| Perovskite transition metal oxides are a class of fascinating materials since they can exhibit a diverse range of physical properties, from insulating and metallic conduction to superconductivity, from paramagnetism and ferromagnetism to antiferromagnetism, from dielectricity and piezoelectricity to ferroelectricity. With recent advances in deposition techniques particularly pulsed-laser deposition (PLD) and molecular-beam epitaxy (MBE), people now can synthesis these materials on an atomic-scale level and make high quality of oxide heterostructures and ultrathin films. In2004, A.Ohtomo and H. Y. Hwang reported the existence of a high-mobility electron gas at the interface between two band insulators, SrTiO3(LAO) and LaAlO3(LAO). This work has intrigued a surge of researchers in oxide heterointerfaces. Many extraordinary phenomenon have been found in oxide heterointerfaces. During my Ph.D study, I have focused on LaAlO3/SrTiO3heterointerfaces and the magnetic properties of La2/3Sr1/3MnO3(LSMO) ultrathin films, which grown by using pulsed-laser deposition. My thesis mainly contains the following parts:To study such metal oxide interfaces and ultrathin films, an ability to grow epitaxial films on the atomic-scale level is primarily required. Utilizing a PLD system designed by ourselves, which combined with an in situ RHEED, we have achieved this primary requirement. On the other hand, to get well-defined interfaces or ultrathin films with atomic level flatness, the substrates must have an atomically smooth surface. By using etching and annealing methods, we have obtained atomically smooth STO(001) surfaces terminated by TiO2ionic plane.LaAlO3/SrTiO3heterointerfaces:for the mechanism of interfacial conduction, by adding La1-xSrx MnO3(LSMO-x) capping layer, whose formal polarization can be tuned by Sr doping, we can tune the carrier density of LAO/STO interface, without changing carrier mobility; utilizing an experimental fact that the formation of oxygen vacancies in STO substrates could be strongly suppressed during PLD process when STO is capped with just a few unit cells of La2/3Sr1/3MnO3film, we have constructed a new LAO/STO/LSMO/STO (substrate) interface, we found that the interfacial conduction is not only dependent on the thickness of LaA103, but also on the thickness of SrTiO3, partly indicating that carrier of LaA103/SrTiO3may be derived from oxygen vacancies in STO. For the electric transport measurements, we observed a significant negative magnetoresistance behavior. Further analysis shows that such magnetoresistance is more likely to be caused by weak localization, rather than the interfacial magnetism; For magnetic property measurements, using SQUID, we found an abnormal LaA103thickness dependence of the saturation magnetization of LaA103/SrTiO3systems, which is believed to be caused by the oxygen vacancies, not by the intrinsic interfacical magnetism.Lao.7Sro.3MO3ultrathin films:we studied "dead layer" behavior in ultrathin LSMO films. Such "dead layer" is of a few unit cells thickness and shows a large decrease of the magnetism and conductivity. We prepared Co/LSMO bilayers and observed exchange bias effect at low temperature only for the LSMO thickness is1-2unit cells, suggesting the existence of antiferromagnetic (AFM) phase in LSMO dead layer. Finally, based on the theory that the dead layer may be caused by Mn (LSMO) and Ti (STO)3d orbital hybridization at the interface, we tried to use LaA103as an interval layer to enhance the magnetism of LSMO ultrathin films. However, in contrast, LaA103interval layer causes a deterioration of LSMO magnetism, suggesting that the origin of dead layer may be much more complicated. |
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