A Study On The Nanometer Size Effect Of Double Perovskite La₂NiMnO₆

There is a surge of interest in perovskite manganite because of its multiple physical properties and possible application. It would present more novel properties with decreasing its particles to nanoscale. In present work, the sol-gel method was used to synthesize La2NiMnO6 nanoparticles, and we investigate their properties of the structure, magnetism, ESR signal and Raman spectrum.In Chap. 1, we introduce some basic properties of perovskite manganite, generally. For example, the materials'structures, structure of outer electrons, transportation properties, and the colossal magnetroresistivity effect, as well as the promising application.In Chap. 2, we introduce some breakthrough in the research of double perovskite La2NiMnO6 and La2CoMnO6, and summarize some basic properties. Through the investigation of La2NiMnO6 nanoparitcles, which are synthesized by the sol-gel method, we obtain five samples with different grain sizes. The structural studies reveal that the orthorhombic structure fit the XRD patterns well, and the lattice parameters do not change too much with decreasing the particle sizes. The results obtained from TEM are consistent with the grain size distribution gained from XRD patterns. High resolution TEM was also used, while no bi-domain structure was observed.In Chap. 3, the temperature dependence of DC magnetization for La2NiMnO6 of different particles sizes was measured, and it is found that the rates of disordered volume increase with decreasing the particles sizes. The higher Curie temperature Tc1 shifts to low temperature side while the Tc2 shifts to the opposite side. There are also the occurance of glasslike behaviors, and the TP shifts to the higher temperature side. It is deduced that the rates of disordered volume are not changed greatly by analysing the saturation magnetization for the five different samples. ESR signals indicate that, for 16-nm and 57-nm La2NiMnO6 samples, there are distinguished properties, comparing with the bulk samples, because they have different T*, which indicates the transition of short-range order.In Chap. 4, Raman spectrum was used to investigate the changes of the two main peaks, which are around 535 and 670cm-1, respectively. With decreasing the particles sizes, blue and red shifts are found respectively, while the FWHM increase. All the results could be understood by the increase of Jahn-Teller ions which are brought by the trend of Mn 4 + + Ni 2 + ??→Mn 3 ++ Ni3+ .