Syntheses And Magnetic Properties For New Kagome Lattice And Spin Cluster Antiferromagnets

In recent years,low-dimensional transition mental oxides have attracted considerable interests owing to their rich and exotic magnetic phenomena,such as quantum spin liquid state,quantum magnetic plateau,valence-bond condensation as well as Bose-Einstein condensation.The magnetism of materials is related to the types of magnetic ions and the topological structures formed by magnetic ions.Thus,exploring and synthesizing new quantum magnets play an important role in the development of quantum magnetism.In this paper,we synthesized a series of new two-dimensional quantum magnets by hydrothermal method,including Kagome lattice compound Cu₄?OH?₆FCl,spin pentamer compound Na₃Cu₅?PO₄?₄F?4H₂O and spin hexamer compounds Sr₂M₃?C₂O₄?₃?OH?₄·3H₂O?M=Co,Ni?.Susceptibility,specific heat,pulsed magnetic field magnetization and electronic spin resonance measurements have been taken to research the magnetism of these compounds.Combining with theoretical analysis,the ground state and magnetic coupling of these compounds have been studied reasonably.In addition,the reduced-size effect of Haldane chain system SrNi_2-xMg_xV₂O₈?x=0,0.1?have been reported.The main contents of this paper are as follow:1.A new spin-1/2 antiferromagnet Cu₄?OH?₆FCl with perfect Kagome lattice was synthesized by hydrothermal method.The analyses of single crystal structure show Cu₄?OH?₆FCl contains a geometrically perfect Kagome lattice formed by Cu²⁺ions.The magnetic susceptibility gives a Curie-Wiess temperature?_CW=-135 K,nearest-neighbor exchange J=-166 K,and two magnetic anomalies at T₁=15 K and T₂=4.8 K with weak ferromagnetism.The high-field ESR data are strongly correlated with the magnetism.There are a gapless mode and two gapped modes,corresponding to different temperature regions.2.A new spin pentamer compound Na₃Cu₅?PO₄?₄F?4H₂O has been synthesized using a conventional hydrothermal method.The analyses of single crystal structure show the basic magnetic unit of this compound is a{Cu₅?PO₄?₄F}?Cu₅ cluster?.These Cu₅clusters are connected by phosphate group forming a layered structure along ab plane.Susceptibility and specific heat exhibit a short-range ordering at 19.2 K followed by a long-range antiferromagnetic ordering at 11.5 K.High-field magnetization measurement demonstrates a 2/5 quantum magnetization plateau above 40 T.The ESR data indicate the presence of magnetic anisotropy,in accordance with the two-dimensional structure of the system.3.Two new isostructural spin hexamer compounds Sr₂M₃?C₂O₄?₃?OH?₄·3H₂O?M=Co,Ni?were synthesized using a conventional hydrothermal method.The analyses of single crystal structure show that the basic magnetic unit of these compounds is a Co₆O₂₄/Ni₆O₂₄?Co₆/Ni₆ cluster?.For Sr₂Co₃?C₂O₄?₃?OH?₄·3H₂O,susceptibility and specific heat exhibit a long-range antiferromagnetic ordering at 9.97 K.High-field magnetization measurement demonstrates two magnetic transitions at H₁=13.7 T and H₂=38.1 T,respectively.While no long-range antiferromagnetic ordering occurs at low temperature for Sr₂Ni₃?C₂O₄?₃?OH?₄·3H₂O,indicating a spin-singlet like behavior.4.Nanoparticles of Haldane chain system SrNi_2-xMg_xV₂O₈?x=0,0.1?were synthesized by ball milling method.Powder XRD and Fullprof refinements were taken to determine the structure parameters and grain size of all samples.Magnetization and ESR results imply that a reduced sample grain size breaks the long chain and creates a considerable number of S=1/2 edge spins,resulting in the enhancement of magnetization and the decrease of Haldane gap in the nanoparticles.