Study Of The Properties Of Nano Layered Perovskite Cobaltite And Its Composites

This paper focuses on the studies of structure, morphology, magnetic and transport properties of nano layered perovskite GdBaCo2O5+? and (1-x)GdBaCo2O5+?/xBi2Se3 composites with different volume ratio.Layered perovskite GdBaCo2O5+? in different sintering temperature were prepared by sol-gel method. The X-ray diffraction, Transmission Electron Microscopy and Energy Dispersive X-Ray Spectroscopy show that all samples are single phase with layered perovskite structure and the grain size of sample sintered in 900? is below 100nm. Iodometric method has been used to make oxygen content clear. The oxygen contents are 5.44, 5.26 and 5.25 for GdBaCo2O5+? sintered in 900?, 1000? and 1100? respectively.The temperature dependence of magnetization shows that sample sintered in 900? has a similar M-T curve with the sample parpered by solid-state method because of the similarity in oxygen content between them. The value of magnetization for samples sintered in 1000? and 1100? with less oxygen content is smaller than that for the sample sintered in 900? because of isolated ferromagnetic clusters imbedding in the anti-ferromagnetic matrix around ferromagnetism-antiferromagnetism transition temperature. These clusters demonstrate curious combination of superparamagnetic and ferromagnetic features below Tc. There also exists bifurcation in M-T curves in all the samples with different measurement mode of Zero Field Cooling and Field Cooling on account of the competition between ferromagnetic exchange interaction and anti-ferromagnetic exchange interaction.The temperature dependence of resistivity manifests that all the samples exhibit insulator behavior in the whole temperature range and the resistivity increases by orders of magnitude with the decrease of oxygen content. Magnetoelectrical transport results show that there exists large magnetoresistance at low temperature for GdBaCo2O5+? with small particles, especially for the 900? sample with nano-particles. This large magnetoresistance can attribute to spin polarized tunneling mechanism in polycrystalline samples with small particles.(1-x)GdBaCo2O5+?/xBi2Se3 composites with different volume ratio have been prepared successfully in this study. The temperature dependence of resistivity shows that all the sample exhibit metallic behavior and the resistivity decreases with the increase of volume ratio of Bi2Se3. Magnetoelectrical transport data shows that there exists positive magnetoresistance for all samples during the measuring temperature regions. The magnetoresistance behavior becomes weaker with the increases of temperature. Linear MR-H curve in high magnetic field indicates that there still retains the nature of topological insulators on the surface of Bi2Se3 for all composites. The large magnetoresistance has been observed for the x= 0.7 and x= 0.8 composites at 150 K and 300 K, which probably due to the interface effect between Bi2Se3 and GdBaCo2O5+?.