Preparation Of Nano-scaled Manganites With Pervoskite Structure And Study On Magnetic Properties

Manganites with pervoskite structure have been investigated experimentally and theoretically due to the interesting and rich electronic and magnetic properties and can be used in various areas. Nanomaterials have some unique and novel physical and chemical properties and are promising in applications, therefore these materials have been the focus of condensed matter physics, material science and chemisty. It is meaningful to investigate the manganites in nanoscale either from theoretical or practical application point of view.In this dissertation the controllable synthesizing and the physical properties such as magnetic and magnetocaloric properties of manganites was investigated. The results are summarized as follows:(1)La0.8Ca0.2MnO3nanoparticles with different sizes have been prepared using the sol-gel method. The existence of blocking of the superparamagnetism (SPM), freezing of super-spin-glass and surface-spin-glass are evidenced. It is found that a core shell structure can be responsible for the magnetism behavior of the nanoparticles. The phase transition from paramagnetism (PM) to ferromagnetism (FM) has been modified from first order to second order as the particle size reduced. The magnetocaloric effect (MCE) thus has been modified by the changed magnetism. The temperature interval of observed magnetic entropy change has broadened as the particle size reduced. The relative cooling power (RCP) can be tuned dramatically by particle size due to the change of magnetism in the superparamagnetic core. The magnetic entropy change of superparamagnetic particles has been calculated based on the core shell model. We observed irreversibility in high magnetic field. The surface spin-glass behavior as well as the high-field irreversibility is suppressed by increasing particle size while the freezing temperature TF does not change with particle size. The enhanced coercivity has been observed in the particles and we attributed it to the large surface anisotropy. We have disclosed a clear relationship between the particle size, the thickness of the shell, and the saturation magnetization of the particles. The large reduction of the saturation magnetization of the samples is found to be induced by the increase of large nonmagnetic surface since the thickness of the spin-disordered surface layer increases with a decrease in the particle size. Due to the reduction of the magnetization, the magnetocaloric effect (MCE) has been reduced by the decreased particle size since the nomagnetic surface contributes little to the MCE. Based on the core-shell structure large RCPs of180J/kg and471J/kg were predicted for a field change of2.0T and4.5T respectively in the small particles with thin spin-glass layer.(2)We report the observation of a tunable magnetocaloric effect near room temperature in the manganites La0.67Ba0.33Mn03, The suppression of TC which induces the tunable entropy change was attributed to the finite size effect in nanopatticles. A ferrimagnetic-paramagnetic transition temperature decreased from337to295K as the particle size reduced from60nm to17nm. The rounding of the phase transition was also found which may tune the temperature span of the refrigeration. this rounding is also attributed to the finite size effect. Considering its low-cost and innocuous raw materials, Mn-based perovskite compounds are suggested to be appropriate for pursuing new materials with larger MCE.(3) LaMnO3and La0.8Ca0.2MnO3nanoparticle and nanowire were synthetized by hydrothermal method. The effect of synthetize conditions such as the reaction temrature and reaction time, the concentration of the precursor material on the microstructure and phase purity was investigated. The samples were analyzed by the SEM and XRD. By controlling the reaction condition such as reaction time and the concentration of the precursor solution the nanoparticle and nanowire were prepared.