The Study On Properties Of Ruthenate Oxide Superlattices

Perovskite materials exhibit many interesting and intriguing properties from both the theoretical and the application point of view.These compounds are used as catalyst electrodes in certain types of fuel cells and sensors and are candidates for spintronics and memory devices applications.Heterostructures is one way of the most important and commonly used to study the thin films.Due to the reversal symmetry breaking and dimension reducing,the electronic correlation will be further enhanced.And the reconstruction of spin,orbit,and charge through crystal field,stress field,interfacial coupling at the interface could induce emergent behaviors,such as conductivity,magnetoelectric coupling and quantum Hall effect,etc.which are not observed in bulk materials.The development of advanced methods for a layer-by-layer growth and in situ monitoring technology promote the realization of smooth interface on the scale of one atom.Superlattices comprise many interfaces while maintaining a perfect crystal structure.Combining different materials into desired superlattices can produce new electronic states at the interface and the opportunity to testify the validity and feasibility of theoretical model in the field of fundamental research.This thesis focuses on the superlattices formed by 4d Ruthenium oxide and other perovskite materials.Metal-insulator transitions?MITs?and interlayer exchange coupling?IEC?are the main research contents.This thesis is divided into 6 chapters.Chapter 1:This chapter introduces some background involved in this thesis.First,we show the structure,distortion of oxygen octahedron and coupling mechanism in electron strong correlation systems,i.e.,perovskite oxides.The importance of coupling of octahedron at interface was emphasized.Second,we introduce some novel phenomena at the interface of complex oxides.Third,we introduce the transport behavirs,including MITs,weak localization,spin-orbit coupling,and so on.Finally,we describe several mechanisms of IEC.Chapter 2:This chapter mainly introduces the experimental methods,apparatus and equipment,and some basic theory.Chapter 3:In this chapter,the IEC in CaRu_0.5Ti_0.5O₃/La_0.7Sr_0.3MnO₃system was investigated.We realized the indirect IEC effects by optimized growth conditions in a small window.And focused on the switching mechanisms of La_0.7Sr_0.3MnO₃layers of superlattices with different periods.Phase diagram was delineated by the value of fieldin descending branch extracted from M-Hloops at different temperatures.Three models were utilized to fit the dependency between the IEC strength and temperature.The combined model 3 gives the best fitting to the experimental data,indicating a common role played by spacer and ferromagnetic layers.Chapter 4:In this chapter,the IEC in SrRu_1-xTi_xO₃/La_0.7Sr_0.3MnO₃system was investigated.Unlike CaRu_0.5Ti_0.5O₃/La_0.7Sr_0.3MnO₃system,biquadratic IEC play a key role in SrRu_1-xTi_xO₃/La_0.7Sr_0.3MnO₃system.First,we studied the structure,electrical transport and magnetic properties of SrRu_1-xTi_xO₃/LSAT thin films.Then we described the type of IEC in SrRu_1-xTi_xO₃/La_0.7Sr_0.3MnO₃system grown on NGO?001?.Likewise,several model were utilized to fit the dependence of IEC strength,and the influences of bilinear and biquadratic IEC upon magnetization were analyzed.Finally,we found that the cubic anisotropy presented by LSAT substrate could help to form biquadratic IEC,and the cooperation of two type IEC would lead a slant alignment.Chapter 5:To maintain the connectivity of octahedron at the interface,the tilt/rotation angles of components will deviate from bulk value,and form a transition region where the novel physics properties can perform.In CaRuO₃/SmFeO₃ system,enhanced conductivity of CaRuO₃ ultrathin films was observed.As thethickness of CaRuO₃ decreases in CaRuO₃/SmFeO₃ superlattices,evolution from a metallic to an insulating state coinciding with change in magnetoresistance signs can be attributed to Anderson localization effects with spin-orbit coupling interaction.1.6 nm of CaRuO₃ is a critical thickness to separate the weak localization and strong localization region.