Exchange Bias In Charge Ordering Manganite Nanoparticles

Due to the strong correlation among spin, electron and orbital degree of freedom, doped perovskite manganites exhibit unusual magnetic and electronic properties, such as colossal magnetoresistance, charge ordering (CO), orbital ordering, phase separation (PS) and so on. The charge ordering state may be'melted'by the applied magnetic field or electric field. Recently, in some experiments, the phenomenon that stability of the charge ordering state in doped perovskite manganites was weakened and appearance of the ferromagnetism with the reducing of particles'size was found. In this paper, charge ordering perovskite manganite La_0.4Ca_0.6MnO₃,La_0.5Ca_0.5MnO₃ and Nd_0.5Ca_0.5MnO₃ nanoparticles was studied focusing on the effect of particle size on the magnetic properties of charge ordering perovskite manganite and its exchange bias effect. As follows:Firstly, an brief introduction about the background of the exchange bias effect was presented, focusing on the conception, basic theory and advancement of the exchange bias effect as well as the charge ordering and phase separation in perovskite manganites. Based on the introduction, the basis of subject selection and study significance was put forward.Secondly, charge ordering perovskite manganite nanoparticles were prepared by self-propagating high temperature synthesis sol-gel method. The particle size was controlled by adjusting the sintering temperature and time.Thirdly, the crystal structure and magnetic properties of La_0.4Ca_0.6MnO₃ nanoparticles were studied. With the appearance of FM phase and spin glass like phase on the surface of nanoparticles, surface phase separation and the exchange bias effect were found in the samples. The effect of nanoparticles'size and measuring field on the magnetic of La_0.4Ca_0.6MnO₃ sample was studied, at the same time, possible cause of the appearance of exchange bias effect in samples was brought forth.Lastly, magnetic properties of the half-doped narrow-band perovskite manganites La_0.5Ca_0.5MnO₃ and Nd_0.5Ca_0.5MnO₃ nanoparticles were studied. The difference of crystal structure and magnetic properties between La_0.5Ca_0.5MnO₃ nanoparticles and La_0.4Ca_0.6MnO₃ nanoparticles was investigated, moreover, effect of the measuring field on La_0.5Ca_0.5MnO₃ nanoparticles was studied. At the same time, magnetic properties of the Nd_0.5Ca_0.5MnO₃ nanoparticles were also studied. The effect of nanoparticles'size and measuring field on the exchange bias effect was quested in Nd_0.5Ca_0.5MnO₃ nanoparticles. It's worth noting that the effect of nanoparticles'size on Nd_0.5Ca_0.5MnO₃ nanoparticles was entirely different.