetic fields up to 15 T in the temperature range from 1.2 K down to 50 mK. From the splitting of NMR lines, we determine the phase boundary and the order parameter of the low-temperature (three-dimensional) long-range-ordered phase. In the Tomonaga-Luttinger regime above the ordered phase, NMR relaxation reflects characteristic power-law decay of spin correlation functions as 1/T1∝T(1/2K-1), which allows us to determine the interaction parameter K as a function of field. We find that field-dependent K varies within the 1<K<2 range, which signifies attractive interaction between the spinless fermions in the Tomonaga-Luttinger liquid.

Spectrum of a magnetized strong-leg quantum spin ladder

Inelastic neutron scattering is used to measure the spin excitation spectrum of the Heisenberg S=1/2 ladder material (C7H10N)2CuBr4 in its entirety, both in the gapped spin liquid and the magnetic field-induced Tomonaga-Luttinger spin liquid regimes. A fundamental change of the spin dynamics is observed between these two regimes. Density matrix renormalization group calculations quantitatively reproduce and help understand the observed commensurate and incommensurate excitations. The results validate long-standing quantum field-theoretical predictions but also test the limits of that approach.