Synthesis And Characterization Of Rare-earth Doped Low-dimensional Perovskite Manganite Oxide Nanostructures

Rare-earth doped perovskite manganites display rich physical phenomena and complex phase structures due to the complex interactions among the spin,charge,orbital,and lattice degrees of freedom.Nanostructured perovskite manganites exhibit some novel physical properties that are different from their bulk counterpart due to their larger specific surface areas.This not only provides abundant materials for theoretical research,but also offers the possibility for technical applications.In the previous works,much attention was mainly focused on the lattice distortions caused by the rare earth ions-doping,leading to the changes of the magnetic properties of perovskite manganites,whereas the influence of the magnetic moments of rare earth ions on the magnetic properties was rarely involved.At the same time,there are few systematic studies on different rare earth elements and their different doping ratios,therefore,a lot of experimental data are needed.In this thesis,La_0.67Ca_0.33Mn O₃ is used as a matrix to investigate the influence of rare earth doping on the crystal structures and physical properties of perovskite manganite nanoparticles,and the related physical mechanisms underlying them are also explored.Therefore,the first part of this thesis is to synthesize perovskite Ln_0.67Ca_0.33Mn O₃?Ln=La,Pr,Nd,Sm?nanoparticles,and explore the interactions and competitions between the ferromagnetism?FM?and antiferromagnetism?AFM?in the nanoparticles.The second part is the synthesis of rare earth-doped(La_1-xLn_x)_0.67Ca_0.33Mn O₃?Ln=Pr,Nd,Sm?nanoparticles.The effects of different rare earth elements and their different doping ratios on the magnetic properties are systematically studied,and a theoretical physical model is established to give a reasonable explanation.The third part is controlling the morphology of(La_0.6Pr_0.4)_0.67Ca_0.33Mn O₃low dimensional nanostructures effectively by adjusting the processing parameters of hydrothermal process.The main achievements of this thesis are summarized as followings:1.High purity Ln_0.67Ca_0.33Mn O₃?Ln=La,Pr,Nd,Sm?nanoparticles were synthesized by sol-gel method,and the particle sizes were distributed in the range of 20-100 nm.The interactions and competitions between the FM and AFM in the nanoparticles are interpreted by a core-shell model,where the core part of the La_0.67Ca_0.33Mn O₃ nanoparticles has a large component of FM phase whereas the shell part has surface spin glass state.That is ascribed to that the surface defects and other reasons destroy the double exchange intercation and produce the super exchange interaction and the frustration,leading to the formation of surface spin glass states in the shell part.The spin structures of the cores in the Pr_0.67Ca_0.33Mn O₃,Nd_0.67Ca_0.33Mn O₃,Sm_0.67Ca_0.33Mn O₃ nanoparticles are antiferromagnetic,and the shell is ferromagnetic.The surface effect leads to the disordered surface spins,which are partially aligned along the direction of the external applied magnetic field,exhibiting a weak ferromagnetism.As compared with the bulk counterpart,the charge ordering transition and antiferromagnetic transition in these nanoparticles are are highly depressed.2.High purity(La_1-xLn_x)_0.67Ca_0.33Mn O₃?Ln=Pr,Nd,Sm?nanoparticles were synthesized by sol-gel method,and the particle size was in the range of 5-90 nm.With increasing the doping contents of rare earth ions(Ln³⁺),the transition temperature from ferromagnetism to paramagnetism shifts to the low temperature,and the magnetic behavior is also different.When the doping content of the Ln³⁺ion is low,the magnetic behavior of the nanoparticle is similar to that of the parent La_0.67Ca_0.33Mn O₃,but due to the influence of lattice distortion and magnetic moment,the double exchange effect is weakened and the long-range order of ferromagnetic phase is destroyed;when the doping content of Ln³⁺ion is high,the ZFC and FC curves cross and the magnetic hysteresis loops also cross in high magnetic field.This phenomenon is explained by the phase separation model of ferromagnetic clusters,charge ordering and antiferromagnetic clusters,where an energy barrier exists among them.When the external field such as temperature or magnetic field is high enough,mutual percolation between the clusters will occur due to the energy of cluster exceeding its transition energy barrier.3.With the synthesis of(La_0.6Pr_0.4)_0.67Ca_0.33Mn O₃low-dimensional nanostructures as an example,the effects of hydrothermal processing paramters?such as hydrothermal synthesis temperature,holding time,mineralizer concentration,cleaning process,annealing temperature and time?on the morphologies of low-dimensional nanostructures were systematically investigated,and the controllable fabrication of low-dimensional nanostructures is achieved.The hydrothermal method used here is a two-step method that requires subsequent annealing at high temperature after the normal hydrothermal process.Under the hydrothermal synthesized temperature of170^oC and low concentration of mineralizer?p H value just equal to 14?,the(La_0.6Pr_0.4)_0.67Ca_0.33Mn O₃nanoparticles with good morphology were synthesized by two-step hydrothermal method,while one-dimensional(La_0.6Pr_0.4)_0.67Ca_0.33Mn O₃nanostructures were obtained under the hydrothermal temperature of 210^oC and mineralizer concentration of 20 mol/L.The diameters of the(La_0.6Pr_0.4)_0.67Ca_0.33Mn O₃nanowires was in the range of 60-120 nm,and the length was about 2mm.Their transition temperature?T_C?from paramagnetism to ferromagnetism was 224K,and the remamnent magnetization?M_r?and coercive field?H_C?were determined to be 17.72emu/g and 419.88 Oe,respectively.