The Study Of Magnetoelectric Properties In La_0.7Sr_0.3MnO₃ And LaNiO₃ Films

In recent years,some novel physical properties often occur in the perovskite heterostructure,which is still a hot topic in condensed matter physics and material science.Among them,La_0.7Sr_0.3MnO₃?LSMO?material has attracted much attention due to its half metallicity,high Curie temperature and colossal magnetoresistance effect.And LaNiO₃?LNO?has also become the focus for individuals because it is the only metal and paramagnetic material in the perovskite rare-earth nickelates family.It is found that ensuring the atomic flatness of substrate surface is the key to preparation of high quality epitaxial films in the research of film growth.Based on that,the magnetic and electrical properties in two materials can be controlled by means of size and heterostructure.In the paper,the perovskite materials LSMO and LNO are used as the research object to the preparation of the ultrathin films and heterostructures.The transport properties and magnetic properties are regulated by the size effect and the design of the heterostructure system and find that metal-insulation transition with the change of thickness in the ultrathin films and the novel exchange bias in the heterostructure.The specific contents are as follows:?1?In order to get high-quality epitaxial film samples,we need to ensure that the substrate surface reaches the evenness of atomic level,therefore,we treat SrTiO₃?STO?substrate by etching in the HF-NH₄F buffer solution first and then annealing in oxygen environment.The substrate of TiO₂ terminated layer and regular steps are obtained by controlling conditions of etching time,annealing temperature and annealing time,which lays a foundation for the high quality epitaxial growth of subsequent films.?2?A series of LSMO and LNO films are prepared by pulsed laser deposition,we have studied the magnetic and electronic transport properties of them,and found that the Curie temperature of LSMO thin films grown on SrTiO₃ substrate from thin to thick in the process are almost not affected by the orientation,about 331 K.Moreover,the transport properties of LSMO films and LNO films on SrTiO₃?001?substrate undergo metal-insulator transition with decreasing thickness for thin films,which may be caused by the localized effect.In addition,an exchange bias is observed in LSMO film grown on LaAlO₃?001?substrate,and the exchange bias field is up to 642 Oe,which may be derived from the coupling between ferromagnetic LSMO and interfacial dead layer just above LaAlO₃.?3?In order to further study the magnetic properties of heterostructure,we design and prepare bilayer films consisted of ferromagnetic LSMO and paramagnetic LNO materials,the exchange bias has been observed.And the stack order of bilayer heterostructure plays a crucial role in the interfacial coupling effect,and the coupling effect in the bilayer interface of LSMO?top layer?/LNO?bottom layer?is larger than that of LNO?top layer?/LSMO?bottom layer?.In addition,the spin glass state has been observed at the LSMO/LNO bilayer interface,which is mainly based on:Firstly,the dependence of the irreversible temperature on the cooling field follows Almeida-Thouless line.Secondly,the relaxation of the thermal remanence below the freezing temperature of the spin glass can be fitted by the stretched exponential function.It indicates that the exchange bias is closely related to the interfacial spin glass state.