Spin-gap Behavior Research Articles

Two Distinct Cu(II)-V(IV) Superexchange Interactions with Similar Bond Angles in a Triangular "CuV2" Fragment.

The strength and sign of superexchange interactions are often predicted on the basis of the bond angles between magnetic ions, but complications may arise in situations with a nontrivial arrangement of the magnetic orbitals. We report on a novel molecular tetramer compound [Cu(H2O)dmbpy]2[V2O2F8] (dmbpy = 4,4'-dimethyl-2,2'-bipyridyl) that is composed of triangular "CuV2" fragments and displays a spin gap behavior. By combining first-principles calculations and electronic models, we reveal that superexchange Cu-V interactions carry drastically different coupling strengths along two Cu-F-V pathways with comparable bond angles in the triangular "CuV2" fragment. Counterintuitively, their strong disparity is found to originate from the restricted symmetry of the half-filled Cu dx2-y2 orbital stabilized by the crystal field, leading to one dominating antiferromagnetic Cu-V coupling in each fragment. We revisit the magnetic properties of the reported spin-gapped chain compound [enH2]Cu(H2O)2[V2O2F8] (enH2 = ethylene diammonium) containing similar triangular "CuV2" fragments, and the magnetic behavior of the molecular tetramer and the chain compounds is rationalized as that of weakly coupled spin dimers and spin trimers, respectively. This work demonstrates that fundamentally different magnetic couplings can be observed between magnetic ions with similar bond angles in a single spin motif, thus providing a strategy to introduce various exchange interactions combined with low dimensionality in heterometallic Cu(II)-V(IV) compounds.

Synthesis, structure and magnetic properties of a new spin-dimer compound CaCu(SeO3)2

A new selenite compound CaCu(SeO3)2 was obtained by a hydrothermal method. This compound crystallizes in the monoclinic system of space group P21/c, featuring a typical spin-dimer structure. The results of magnetic and heat capacity measurements confirm that this compound has a nonmagnetic singlet ground state, while a fit of magnetic susceptibility using the spin-dimer model suggests a large spin gap of ∼329 ​K in the system. Although CaCu(SeO3)2 and HgCu(SeO3)2 have a similar spin-dimer structure built by [Cu2O8] dimers and SeO3 groups, the spin gap of CaCu(SeO3)2 is much larger than that of HgCu(SeO3)2. Such different spin-gap behaviors may be due to a change on the bonding geometry of spin dimers with a substitution of Ca2+ ions for Hg2+ ions, leading to stronger intradimer and weaker interdimer interaction exchanges in CaCu(SeO3)2. Also, the solid-state UV–vis diffuse reflectance spectrum shows an optical band gap of 2.71 ​eV in the system.

Unusual spin pseudogap behavior in the spin web lattice Cu3TeO6 probed by Te125 nuclear magnetic resonance

We present a $^{125}\mathrm{Te}$ nuclear magnetic resonance (NMR) study in the three-dimensional spin web lattice ${\mathrm{Cu}}_{3}\mathrm{Te}{\mathrm{O}}_{6}$ which harbors topological magnons. The $^{125}\mathrm{Te}$ NMR spectra and the Knight-shift $\mathcal{K}$ as a function of temperature show a drastic change at ${T}_{\mathrm{S}}\ensuremath{\sim}40\phantom{\rule{0.28em}{0ex}}\mathrm{K}$ much lower than the N\'eel ordering temperature ${T}_{\mathrm{N}}\ensuremath{\sim}61\phantom{\rule{0.28em}{0ex}}\mathrm{K}$, providing evidence for the first-order structural phase transition within the magnetically ordered state. Most remarkably, the temperature dependence of the spin-lattice relaxation rate ${T}_{1}^{\ensuremath{-}1}$ unravels spin-gap-like magnetic excitations, which sharply sets in at ${T}^{*}\ensuremath{\sim}75\phantom{\rule{0.28em}{0ex}}\mathrm{K}$, the temperature well above ${T}_{\mathrm{N}}$. The spin-gap behavior may be understood by weakly dispersive optical magnon branches of high-energy spin excitations originating from the unique corner-sharing Cu hexagon spin-1/2 network with low coordination number.

A review on crystal structure and properties of 3d transition metal (II) orthophosphates M3(PO4)2

Abstract A family of anhydrous 3d transition metal (II) orthophosphates M3(PO4)2 (M = Cr - Zn) counts fourteen members adopting amazing variety of eight structural types. Even more diverse are structural types in mixed cation orthophosphates A3-nMn(PO4)2 (A = Sr, Ca, Ba, Pb). Rich crystal chemistry of these species is associated with flexibility of basic structures of sarcopside Fe3(PO4)2, graftonite (Fe,Mn,Ca)3(PO4)2 and farringtonite Mg3(PO4)2 minerals. The low temperature magnetic properties of these compounds are quite peculiar showing either spin gap behavior due to the presence of even spin clusters in the crystal structure or formation of non-collinear magnetic structures with temperature driven succession of magnetic order phase transitions, multiple magnetization reversal and well defined plateaus in magnetization at low temperatures. The correspondence between crystal structure and physical properties in the vast family of transition metal orthophosphates is reviewed. In addition, the structural relationships between different members of the title family are discussed.

Magnetism and spin-gap behaviour in the layered Pr3T4Al12 (T = Fe, Ru, Os) compounds with the distorted Kagomé lattice

We have investigated the physical and magnetic properties of the distorted Kagomé lattice structure compounds; PrAl12 (T = Fe, Ru, Os) which crystallize in the hexagonal Gd3Ru4Al12-type structure with space group P63/mmc (No. 194). The three compounds show long-range magnetic orderings of FM type at TC = 36.5 K (Fe), 39 K (Ru) and 37 K (Os) as indicated by the temperature dependences of magnetic susceptibility, , specific heat, Cp (T) and electrical resistivity, . Pr3Ru4Al12 shows an additional anomaly at 7 K which is associated to possible spin reorientation of some of the Pr moments. We also observed an additional anomaly in Pr3Fe4Al12 at 132 K which might be associated to the formation of a spin density wave. The Sommerfeld coefficient, extracted from Cp (T) in the temperature range immediately above TC indicate heavy fermion behaviours in PrAl12 (T = Fe, Ru, Os). of the three compounds show spin-gap behaviours below TC of comparable energy gaps magnitude.

Tabby graphene: Dimensional magnetic crossover in fluorinated graphite

Tabby is a pattern of short irregular stripes, usually related to domestic cats. We have produced Tabby patterns on graphene by attaching fluorine atoms running as monoatomic chains in crystallographic directions. Separated by non-fluorinated sp2 carbon ribbons, sp3-hybridized carbon atoms bonded to zigzag fluorine chains produce sp2-sp3 interfaces and spin-polarized edge states localized on both sides of the chains. We have compared two kinds of fluorinated graphite samples C2Fx, with x near to 1 and x substantially below 1. The magnetic susceptibility of C2Fx (x < 1) shows a broad maximum and a thermally activated spin gap behaviour that can be understood in a two-leg spin ladder model with ferromagnetic legs and antiferromagnetic rungs; the spin gap constitutes about 450 K. Besides, stable room-temperature ferromagnetism is observed in C2Fx (x < 1) samples: the crossover to a three-dimensional magnetic behaviour is due to the onset of interlayer interactions. Similarly prepared C2Fx (x ≈ 1) samples demonstrate features of two-dimensional magnetism without signs of high-temperature magnetic ordering, but with transition to a superparamagnetic state below 40 K instead. The magnetism of the Tabby graphene is stable until 520 K, which is the temperature of the structural reconstruction of fluorinated graphite.

Metal-insulator transition and magnetic fluctuations in polycrystalline Ru1−xRhxP investigated by P31 NMR

$^{31}\mathrm{P}$ NMR measurements have been made on polycrystalline samples to study a metal-insulator (MI) transition and magnetic fluctuations in ${\mathrm{Ru}}_{1\ensuremath{-}x}{\mathrm{Rh}}_{x}\mathrm{P}$ which has metallic (M), pseudogap (PG), insulating (I), and superconducting (SC) phases. We find that RuP undergoes a crossover from the high-temperature (high-$T$) M phase to the PG phase with the pseudo spin-gap behavior probed by the nuclear spin-lattice relaxation rate at ${T}_{\mathrm{PG}}=330\phantom{\rule{4pt}{0ex}}\mathrm{K}$. The first-order MI transition is observed to take place from the PG phase to the low-$T$ nonmagnetic I phase with the spin-gap energy of 1250 K at ${T}_{\mathrm{MI}}=270\phantom{\rule{4pt}{0ex}}\mathrm{K}$. In the PG phase of ${\mathrm{Ru}}_{1\ensuremath{-}x}{\mathrm{Rh}}_{x}\mathrm{P}$ with $0\ensuremath{\le}x&lt;0.45$, an analysis based on the modified Korringa relation, which is applicable to an itinerant paramagnet with weak electron correlation, shows that antiferromagnetic (AFM) fluctuations described in the random-phase approximation are enhanced in the low-$T$ and low-$x$ region. Around the PG-M phase boundary at ${x}_{\mathrm{c}}\ensuremath{\sim}0.45$, there is the SC phase whose normal state has negligible electron-electron interaction. We discuss the MI transition, the crossover from the M phase to the PG phase, and the magnetic properties of each phase based on the band structure.

Magnetic Phase Diagram of the Breathing Pyrochlore Antiferromagnet LiGa1−xInxCr4O8

The spinel oxides LiGaCr4O8 and LiInCr4O8 contain size-alternating pyrochlore lattices of spin-3/2 Cr3+ tetrahedra with different magnitudes of alternation. We show here that the solid solutions LiGa1-xInxCr4O8 between these two 'breathing' pyrochlore compounds display (i) rapid suppression of magnetic and structural transitions upon doping the end members, (ii) spin-glass-like freezing above 2 K in the range 0.1 $\lesssim$ x $\lesssim$ 0.6, and (iii) apparent spin-gap behavior for x $\gtrsim$ 0.7. Furthermore, no transitions are observed above 2 K at x ~ 0.9, where magnetic susceptibility remains finite at 2 K and magnetic heat capacity shows a quadratic temperature dependence at 1-5 K. Our work shows that breathing pyrochlore compounds provide a unique opportunity for studying both geometrical frustration and bond alternation.

Local magnetic properties of $$L\times L$$ L × L square lattice with an Extended Hubbard Model

Within an analytical diagonalization, it is possible to exactly solve a large class of models useful for the study of strongly correlated electrons systems. Here, we perform an exact diagonalization of the two-dimensional Extended Hubbard Model with first nearest neighbor repulsive $$V$$ and spin-spin interaction $$J$$ at one-eight filling. The analysis of the ground state energy and spin gap behaviors as a function of $$V$$ and $$J$$ terms allows us to study some local magnetic properties of $$2D$$ ferromagnetic correlated electrons system distributed in finite square lattice. The obtained results show the existence of competition between Spin Density Wave and Charge Density Wave with the possibility to have a phase transition between these two waves at a critical value $$V_{c}$$

Ionic size effect on the spin gap nature of SrCu2(TeO3)2Cl2

By means of magnetic susceptibility measurements, we report the effects of partial substitution of Br onto the Cl sites of SrCu2(TeO3)2Cl2. This material has been a subject of current interest due to indications of a spin gap behavior. However, the magnetic spin couplings responsible for the spin gap nature remain unclear. For each composition of SrCu2(TeO3)2(Cl1−xBrx)2, the temperature-dependent susceptibility exhibits a character of low-dimensional magnetism with a broad maximum at Tmax. The magnitude of Tmax slightly shifts to higher temperatures with increasing Br concentration. The experimental data can be well fitted to a coupled spin dimer model, yielding an increasing trend for the spin gap size from 172 to 188 K. This indicates that the substitution of Br ions has the effect of reducing the distance between Cu(1) and Cu(2) ions, leading to a stronger antiferromagnetic spin interaction. On the other hand, the variation of the interdimer coupling is marginal, indicative of substituting Br ions having little effect on the distance changes of Cu(1)-Cu(1) and Cu(2)-Cu(2) in SrCu2(TeO3)2Cl2.

Nonmagnetic impurity effect of the S = 1/2 spin ladder system (pipdH)2Cu1−x Zn x Br4

We report the synthesis and the magnetic susceptibility of (pipdH)2Cu1-xZnxBr4, which is a nonmagnetic impurity-doped S=1/2 spin ladder system. The samples were synthesized from a solution by using a slow evaporation method. Samples were confirmed to be in a single phase and to have the same crystal structure as the pure system (pipdH)2CuBr4 by using X-ray diffraction measurements. To check the magnetic properties, we performed magnetic susceptibility and magnetization measurements with a SQUID magnetometer. A plot of the inverse magnetic susceptibility indicates the presence of dominant antiferromagnetic coupling. The magnetic susceptibility shows a broad maximum due to low dimensionality and a spin gap behavior related to the two-leg spin ladder at low temperature. The spin gap and the Curie constants of Zn-doped samples, as estimated from an analysis of the magnetic susceptibility, monotonically decrease as the Zn concentration decreases. A nonmagnetic impurity of S=1/2 spin ladder system affects the spin gap.

Breathing Pyrochlore Lattice Realized inA-Site Ordered Spinel OxidesLiGaCr4O8andLiInCr4O8

A unique type of frustrated lattice is found in two A-site ordered spinel oxides, LiGaCr(4)O(8) and LiInCr(4)O(8). Because of the large size mismatch between Li(+) and Ga(3+)/In(3+) ions at the A site, the pyrochlore lattice, made up of Cr(3+) ions carrying spin 3/2, becomes an alternating array of small and large tetrahedra, i.e., a "breathing" pyrochlore lattice. We introduce a parameter, the breathing factor B(f), which quantifies the degree of frustration in the pyrochlore lattice: B(f) is defined as J'/J, where J' and J are nearest-neighbor magnetic interactions in the large and small tetrahedra, respectively. LiGaCr(4)O(8) with B(f)~0.6 shows magnetic susceptibility similar to that of conventional Cr spinel oxides such as ZnCr(2)O(4). In contrast, LiInCr(4)O(8) with a small B(f)~0.1 exhibits a spin-gap behavior in its magnetic susceptibility, suggesting a proximity to an exotic singlet ground state. Magnetic long-range order occurs at 13.8 and 15.9 K for LiGaCr(4)O(8) and LiInCr(4)O(8), respectively, in both cases likely owing to the coupling to structural distortions.

Tuning of magnetic ground state of the spin-12square-lattice compound Zn2VO(PO4)2through chemical substitution

Using first-principles density-functional-theory-based calculations, we derive the low-energy spin model of the compound Zn${}_{2}$VO(PO${}_{4}$)${}_{2}$, and $\frac{1}{4}$th Ti-substituted compound Zn${}_{8}$TiV${}_{3}$O${}_{4}$(PO${}_{4}$)${}_{8}$ which is yet to be synthesized. We compute the thermodynamics properties of the proposed spin models by the quantum Monte Carlo technique. For the pristine compound Zn${}_{2}$VO(PO${}_{4}$)${}_{2}$ our computed susceptibility is found to be in good agreement with the available experimental data and is in accordance with the earlier proposed spin model of the spin-$\frac{1}{2}$ antiferromagnetic square lattice. Upon Ti substitution, which may be viewed as $\frac{1}{4}$-spin depletion, the two-dimensional antiferromagnetic square-lattice behavior of the parent compound is found to be altered significantly, giving rise to spin-gap behavior. We hope that our work will stimulate further experimental studies on this compound.

On the cause for the no spin-gap behavior of the triangular spin tube system CsCrF4

The spin exchange interactions of the triangular spin tube system CsCrF4 were evaluated by performing mapping analysis on the basis of density functional theory calculations. Our results show that the exchange J1 for the triangular ring and the exchange J2 along the chain are both antiferromagnetic with the ratio J1/J2≈0.5, so the spin gap of CsCrF4 is too small to be experimentally detected. This finding is consistent with the experimental observation and theoretical analysis. The possible ways of preparing a triangular spin tube with observable spin gap was discussed.

Characterization of the spin gap nature in Na3Cu2SbO6 using 23Na NMR

We report the results of a Na23 nuclear magnetic resonance (NMR) study on Na3Cu2SbO6 at temperatures between 4 and 300K. This material has been a subject of current interest due to indications of spin gap behavior. The temperature-dependent NMR shift exhibits a character of low-dimensional magnetism with a broad maximum at Tmax≃95K. While the NMR shift data can be fitted well to both antiferromagnetic–antiferromagnetic (AF–AF) and antiferromagnetic–ferromagnetic (AF–F) alternating chain models, the former yields a spin gap of 62K which is too small as compared to the value obtained from the neutron scattering measurement. On the other hand, a spin gap of about 112K based on the AF–F alternating chain scenario is quite consistent with the reported value. In addition, the employment of the AF–F alternating chain picture to analyze the spin-lattice relaxation rate reveals a gap size of 105K, in good agreement with the 8.9meV peak observed in the inelastic neutron scattering experiment. With this accordance, we point out that the AF–F alternating chain picture is more appropriate for the understanding of the spin gap nature in Na3Cu2SbO6.

13C-NMR studies of the paramagnetic and charge-ordered states of the organic superconductorβ′′-(BEDT−TTF)3Cl2·2H2O under pressure

${\ensuremath{\beta}}^{\ensuremath{'}\ensuremath{'}}$-(BEDT$\ensuremath{-}$TTF)${}_{3}$Cl${}_{2}$$\ifmmode\cdot\else\textperiodcentered\fi{}$2H${}_{2}$O [BEDT$\ensuremath{-}$TTF: $\underline{\mathrm{b}}$is-($\underline{\mathrm{e}}$thylene$\underline{\mathrm{d}}$i$\underline{\mathrm{t}}$hio)$\underline{\mathrm{t}}$etra$\underline{\mathrm{t}}$hia$\underline{\mathrm{f}}$ulvalene] is superconductive under pressures, whereas the salt exhibits metal-insulator (MI) transition under ambient pressure. The insulator phase in the salt was examined using the charge density wave (CDW) phase that was obtained from band calculation. The charge-ordered (CO) state was recently proposed as the insulator phase of the salt, and the mechanism of superconductivity intermediated by charge fluctuation was suggested. We accessed ${}^{13}$C-NMR on ${\ensuremath{\beta}}^{\ensuremath{'}\ensuremath{'}}$-(BEDT$\ensuremath{-}$TTF)${}_{3}$Cl${}_{2}$$\ifmmode\cdot\else\textperiodcentered\fi{}$2H${}_{2}$O at ambient pressure and under pressure up to 1.6 GPa. At ambient pressure, the NMR spectrum changed at approximately 100 K. Three isolated peaks appeared at low temperatures, suggesting that the CO state exists below 100 K, and spin-gap behavior was observed. By analyzing the chemical shift, the charges on the three sites were estimated as $\ensuremath{\sim}$+0.4$e$, $\ensuremath{\sim}$+0.6$e$, and $\ensuremath{\sim}$+1.0$e$. The ratio of peak intensity and unsymmetrical peak position suggest the CO state with some symmetry breaking. When pressure is applied, the splitting of the NMR peaks in the CO state is reduced. The salt finally exhibits superconductivity at 1.6 GPa, spin-gap behavior observed at ${({T}_{1}T)}^{\ensuremath{-}1}$ below 1.3 GPa suddenly disappears, whereas the NMR spectrum predicts that charge disproportionation coexists with superconductivity. The suppression of the spin-singlet formation observed in ${({T}_{1}T)}^{\ensuremath{-}1}$ at 1.6 GPa suggests the metallic state with the charge disproportionation and the CO instability with some symmetry breaking.

QUANTUM PHASE TRANSITION OF A TRIANGULAR LATTICE SPIN TUBE AND EDGE SPIN EFFECTS

We study the low-energy excitation of the quantum spin tube with the triangular lattice structure, using density matrix renormalization group. Taking account of the edge spin effect, we particularly investigate the spin gap behavior and the low-field magnetization curve near the quantum phase transition point in contrast with the usual free boundary condition. We then find that the bulk behavior of the spin tube can be extracted easier for the single spin termination.

Structural distortion and frustrated magnetic interactions in the layered copper oxychloride(CuCl)LaNb2O7

We present a computational study of the layered copper oxychloride $(\text{CuCl}){\text{LaNb}}_{2}{\text{O}}_{7}$ that has been recently proposed as a spin-1/2-frustrated square lattice compound. Our results evidence an orbitally degenerate ground state for the reported tetragonal crystal structure and reveal a Jahn-Teller-type structural distortion. This distortion heavily changes the local environment of copper---${\text{CuO}}_{2}{\text{Cl}}_{2}$ plaquettes are formed instead of ${\text{CuO}}_{2}{\text{Cl}}_{4}$ octahedra---and restores the single-orbital scenario typical for copper oxides and oxyhalides. The calculated distortion is consistent with the available diffraction data and the experimental results on the electric field gradients for the Cu and Cl sites. The band structure suggests a complex three-dimensional spin model with the interactions up to the fourth neighbors. Despite the layered structure of $(\text{CuCl}){\text{LaNb}}_{2}{\text{O}}_{7}$, the spin system has pronounced one-dimensional features. Yet, sizable interchain interactions lead to the strong frustration and likely cause the spin-gap behavior. Computational estimates of individual exchange couplings are in qualitative agreement with the experimental data.

3D network structure and spin gap behavior in two new molecular magnets based on bis(maleonitriledithiolato)nickelate monoanion

Two new molecular magnets, based on [Ni(mnt) 2] − monoanion, [DiBrBzPy][Ni(mnt) 2] ( 1) and [DiBrBzIQl][Ni(mnt) 2] ( 2) ([DiBrBzPy] + = 1-(3′,5′-dibromobenzyl)pyridinium, [DiBrBzIQl] + = 1-(3′,5′-dibromobenzyl)isoquinolinium and mnt 2− = maleonitriledithiolate), were prepared and characterized by elemental analyses, IR, ESI-MS spectra, single crystal X-ray diffraction and magnetic measurements. The [Ni(mnt) 2] anions and the cations of 1 and 2 are alternately stacked and form 1D column via π⋯π stacking interactions between the [Ni(mnt) 2] anions and the neighboring cations. Some weak Ni⋯N, C⋯N interactions and C H⋯Br, C H⋯N hydrogen bonds between the adjacent columns further generate a 3D network structure. Magnetic susceptibility measurements show that both 1 and 2 exhibit the typical magnetic behavior of a spin gap system with an energy gap of 1151.9 K for 1 and 73.9 K for 2.

The Spin Gap Transition around 180 K Observed in a New Molecular Magnet based on the [Ni(mnt)2]– Monoanion and Substituted Benzylpyridinium

AbstractA new molecular magnet, [NO2BrBzPyCH3][Ni(mnt)2] (1) ([NO2BrBzPyCH3]+ = 1‐(2′‐bromo‐4′‐nitrobenzyl)‐2‐methylpyridinium, and mnt2– = maleonitriledithiolate), has been prepared and characterized by single crystal X‐ray diffraction and magnetic measurements. The Ni(III) ions of 1 form a quasi‐one‐dimensional alternating zig‐zag magnetic chain within a Ni(mnt)2– column by intermolecular Ni···S, Ni···Ni or π···π interactions, and the [NO2BrBzPyCH3]+ cations stack into a column via weak Br···O interaction, p···π stacking interactions and C‐H···O hydrogen bonds between the cations. Magnetic susceptibility measurements in the temperature range 1.8‐300 K show that 1 exhibits a spin‐gap transition around 180 K, and an antiferromagnetic interaction in the high‐temperature phase (HT) and spin gap behavior in the low‐temperature phase (LT). The transition for 1 is a second‐order phase transition as determined by DSC analyses.