Preparation And Magnetic Studies Of Spin-Chain Compounds R₂BaNiO₅（R=Y,Gd）Nanocrystals

The Haldane chain compounds R₂BaNiO₅（R=rare earths or Y）were recently recognized as classical model systems in condensed matter physics to study quasi-one-dimensional magnetism,because of the exotic physical properties（Haldane gap,anisotropy,multiferroic and magnetoelectric effect）.Up to date,most of the magnetic studies on R₂BaNiO₅ are focused on the doping element and doping amount,but there are few studies on the nanoparticles.In this thesis,nano-size effect is introduced in the well-known Haldane system Y₂BaNiO₅ and Gd₂BaNiO₅.The magnetic properties of bulk and nanocrystalline materials were studied by magnetization and pulsed-field electron spin resonance（ESR）measurements.The main investigations of this thesis are as follows.（1）The basic concepts of Haldane spin-chain systems were presented and a brief introduction about the compounds R₂BaNiO₅ was described.Then we introduced the chemical doping effects on low dimensional quantum magnetic and the nano-size effect.（2）The introduction for sample synthesis,experimental procedure for physical property measurements is presented,including the parameters of the measuring instrument and measuring methods under high magnetic field.（3）Polycrystal and nanoparticles of Y₂BaNiO₅ was obtained by the solid-state reaction method and mechanically milling method,respectively.XRD spectra show that all the samples are single phase.The reduction of grain size with milling time were showed by analyzing the full width at half maximum（FWHM）of the peaks,and can be further confirmed by the SEM images.The magnetization of nanoparticles measured by SQUID is much enhanced as compared with that of the bulk material.The Haldane gap in the bulk of Y₂BaNiO₅ tends to be suppressed as the grain size is reduced down to the nanometer scale.The chain-end S-1/2 spins have an important contribution to the magnetism of nanoparticles.The uncompensated and canted spins enhance the paramagnetic contribution,also resulting in the magnetic enhancement.High field ESR measurements show that the single ESR peak observed in the bulk sample becomes broadened as the system converted into nano meter scale.The frequency-field（f-H）relationship is linear and passes through the origin.Shorter chains as well as S=1/2edge-chain spins play an important role.（4）The Gd₂BaNiO₅ polycrystal and nanoparticles ware prepared by the same methods with Y₂BaNiO₅.Structure and particle size of the Gd₂BaNiO₅ crystallites were analyzed by XRD.The morphology of the Gd₂BaNiO₅ crystallites was observed by SEM.The magnetization of nanoparticles measured by SQUID is much enhanced as compared with that of the bulk material.The spinflop in the bulk of Gd₂BaNiO₅ form tends to be suppressed as the grain size is reduced down to the nanometer scale.The ESR data show that the magnetization of nanoparticles originates from additional“paramagnetic”Ni ions and the“paramagnetic”Gd ions.