Deposition,Electrical And Magnetic Properties Of Two Perovskite Thin Films

As a typical strongly correlated electron system,oxides with ABO3 perovskite structure exhibit unique physical phenomena such as metal-insulator transition,phase separation and Jahn-Teller Effect.Because of properties such as ferroelectricity,ferromagnetism,multiferroics and non-linear optics,great attention has been paid to these materials to elucidate the relationships among their structure,chemical compositions and physical properties since 1950s.In recent years,the discovery of colossal magnetoresistance makes these researches revivided,which not only explores materials science and the fundamental physical problems,but also expands their potential applications in magnetic memory,sensor,catalysis,solar cell,refrigeration and other devices.As far as perovskite thin films are concerned,the existence of strain,dopping and interface in epitaxial thin films may lead to electrical and magnetic properties that are different from bulk materials,and manipulatable physical properties.In this thesis,we prepare two kinds of perovskite oxide thin films,investigate their structures,valence states,electrical and magnetic properties.Major achievements are:1 Using pulsed laser deposition,Ba0.95La0.05PbO3(BLPO)thin films with different substrate temperatures(550-750 0C)and oxygen pressures(5-20 Pa)were prepared on SrTiO3(001)substrates.Both factor have negligible influence on microstructure by XRD results.The charge valences of Ba and Pb cations in all the films are determined to be +2 and +4 by x-ray photoelectron spectroscopy.However,surface roughness of the films increases with increasing substrate temperature and oxygen pressure.The resistivity is significantly affected by substrate temperature and oxygen pressure,which increases with the former and decreases with the latter.Especially,the films deposited under 600? and 10 Pa oxygen pressure show metallic transport behavior with nearly temperature-insensitive values,as evidenced by the resistivity of 6.24×10-5 ? m and 5.41×10-5 ? m at 300 K and 15 K,respectively.This strongly substrate-temperature and oxygen-pressure dependent resistivity is explained by the extrinsic microstructural effect and intrinsic oxygen vacancy effect.Based on the structure and transport measurements,the optimized deposition conditions are determined to be close to 600 ? and 10 Pa.2 Different amounts(0.025-0.1)of paramagnetic,low-spin Co3+/Co2+ species were doped into epitaxial La0.7Sr0.3MnO3 thin films by pulsed laser deposition.The evolution of electrical,magnetic,magneto-transport and valence state of this series of Co-doped La0.7Sr0.3MnO3 have been systematically characterized.Structure characterizations show that all doped films are epitaxially grown on SrTiO3(001)substrates and have uniform interface.The XAS results suggest the chemical state of Co to be +2/+3 due to its low spin state.Magnetic measurements show ferromagnetism feature and decreased magnetization as Co amount increases.Remarkably,MR-T curves demonstrate that magnetoresistance in 0.1Co-doped La0.7Sr0.3MnO3 reaches 35%within an extremely large temperature window(from 40 to 200 K).Other Co-doped epitaxial La0.7Sr0.3MnO3 thin films show similar sharp MR-peaks around Curie temperature as in La0.7Sr0.3MnO3 matrix.The temperature-insensitive MR of 0.1Co-doped La0.7Sr0.3MnO3 may be attributed to relaxor-like magnetic state.