Synthesize, Structure And Magnetic Properties Of Coupled Spin Clusters With （SeO₃）^2- Or （TeO₃）^2- Groups

A finite number of spins linked together via exchange interactions and containing weak intercluster couplings can be regarded as coupled spin cluster compound.Coupled spin cluster compounds are able to achieve the crossover between zero-and three-dimensionalities and can be considered as perfect candidates to investigate quantum effects and exotic physical behaviors,such as quantum criticality,Higgs excitations and so on.In order to study the magnetism of coupled spin cluster systems,it is crucial to synthesize new coupled spin cluster materials and obtain the information of exchange interactions.It was reported that the groups of（SeO₃）^2-and（TeO₃）^2-play an important role in the synthesis of coupled spin cluster compounds.In this dissertation,four kinds of coupled spin cluster compounds,containing（SeO₃）^2-or（TeO₃）^2-groups,were successfully synthesized via traditional hydrothermal method and solid-state reaction method.The magnetic properties of them were systematically investigated by pulsed high magnetic field magnetization and ESR measurements together with theoretical calculations.The gist of this dissertation can be summarized as follows:Firstly,the single crystals and powder samples of S_eff=1/2 BaCo₂（SeO₃）₃·3H₂O with spin dimer were successfully synthesized via a traditional hydrothermal method.The compound crystallizes in hexagonal lattice with space group P6₃（No.173）.The spin-dimers connected via the（SeO₃）^2-groups constitute a three-dimensional honeycomb lattice.The magnetic susceptibility and heat capacity measurements indicate that this compound undergoes antiferromagnetic（AFM）order at T_N=7.6 K.The magnetization M（H）curve at2 K presents a spin-flop transition within 2.6-3.0 T when the magnetic field is applied along the ab plane（H⊥c）,and a concave-upward before magnetic saturation around 22 T,which pointed to the existence of easy-axis type of anisotropy and quantum effect.The results of magnetization and ESR measurements suggest that the anisotropy of magnetization and g factor of this material is negligible.The fittings and simulations of the M（H）curves using exact diagonalization and quantum Monte Carlo methods give the exchange interactions J₁/k _B=-20.0 K（intradimer）,J₂/k _B=-14.6 K（the nearest-neighboring interdimer）,and J₃/k _B=-6.2 K（the next-nearest-neighboring interdimer）.Thus,BaCo₂（SeO₃）₃?3H₂O is strong coupled dimer materials,although structurally it contains dimer chain.Secondly,the new S=1 spin-dimer compound BaNi₂（SeO₃）₃·3H₂O was successfully synthesized via a traditional hydrothermal method.This sample is an isostructural compound of BaCo₂（SeO₃）₃·3H₂O and crystallizes in hexagonal lattice with space group P6₃（No.173）.The results of magnetic measurements suggest that this compound undergoes an AFM order at T_N=26.0 K,and presents a field induced spin-flop transition at H_sf=3.6T.The M（H）curve measured under the pulsed high magnetic field exhibits a weak 1/2magnetization plateau around 47.6 T.Thus,BaNi₂（SeO₃）₃·3H₂O is a spin-dimer compound with strong interdimer coupling.Thirdly,the S=1/2 spin-dimer compound BaCu（SeO₃）₂ has been successfully synthesized via the solid-state reaction method.It crystallizes in the orthorhombic crystal system with space group Pnm2₁（No.31）.The spin dimers were connected by the（SeO₃）^2-groups,forming a three-dimensional honeycomb lattice.The results of magnetic measurements demonstrate that this compound experiences an AFM short-range order at T₁=3 K,a short-range ferromagnetic（FM）correlation below T_G=62 K,and a field-induced magnetic transition at 0.4 T.At high temperature region,the inverse magnetic susceptibility?^-1（T）curve follows the Curie-Weiss law withθ_CW=11.3 K.The?^-1（T）curves measured under different magnetic fields deviate downward from paramagnetic Curie-Weiss behavior withinθ_CW<T<T_G,mimicking the Griffiths-like phase.The density function theory calculations and the quantum Monte Carlo simulations of M（H）curve demonstrate that the intradimer interaction are strong FM（J₁/k _B=39.09 K）and the interdimer couplings are weak AFM（J₂/k _B=-1.51 K,J₃/k _B=-1.68 K）,respectively.The competing between the exchange interactions are proposed to be responsible for the formation of Griffiths-like phase in weak coupling FM spin-dimer compound BaCu（SeO₃）₂.Fourthly,a new S=1/2 coupled spin cluster compound Y₂Cu₇（TeO₃）₆Cl₆（OH）₂ has been successfully synthesized via a traditional hydrothermal method.It crystallizes in the triclinic crystal system with space group-1（No.1）.The magnetic ions constitute a two-dimensional layered lattice in which the Cu₄ clusters form diamond chains and these chains are further connected by the Cu₃ clusters.The results of magnetic measurements show that the compound undergoes an AFM long range order at T_N=4.1 K and exhibits a field induced magnetic transition around H_c=0.2 T.The high-field magnetization and ESR measurements suggest that this compound exhibits a wide 3/7-like magnetization plateau above 7 T.The discussion on possible mechanism of the magnetization plateau suggested that the intercluster couplings are strong.Thus,the new compound Y₂Cu₇（TeO₃）₆Cl₆（OH）₂ formed by coupled spin clusters of Cu₃ trimer and Cu₄ tetramer,is a two-dimensional layered antiferromagnet.