Interfacial Properties And Low-temperature Insulating Phase In Ultrathin La_0.8Sr_0.2MnO₃ Films

Since the interface region of manganite exhibits essential deviations from the bulk in both crystal and electronic structures,the ultrathin manganite films with a thickness of several unit cells often display unusual and intriguing properties.Thus,a thorough understanding of the near-interface magnetism in manganite is significant not only to the theoretical development but to the future applications of manganite thin films.This thesis mainly focuses on the interfacial properties of the La_0.8Sr_0.2MnO₃ thin films and the physical mechanism of the low-temperature insulating phase appeared in the ultrathin La_0.8Sr_0.2MnO₃ films.The main results are listed as follows:1.Firstly,we carried out an investigation on the crucial role of atomically flat terraces on the surface of SrTiO₃ in the properties of high quality manganite films.The results show that the atomically flat terraces on the surface of TiO₂-terminated SrTiO₃ can greatly enhance the Curie temperature and conductivities of the ultrathin La_0.8Sr_0.2MnO₃ films with thickness less than 50 unit cells,due to the perfect layer-by-layer epitaxial growth in the initial growth process and less substrate-induced disorder at the interface.However,when the La_0.8Sr_0.2MnO₃ films become thicker,the properties of the films are less affected by the interface defects and gradually become bulk-like.Therefore,no obvious difference is detected in the magnetic and transport properties of the 80 unit-cell thick films grown on the as-received SrTiO₃ and TiO₂-terminated SrTiO₃.Similar results can be applied to the growth of manganite films on other substrates with perovskite structure.2.Secondly,we systematically studied the insulating phase at low temperature appeared in the La_0.8Sr_0.2MnO₃ films with thicknesses varying from 6 to 10 unit cells.Through analyzing the transport data,we have found that the low temperature insulating behaviors of these ultrathin films can be well described by the Mott variable range hopping law with a decrease of the localization length in thinner films.This result suggests that the localization of the electrons leads to the low-temperature insulating phase in the ultrathin La_0.8Sr_0.2MnO₃ films,and this localization effect is increased with decreased thickness.3.A deficiency of oxygen content and a resulting decrease of the Mn valence have been observed in the ultrathin films with thickness smaller than or equal to 10 unit cells by studying the aberration corrected scanning transmission electron microscopy and electron energy loss spectroscopy of the films.These results suggest that the existence of the oxygen vacancies in thinner films suppresses the double-exchange mechanism and contributes to the suppression of both the magnetic and transport properties in thinner films.The above results show that,reducing the dimensionality of the La_0.8Sr_0.2MnO₃ films can contribute to the enhancement of disorder in the films,such as oxygen vacancies and disorder of magnetic moment.This phenomenon leads to the localization of the electrons and the appearance of the low-temperature insulating phase in ultrathin La_0.8Sr_0.2MnO₃ films.