Strong Spin Frustration Research Articles

Ground-state tuning of metal-insulator transition by compositional variations inBaIr1−xRuxO3(0≤x≤1)

BaIrO3 is a magnetic insulator driven by the spin-orbit interaction (SOI), whereas BaRuO3 is a paramagnet and exhibits a crossover from a metallic to an insulating regime. Our investigation of structural, magnetic, transport and thermal properties reveals that substitution of Ru4+ (4d4) ions for Ir5+ (5d5) ions in BaIrO3 reduces the magnitudes of the SOI and a monoclinic structural distortion, and rebalances the competition between the SOC and the lattice degrees freedom to generate a rich phase diagram for BaIr1-xRuxO3 (0< x <1). There are two major effects of Ru additions: (1) Light Ru doping (0 < x < 0.15) prompts simultaneous, precipitous drops in both the magnetic ordering temperature TN and the electrical resistivity, which exhibits a crossover behavior from a metallic to an insulating state near TN. (2) Heavier Ru doping (0.41< x < 0.9) induces a robust metallic state with a strong spin frustration generated by competing antiferromagnetic and ferromagnetic interactions.

Effective magnetic coupling with strong spin frustration in (Ph3MeP+)(C60˙−) and reversible C60˙− dimerization in (Ph3MeP+)(C60˙−)·C6H5CN. Effect of solvent on structure and properties

Antiferromagnetic coupling of spins with spin frustration and fullerene dimerization are observed for (Ph3MeP+)(C60˙−) (left) and (Ph3MeP+)(C60˙−)·C6H5CN (right).

Strong spin frustration from isolated triangular Cu(ii) trimers in SrCu(OH)3Cl with a novel cuprate layer

Strontium oxocuprates such as Sr2CuO2Cl2 have been extensively investigated for their two-dimensional (2D) planar CuO2 square layers that are typical of high-temperature cuprate superconductors. A new synthetic strontium cuprate SrCu(OH)3Cl features a novel cuprate layer with isolated triangular Cu(II) trimers. The crystal structure of the title compound can be derived from breaking the cuprate planar layer to form isolated, nonplanar triangular copper trimers ({[Cu3(μ3-Cl)(OH)3](OH)6Cl2}6−), which are connected by strontium coordination spheres to form a novel framework structure. The copper trimers exhibit an unprecedented array that has triangular Cu3-planes perpendicular not only to their own layer but also to their counterparts in the next layers, which is thus different from the parallel array in the Kagomé layer of ZnCu3(OH)6Cl2. This cross-orientation arrangement of the triangular Cu(II) trimers is proposed to be responsible for a strong antiferromagnetic exchange (J = −233(2) cm−1) in the triangles characterized by a Weiss temperature θcw = −135 K but virtually zero (or negligible) magnetic coupling between the triangles. Such remarkable magnetic properties make the title compound a Mott insulator – an ordered inorganic solid with Cu spin-1/2 spins forming an intrinsic “molecular magnet”.

Unusual spin fluctuations and magnetic frustration in olivine and non-olivine LiCoPO4detected byP31andLi7nuclear magnetic resonance

We report $^{31}\mathrm{P}$ and $^{7}\mathrm{Li}$ nuclear magnetic resonance (NMR) studies in new non-olivine LiZnPO${}_{4}$-type LiCoPO${}_{4}^{\mathrm{tetra}}$ microcrystals, where the Co${}^{2+}$ ions are tetrahedrally coordinated. Olivine LiCoPO${}_{4}$, which was directly transformed from LiCoPO${}_{4}^{\mathrm{tetra}}$ by an annealing process, was also studied and compared. The uniform bulk magnetic susceptibility and the $^{31}\mathrm{P}$ Knight shift obey the Curie-Weiss law for both materials with a high spin Co${}^{2+}$ ($3{d}^{7}$, $S=3/2$), but the Weiss temperature $\ensuremath{\Theta}$ and the effective magnetic moment ${\ensuremath{\mu}}_{\mathrm{eff}}$ are considerably smaller in LiCoPO${}_{4}^{\mathrm{tetra}}$. The spin-lattice relaxation rate ${T}_{1}^{\ensuremath{-}1}$ reveals a quite different nature of the spin dynamics in the paramagnetic state of both materials. Our NMR results imply that strong geometrical spin frustration occurs in tetrahedrally coordinated LiCoPO${}_{4}$, which may lead to the incommensurate magnetic ordering.

Development of π-Electron Systems Based on [M(dmit)2] (M = Ni and Pd; dmit: 1,3-dithiole-2-thione-4,5-dithiolate) Anion Radicals

Abstract Molecular conductors based on [M(dmit)2] (M = Ni and Pd) present a variety of π electron systems that pave the way for a higher stage of solid-state science. Supramolecular interactions between [Ni(dmit)2] anion and halogen-containing cations provide bilayer systems that are characterized by coexistence of two crystallographically independent anion layers with different molecular arrangements and contrasting (for example, metal/insulator and ferromagnetic/antiferromagnetic) properties. In [Pd(dmit)2] salts in the Mott insulating state, a small energy difference between HOMO and LUMO coupled with strong dimerization affords HOMO–LUMO band inversion. The dimer units [Pd(dmit)2]2− form a triangular lattice, and interplay of strong electron correlation and spin frustration generates a wide variety of magnetic/charge states including antiferromagnetic long-range order, quantum spin liquid, charge order, and valence bond order, depending on counter cations. The cation dependence is attributed to a systematic arch-shaped molecular distortion that tunes the anisotropy of interdimer transfer integrals. This means that the [Pd(dmit)2] molecular skeleton is sufficiently flexible within the crystal field, and molecular degrees of freedom play an important role in fine tuning of the electronic state.

Transition metal phosphite complexes: from one-dimensional chain, two-dimensional sheet, to three-dimensional architecture with unusual magnetic properties

Four new compounds, [Ni(HPO3)(4,4′-bpy)(H2O)3]·4H2O (1), [Co2(HPO3)2(4,4′-bpy)2(H2O)6]·9H2O (2), [Zn(HPO3)(4,4′-bpy)0.5]·H2O (3), and [Co3(PO3)2(4,4′-bpy)3(H2O)6]·3H2O (4), have been synthesized. Compounds 1 and 2 are linear chains and isostructural except for the distinction of the metal ion and the number of lattice water molecules. Compound 3 consists of a 2D sheet structure. Compound 4 is a 3D magnetic porous material with 1D channels along the c axis. Magnetic measurements indicate that compound 1 shows paramagnetic behavior, while compound 2 displays antiferromagnetic behavior. Compound 4 exhibits antiferromagnetism with a pronounced field-induced spin–flop transition. A critical field of 3 T at 2.0 K is determined from the derivative dM/dH curve. Meanwhile, compound 4 shows a strong spin frustration arising from the geometric frustration in the kagome lattice. The dehydrated phase 4a displays a characteristic of N2 adsorption with type II isotherm. We performed detailed magnetic measurements for 4a, which exhibits quite different magnetic properties from 4. Compound 4a displays a ferrimagnetic behavior with a lower transition field of 0.2 T, and a weaker spin frustration.

Geometrical Spin Frustration and Ferromagnetic Ordering in (MnxPb2–x)Pb2Sb4Se10

Engineering the atomic structure of an inorganic semiconductor to create isolated one-dimensional (1D) magnetic subunits that are embedded within the semiconducting crystal lattice can enable chemical and electronic manipulation of magnetic ordering within the magnetic domains, paving the way for (1) the investigation of new physical phenomena such as the interactions between electron transport and localized magnetic moments at the atomic scale and (2) the design and fabrication of geometrically frustrated magnetic materials featuring cooperative long-range ordering with large magnetic moments. We report the design, synthesis, crystal structure and magnetic behavior of (MnxPb2-x)Pb2Sb4Se10, a family of three-dimensional manganese-bearing main-group metal selenides featuring quasi-isolated [(MnxPb2-x)3Se30]∞ hexanuclear magnetic ladders coherently embedded and uniformly distributed within a purely inorganic semiconducting framework, [Pb2Sb4Se10]. Careful structural analysis of the magnetic subunit, [(MnxPb2-x)3Se30]∞ and the temperature dependent magnetic susceptibility of (MnxPb2-x)Pb2Sb4Se10, indicate that the compounds are geometrically frustrated 1D ferromagnets. Interestingly, the degree of geometrical spin frustration (f) within the magnetic ladders and the strength of the intrachain antiferromagnetic (AFM) interactions strongly depend on the concentration (x value) and the distribution of the Mn atom within the magnetic substructure. The combination of strong intrachain AFM interactions and geometrical spin frustration in the [(MnxPb2-x)3Se30]∞ ladders results in a cooperative ferromagnetic order with exceptionally high magnetic moment at around 125 K. Magnetotransport study of the Mn2Pb2Sb4Se10 composition over the temperature range from 100 to 200 K revealed negative magnetoresistance (NMR) values and also suggested a strong contribution of magnetic polarons to the observed large effective magnetic moments.

Are trinuclear superhalogens promising candidates for building blocks of novel magnetic materials? A theoretical prospect from combined broken-symmetry density functional theory and ab initio study

The structures, relative stabilities, vertical electron detachment energies, and magnetic properties of a series of trinuclear clusters are explored via combined broken-symmetry density functional theory and ab initio study. Several exchange-correlation functionals are utilized to investigate the effects of different halogen elements and central atoms on the properties of the clusters. These clusters are shown to possess stronger superhalogen properties than previously reported dinuclear superhalogens. The calculated exchange coupling constants indicate the antiferromagnetic coupling between the transition metal ions. Spin density analysis demonstrates the importance of spin delocalization in determining the strengths of various couplings. Spin frustration is shown to occur in some of the trinuclear superhalogens. The coexistence of strong superhalogen properties and spin frustration implies the possibility of trinuclear superhalogens working as the building block of new materials of novel magnetic properties.

Structure and magnetism of S = 1/2 kagome antiferromagnets NiCu3(OH)6Cl2 and CoCu3(OH)6Cl2

We have successfully synthesized S = 1/2 kagome antiferromagnets MCu3(OH)6Cl2 (M = Ni and Co) by a hydrothermal method with a rotating pressure vessel. Structural characterization shows that both compounds have similar crystal structure to ZnCu3(OH)6Cl2 with symmetry. As with ZnCu3(OH)6Cl2, the compounds show no obvious hysteresis at 2 K. A spin-glass transition is found in both NiCu3(OH)6Cl2 and CoCu3(OH)6Cl2 at low temperatures (6.0 and 3.5 K respectively) by AC susceptibility measurements. This indicates no long-range magnetic order and a strong spin frustration. The substitution of Zn2+ by magnetic ions Ni2+ or Co2+ effectively enhances the interlayer exchange coupling and changes the ground state of the kagome spin system.

Spin-glass-like freezing in spin-chain compounds Ca3Co2−xMnxO6: Effect of disorder

We have illustrated the presence of spin-glass-like freezing in the Mn-doped spin-chain compounds Ca3Co2−xMnxO6, which was not reported previously. With increasing Mn composition, the magnetic relaxation due to spin freezing is gradually suppressed. Interestingly, the magnetization in the intermediate Mn compositions relaxes more slowly (characteristic time is significantly enhanced) than those in x = 0.0 and 1.0. This could be understood within the framework of disorder with randomness or imperfections, which plays an important role in the formation of spin-glass-like feature in intermediate Mn compositions, quite different from the cases in x = 0.0 with strong spin frustration and in x ∼ 1.0 with perfect Co/Mn ionic order.

Spin dynamics of S = 1/2 kagome lattice antiferromagnets observed by high‐field ESR

AbstractDue to the existence of strong spin frustration in a system, the spin dynamics of S = 1/2 kagome lattice antiferromagnet at low temperature has attracted much interest. High‐field ESR has been measured on its model substances, Cu3V2O7(OH)2 · 2H2O (volborthite) and BaCu3V2O3(OH)2 (vesignieite), down to 1.8 K using pulsed magnetic fields up to 16 T. The measurements are performed for 160 and 315 GHz using polycrys‐talline samples. Although both samples showed the g‐shift and the change of linewidth at low temperature, volborthite showed a small gap excitation of the order of 40 GHz (1.9 K) while vesignieite showed a paramagnetic behavior down to 1.9 K. Observed difference will be discussed in connection with the crystal structure, and the possible spin liquid state in vesignieite will be discussed.

Incommensurate spin correlation driven by frustration inBiCu2PO6

The magnetic properties of ${\text{BiCu}}_{2}{\text{PO}}_{6}$ have been analyzed by means of magnetic-susceptibility and inelastic neutron-scattering measurements on powder samples by evaluating the spin-exchange interactions on the basis of density-functional calculations and by simulating the inelastic neutron scattering in terms of spin-exchange parameters. ${\text{BiCu}}_{2}{\text{PO}}_{6}$ exhibits magnetic properties described by the two-leg spin ladder with strong spin frustration along each leg chain and has a gapped quantum singlet ground state with excited magnetic states, showing an incommensurate dispersion arising from frustration.

Spin Frustration and Long-Range Ordering in an AlB2-like Metal-Organic Framework with Unprecedented N,N,N-Tris-tetrazol-5-yl-amine Ligand

Solvothermal treatment of a mixture of Co(OAc)(2), NaN(3), NaN(CN)(2), HF, MeCN, and H(2)O generated an AlB(2)-like metal-organic framework, namely, [{(Co(3)F)(3)(trta)(2)(H(2)O)(9)}{Co(Hbta)(3)}(2)].11H(2)O (1) (H(3)trta = N,N,N-tris-tetrazol-5-yl-amine; H(3)bta = N,N-bis-tetrazol-5-yl-amine), in which supertrianglar supramolecular building blocks (SBBs) {(Co(3)F)(3)(trta)(2)} act as 12-connected nodes of hexagonal prisms. The unprecedented ligand N,N,N-tris-tetrazol-5-yl-amine is in situ generated via metal-assisted reactions of NaN(CN)(2), MeCN, and NaN(3), and a possible mechanism involves both 1,3-dipolar cycloaddition and nucleophilic addition. The F-centered Co(3) triangles are bridged by trta groups to result in supertriangular {(Co(3)F)(3)(trta)(2)} SBBs that are further arranged in a frustrated triangular lattice. The long-range ordering temperature is reduced to 4.8 K due to strong spin frustration.

Spin frustration and magnetic exchange in cobalt aluminum oxide spinels

We report on x-ray diffraction, magnetic susceptibility, electron-spin resonance, and heat-capacity studies of $\mathrm{Co}{[{\mathrm{Al}}_{1\ensuremath{-}x}{\mathrm{Co}}_{x}]}_{2}{\mathrm{O}}_{4}$ for Co concentrations $0\ensuremath{\leqslant}x\ensuremath{\leqslant}1$. In this spinel system only the $A$-site ${\mathrm{Co}}^{2+}$ cation is magnetic, while the nonmagnetic $B$-site ${\mathrm{Al}}^{3+}$ is substituted by the low-spin nonmagnetic ${\mathrm{Co}}^{3+}$, and it is possible to investigate the complete phase diagram from ${\mathrm{Co}}^{2+}\mathrm{Al}^{3+}{}_{2}{\mathrm{O}}_{4}$ to ${\mathrm{Co}}^{2+}\mathrm{Co}^{3+}{}_{2}{\mathrm{O}}_{4}$. All samples reveal large negative Curie-Weiss temperatures ${\ensuremath{\Theta}}_{\mathrm{CW}}$ of the order of $\ensuremath{-}110\phantom{\rule{0.3em}{0ex}}\mathrm{K}$ independent of concentration, which is attributed to a high multiplicity of the superexchange interactions between the $A$-site ${\mathrm{Co}}^{2+}$ cations. A pure antiferromagnetic state is found for $x=1.0$ and 0.9 with N\'eel temperatures ${T}_{N}=29.5$ and $21.2\phantom{\rule{0.3em}{0ex}}\mathrm{K}$, respectively, as evidenced by lambdalike anomalies in the specific heat. Compositions with $0.3\ensuremath{\leqslant}x\ensuremath{\leqslant}0.75$ show smeared out strongly reduced magnetic ordering temperatures. At low temperatures, a ${T}^{2.5}$ dependence of the specific heat is indicative of a spin-liquid state. For $x\ensuremath{\leqslant}0.2$ a ${T}^{2}$ dependence of the specific heat and a spin-glass-like behavior of the susceptibility below ${T}_{f}\ensuremath{\sim}4.7\phantom{\rule{0.3em}{0ex}}\mathrm{K}$ are observed. The high value of the frustration parameter $f=\ensuremath{\mid}{\ensuremath{\Theta}}_{\mathrm{CW}}\ensuremath{\mid}∕{T}_{f}g10$ indicates the presence of strong spin frustration at least for $x\ensuremath{\leqslant}0.6$. The frustration mechanism is attributed to competing nearest-neighbor and next-nearest-neighbor superexchange interactions between the $A$-site ${\mathrm{Co}}^{2+}$ ions.

Predicting Exchange Coupling Constants in Frustrated Molecular Magnets Using Density Functional Theory

We study the Heisenberg exchange couplings in polynuclear transition-metal clusters with strong spin frustration using a variety of theoretical techniques. We present results for a trinuclear Cr(III) molecule, a tetranuclear Fe(III) complex, and an octanuclear Fe(III) molecular magnet. We explore the physics of the exchange couplings in these systems using standard broken-symmetry (BS) techniques and a more recently developed constrained density functional theory (C-DFT) approach. The calculations show that the expected picture of localized spin moments on the metal centers is appropriate, and in each case C-DFT predicts coupling constant values in good agreement with experiment. Furthermore, we demonstrate that all of the C-DFT spin states for a given cluster can be reasonably described by a single Heisenberg Hamiltonian. These findings are significant in part because standard BS calculations are in conflict with the experiments on a number of key points. For example, BS-DFT predicts a doublet (rather than quartet) ground state for the Cr(III) cluster while for the Fe(III) complexes BS-DFT predicts some of the exchange couplings to be ferromagnetic whereas the experimentally derived couplings are all antiferromagnetic. Furthermore, for BS-DFT the best-fit exchange parameters can depend significantly on the set of spin configurations chosen. For example, by choosing configurations with Ms closer to Ms(max) the BS-DFT couplings can typically be made somewhat closer to the C-DFT and experimental results. Thus, in these cases, our results consistently support the experimental findings.

1/3 Magnetization Plateau in Spin-1/2 Square Lattice Antiferromagnet (CuBr)Sr2Nb3O10

A new triple-layered perovskite (CuBr)Sr 2 Nb 3 O 10 with the S = 1/2 square lattice has been prepared through a chimie douce route. Presence of strong spin frustration is inferred from zero-field specific heat measurements which exhibit successive phase transitions at 9.3 and 7.5 K. Application of magnetic fields merges the transition temperatures at 3 T, suggesting that the intermediate phase is of magnetic origin with fluctuating spin–spin correlation. Despite the square geometry, magnetization curves have revealed a metamagnetic transition to a novel phase characterized by a 1/3 plateau of the saturated magnetization.

Spin-orbital gap of multiorbital antiferromagnet with geometrical frustration

In order to discuss the spin-gap formation in a multiorbital system, we analyze an e_g-orbital Hubbard model on a geometrically frustrated zigzag chain by using a density-matrix renormalization group method. Due to the appearance of a ferro-orbital arrangement, the system is regarded as a one-orbital system, while the degree of spin frustration is controlled by the spatial anisotropy of the orbital. In the region of strong spin frustration, we observe a finite energy gap between ground and first-excited states, which should be called a spin-orbital gap. The physical meaning is clarified by an effective Heisenberg spin model including correctly the effect of the orbital arrangement influenced by the spin excitation.

Low-field DC-magnetization study of Ho 3+-doped Mn–Zn ferrite ferrofluid

The physical and magnetic properties of magnetic nanoparticles are crucial for their effectiveness and reliability in biomedical applications. In this article, we report the synthesis of a stable Ho-substituted Mn–Zn ferrite ferrofluid and its physical and magnetic properties. Substitution by rare earth metal plays an important role in determining the magneto-crystalline anisotropy in 4f-3d inter-metallic compounds. Ho 3+ substitution not only enhanced the magnetic anisotropy but also produced strong spin frustration at low temperature. The field dependence of blocking temperature shows H 2/3 dependency in the entire range of field, i.e., 10–700 Oe, indicating the emergence of Ising spins characteristics in the present system.

Static impurities in theS=1/2kagome lattice: Dimer freezing and mutual repulsion

We consider the effects of doping the $S=1/2$ kagome lattice with static impurities. We demonstrate that impurities lower the number of low-lying singlet states, induce dimer-dimer correlations of considerable spatial extent, and do not generate free spin degrees of freedom. Most importantly, they experience a highly unconventional mutual repulsion as a direct consequence of the strong spin frustration. These properties are illustrated by exact diagonalization, and are reproduced to semiquantitative accuracy within a dimer resonating-valence-bond description which affords access to longer length scales. We calculate the local magnetization induced by doped impurities, and consider its implications for nuclear magnetic resonance measurements on known kagome systems.

X-,K- and Q-band ESR studies on intercalated Fe 0.9PS 3(Phen) 0.4

The temperature and frequency dependences of electron spin resonance parameters of polycrystalline powders of intercalated Fe 0.9PS 3(Phen) 0.4 have been studied in the temperature range 9–300 K. A very strong and relatively broad single ESR peak in X-,K-and Q-band spectra have been observed. However, a very sharp decrease in X-band ESR signal intensity and accompanying line broadening below 15 K indicates a very strong spin frustration at lower temperatures. K- and Q-band spectra show almost a linear dependence of resonance field on frequency. From the analysis of the frequency-dependent resonance field of ESR signal, the effects of the internal field is refined and thus the spectroscopic g-factor and internal field were determined to be 1.9576 and 197 G, respectively. Also very small non-linearity in H r versus frequency curve was attributed to the external field dependency of the internal field through magnetic susceptibility. Some small and relatively smooth changes at about 40, 80 and 160 K appeared as well, indicating some kind of magnetic anomolies.