CoV2O6 Single Crystals Grown in a Closed Crucible: Unusual Magnetic Behaviors with Large Anisotropy and 1/3 Magnetization Plateau

Quantum annealing of a frustrated magnet

Quantum annealing, which involves quantum tunnelling among possible solutions, has state-of-the-art applications not only in quickly finding the lowest-energy configuration of a complex system, but also in quantum computing. Here we report a single-crystal study of the frustrated magnet α-CoV2O6, consisting of a triangular arrangement of ferromagnetic Ising spin chains without evident structural disorder. We observe quantum annealing phenomena resulting from time-reversal symmetry breaking in a tiny transverse field. Below ~ 1 K, the system exhibits no indication of approaching the lowest-energy state for at least 15 hours in zero transverse field, but quickly converges towards that configuration with a nearly temperature-independent relaxation time of ~ 10 seconds in a transverse field of ~ 3.5 mK. Our many-body simulations show qualitative agreement with the experimental results, and suggest that a tiny transverse field can profoundly enhance quantum spin fluctuations, triggering rapid quantum annealing process from topological metastable Kosterlitz-Thouless phases, at low temperatures.

Large magnetic anisotropy of a decorated spin-chain system K2Co3(MoO4)3(OH)2

The magnetic structure of K2Co3(MoO4)3(OH)2 is studied in detail. The material has a half-sawtooth one-dimensional (1-D) structure containing two unique Co2+ ions, one in the chain backbone and one on the apex of the sawtooth creating a series of isosceles triangles along the b-axis. These triangles can be a source of magnetic frustration. The ability to grow large single crystals enables detailed magnetic measurements with the crystals oriented in a magnetic field along the respective axes. It has a Curie-Weiss temperature θCW of 5.3(2) K with an effective magnetic moment of 4.8(3)μB/Co. The material is highly anisotropic with a sharp antiferromagnetic ordering transition at 7 K with a metamagnetic transition at 2 kOe. Neutron diffraction was used to determine the magnetic structure and revealed a magnetic structure with canted spins along the backbone of the chain while spins along the sawtooth caps maintained a colinear orientation, arranging antiferromagnetically relative to the backbone spins. The parallel chains arrange antiferromagnetically relative to each other along the c-axis and ferromagnetically along the a-axis.

Shape-Controlled First-Row Transition Metal Vanadates for Electrochemical and Photoelectrochemical Water Splitting

Transition metal vanadates (MVs) possess abundant electroactive sites, short ion diffusion pathways, and optical properties that make them suitable for various electrochemical (EC) and photoelectrochemical (PEC) applications. While these materials are commonly used in energy storage devices like batteries and capacitors, their shape-controlled 1D and 2D morphologies have gained equal popularity in water splitting (WS) technology in recent times. This review focuses on recent progress made on various first-row (3d, 4 s) transition metal vanadates (t-MVs) having controlled one-dimensional (fiber, wire, or rod) and two-dimensional (layered or sheet) morphologies with a specific emphasis on copper vanadates (CuV), cobalt vanadates (CoV), iron vanadates (FeV), and nickel vanadates (NiV). The review covers different aspects of shape-controlled 1D and 2D t-MVs including optoelectrical properties, wet chemistry synthesis, and electrochemical (EC-WS) and photoelectrochemical water splitting (PEC-WS) performance in terms of onset potential, overpotential, and long-term stability or high cyclic performance. The review concludes by providing some possible thoughts on how to promote the water-splitting attributes of shape-controlled t-MVs more effectively.

Quantum phase transitions in skewed ladder systems

In this brief review, we introduce a new spin ladder system called skewed spin ladders and discuss the exotic quantum phases of this system. The spin ladders studied are the 5/7, 3/4 and 3/5 systems corresponding to alternately fused 5 and 7 membered rings; 3 and 4 membered rings; and 3 and 5 membered rings. These ladders show completely different behaviour as the Hamiltonian model parameter is changed. When the Hamiltonian parameter is increased the 5/7 ladder switches from an initial singlet ground state to progressively higher spin ground state and then to a reentrant singlet state before finally settling to the highest spin ground state whose spin equals the number of unit cells in the system. The 3/4 ladder goes from a singlet ground state to a high spin ground state with each unit cell contributing spin 1 to the state, as the model parameter is increased. The 3/5 ladder shows a singlet ground state for small parameters and high spin ground state for intermediate values of the parameter and for still higher parameters, a reentrant singlet ground state. They can also show interesting magnetization plateaus as illustrated by studies on a specific spin ladder.

A new compound BaNa2Co7Te3O18 with distorted 2-uniform lattice (T13) showing unusual magnetic behaviors. Chem Commun (Camb) 2022;58:10937-10940. [DOI: 10.1039/d2cc03616a] [Citation(s) in RCA: 0] [Impact Index Per Article: 0]

Search for transition-metal compounds with unique spin lattices is a great challenge. Herein, we report on a successful exploration of a new compound BaNa2Co7Te3O18, featuring a unique spin network of...

Unusual magnetization process and magnetocaloric effect in α-CoV2O6driven by pulsed magnetic fields

In low-dimensional Ising spin systems, an interesting observation is the presence of step magnetization at low temperatures. Here we combine both DC and pulsed magnetic fields to study the 1/3 magnetization plateau and multiple steps in the Ising spin-chain material α-CoV₂O₆. Magnetization in pulsed fields is quite different from that in DC fields, showing multiple steps in an intermediate range of 4.2-6 K, inverted hysteresis below 4.2 K and asymmetric magnetization in negative fields below 11 K. We demonstrate that these unusual behaviors in magnetization are caused by the spin dynamics and the anomalous magnetocaloric effect (MCE) in α-CoV₂O₆, i.e., abrupt changes of sample temperature in adiabatic conditions. We successfully separate the influence between the intrinsic slow spin dynamics and the quasi-extrinsic temperature change. From the MCE, we find that some irreversible behavior is originated from the slow spin dynamics. Two different slow dynamics associated with the metastable steps are observed: one is sensitive to the slow field sweep rate at the order of ∼mT s^-1and weakly depends on temperature, while the other responds to the rapid field sweep rate of ∼kT s^-1and dominates at lowest temperature. We also distinguish that the metastable transition atH₄is the first order and crucial for the ferrimagnetic to ferromagnetic transition. This study is useful to the understanding of multistep magnetization in α-CoV₂O₆and sheds light on recent experimental findings of related compounds.

Observation of a Large Magnetic Anisotropy and a Field-Induced Magnetic State in SrCo(VO4)(OH): A Structure with a Quasi One-Dimensional Magnetic Chain

A new member of the descloizite family, a cobalt vanadate, SrCo(VO₄)(OH), has been synthesized as large single crystals using high-temperature and high-pressure hydrothermal methods. SrCo(VO₄)(OH) crystallizes in the orthorhombic crystal system in space group P2₁2₁2₁ with the following unit cell parameters: a = 6.0157(2) Å, b = 7.645(2) Å, c = 9.291(3) Å, V = 427.29(2) ų, and Z = 4. It contains one-dimensional Co-O-Co chains of edge-sharing CoO₆ octahedra along the a-axis connected to each other via VO₄ tetrahedra along the b-axis forming a three-dimensional structure. The magnetic susceptibility of SrCo(VO₄)(OH) indicates an antiferromagnetic transition at 10 K as well as unusually large spin orbit coupling. Single-crystal magnetic measurements in all three main crystallographic directions displayed a significant anisotropy in both temperature- and field-dependent data. Single-crystal neutron diffraction at 4 K was used to characterize the magnetically ordered state. The Co²⁺ magnetic spins are arranged in a staggered configuration along the chain direction, with a canting angle that follows the tipping of the CoO₆ octahedra. The net magnetization along the chain direction, resulting in ferromagnetic coupling of the a-axis spin components in each chain, is compensated by an antiferromagnetic interaction between nearest neighbor chains. A metamagnetic transition appears in the isothermal magnetization data at 2 K along the chain direction, which seems to correspond to a co-alignment of the spin directions of the nearest neighbor chain. We propose a phenomenological spin Hamiltonian that describes the canted spin configuration of the ground state and the metamagnetic transition in SrCo(VO₄)(OH).

Field-induced magnetic transitions and strong anisotropy in α-CoV2O6 single crystal

The Ising-like antiferromagnet α-CoV₂O₆ has received considerable interests because of stabilized 1/3 magnetization plateau around 5 K under magnetic field applied along magnetic easy c-axis. In this work, this magnetization plateau was studied by varying temperature or rotating magnetic field. As temperature decreased, this stabilized plateau collapsed, and additional magnetic transitions were observed. As a result, a rich magnetic phase diagram was constructed and extended to temperature lower than previously reported. When magnetic field moved from the c to b (or a) axis, the magnetization plateau developed with field directions and vanished finally when the field was restricted in the ab plane. An impressive observation is that this 1/3-plateau can be stabilized and remain robust even when magnetic field deviated from the c axis, accompanied by the evolutions of the magnetic moments and the critical transition fields. We suppose that the origins of these temperature and angular dependences of the 1/3 magnetization plateau are related to strong spin-orbital coupling. Indeed, electron spin resonance (ESR) measurement gives large Landé factor of 8.9, evidencing that there exists strong spin-orbital coupling.

An unprecedented pentanary chalcohalide with Mn atoms in two chemical environments: unique bonding characteristics and magnetic properties

An unprecedented 2D layered pentanary chalcohalide Cs₂[Mn₂Ga₃S₇Cl] features distinguishing structure characteristics with the coexistence of two Mn-coordinated polyhedra and shows interesting ferrimagnetic behaviour.

A facile method to synthesize CoV2O6 as a high-performance supercapacitor cathode

Transition metal oxides can easily lose electrons and thus possess multiple accessible valences. Especially, if two different transition metals are combined, better capacity and cycling stability are achieved. In this study, a binary transition metal oxide, CoV₂O₆, was synthesized via a facile co-precipitation process for use as a supercapacitor cathode; the as-synthesized CoV₂O₆ exhibited high-capacity (306.6 F g⁻¹, 1 A g⁻¹ and 219.2 F g⁻¹, 20 A g⁻¹) and stable cycling stability, retaining 83.3% of its initial specific capacitance after 20 000 cycles. We believe that this facile synthesis process presents an effective method and a new opportunity for promoting the application of electrode materials based on binary transition metal oxides in supercapacitors.

A facile chemical co-precipitation process to synthesize CoV₂O₆, which exhibits high capacity and cycling stability (83.3% after 20 000 cycles).

Synthesis and magnetic properties of new tellurate compounds Na4MTeO6 (M = Co and Ni) with a ferromagnetic spin-chain structure

New tellurate compounds Na₄MTeO₆ (M = Co and Ni) were synthesized by a conventional high-temperature solid-state reaction. The title compounds crystallize in the monoclinic system of a space group P2/c and the triclinic system of a space group P1[combining macron], respectively. Both compounds exhibit a zigzag spin-chain structure, in which Na₄CoTeO₆ shows uniform chains built by Co²⁺ ions and Na₄NiTeO₆ shows alternating chains built by Ni²⁺ ions. Magnetic measurements confirm that Na₄CoTeO₆ and Na₄NiTeO₆ possess an antiferromagnetic ordering at T_N = ∼3 K and ∼6.5 K, respectively, while a metamagnetic transition can be observed in the magnetization curve with an increasing field at 2 K.

Energy Gap of Neutral Excitations Implies Vanishing Charge Susceptibility

In quantum many-body systems with a U(1) symmetry, such as particle number conservation and axial spin conservation, there are two distinct types of excitations: charge-neutral excitations and charged excitations. The energy gaps of these excitations may be independent from each other in strongly correlated systems. The static susceptibility of the U(1) charge vanishes when the charged excitations are all gapped, but its relation to the neutral excitations is not obvious. Here we show that a finite excitation gap of the neutral excitations is, in fact, sufficient to prove that the charge susceptibility vanishes (i.e., the system is incompressible). This result gives a partial explanation for why the celebrated quantization condition n(S-m_{z})∈Z at magnetization plateaus works even in spatial dimensions greater than one.

Homogeneous Cobalt/Vanadium Complexes as Precursors for Functionalized Mixed Oxides in Visible-Light-Driven Water Oxidation

The heterometallic complexes (NH₄ )₂ [Co(H₂ O)₆ ]₂ [V₁₀ O₂₈ ]⋅4 H₂ O (1) and (NH₄ )₂ [Co(H₂ O)₅ (β-HAla)]₂ [V₁₀ O₂₈ ]⋅4 H₂ O (2) have been synthesized and used for the preparation of mixed oxides as catalysts for water oxidation. Thermal decomposition of 1 and 2 at relatively low temperatures (<500 °C) leads to the formation of the solid mixed oxides CoV₂ O₆ /V₂ O₅ (3) and Co₂ V₂ O₇ /V₂ O₅ (4). The complexes (1, 2) and heterogeneous materials (3, 4) act as catalysts for photoinduced water oxidation. A modification of the thermal decomposition procedure allowed the deposition of mixed metal oxides (MMO) on a mesoporous TiO₂ film. The electrodes containing Co/V MMOs in TiO₂ films were used for electrocatalytic water oxidation and showed good stability and sustained anodic currents of about 5 mA cm^-2 at 1.72 V versus relative hydrogen electrode (RHE). This method of functionalizing TiO₂ films with MMOs at relatively low temperatures (<500 °C) can be used to produce other oxides with different functionality for applications in, for example, artificial photosynthesis.

Intra-chain superexchange couplings in quasi-1D 3d transition-metal magnetic compounds

The electronic structure and magnetic properties of the quasi-1D transition-metal borates PbMBO4 (M  =  Ti, V, Cr, Mn, Fe, Co) have been investigated by density functional theory, including electronic correlation. The results evidence PbCrBO4 and PbFeBO4 as antiferromagnetic (AFM) semiconductors (intra-chain AFM and inter-chain FM) and PbMnBO4 as a ferromagnetic (FM) semiconductor (both intra- and inter-chain FM) in accordance with experimental observations. For non-synthesized PbTiBO4, PbVBO4, and PbCoBO4, the ground-state magnetic structures are paramagnetic, FM, and paramagnetic, respectively. In this series of compounds, there are two kinds of superexchange couplings dominating their magnetic properties, i.e. the direction M-M delocalization superexchange and indirect M-O-M correlation superexchange. For PbMBO4 with M (3+) d  (n) , n  ⩽  3 (M  =  V and Cr), the main intra-chain spin coupling is the M-M t 2g-t 2g direct delocalization superexchange, while for PbMBO4 with M (3+) d  (n) , n  >  3 (M  =  Mn and Fe), the main intra-chain spin coupling is the near 90° M-O-M e g-p-e g indirect correlation superexchange.

Pure Single-Crystalline Na1.1V3O7.9 Nanobelts as Superior Cathode Materials for Rechargeable Sodium-Ion Batteries

Pure single-crystalline Na1.1V3O7.9 nanobelts are successfully synthesized for the first time via a facile yet effective strategy. When used as cathode materials for Na-ion batteries, the novel nanobelts exhibit excellent electrochemical performance. Given the ease and effectiveness of the synthesis route as well as the very promising electrochemical performance, the results obtained may be extended to other next-generation cathode materials for Na-ion batteries.

Syntheses and magnetic properties of new tellurite-sulfate compounds M2(TeO3)(SO4)·H2O (M = Co, Mn) with a layer structure showing a distorted honeycomb spin-lattice

New tellurite-sulfate compounds M2(TeO3)(SO4)·H2O (M = Co, Mn) are synthesized by a conventional hydrothermal method. Two compounds are found to exhibit a similar structure, which both crystallize in the orthorhombic system of space group Pbcm. Te(4+) ions are coordinated by three O atoms, forming a quite distorted TeO3 trigonal pyramid with lone-pair electrons, while magnetic Co(2+) or Mn(2+) ions construct a wavelike layer with a distorted honeycomb spin-lattice. Magnetic measurements confirm that two isostructural compounds display different magnetic behaviors, in which Co2(TeO3)(SO4)·H2O shows a canted antiferromagnetic ordering at ∼15 K, while Mn2(TeO3)(SO4)·H2O shows a collinear antiferromagnetic ordering at ∼28 K. The nature of different magnetic behaviors between two isostructural compounds is also discussed.