Research On The Structure And Properties Of Perovskite Manganites And Films

In the perovskite manganite LaMnO₃,when La is partially substituted by divalent alkaline metals Ca,Sr or Ba,colossal magnetoresistance?CMR?effect is to be found in the external magnetic field.This kind of CMR manganites is a typical strongly correlated electron system,with novelty of electron and spin transport,expecting a promising application prospect in the fields of magnetic recording,magnetic probing and magnetic random memorizing.However,there is still a long way to go due to the strong temperature and structure sensibility of the magnetoresistance of these materials.Researchers have shown that the doping of rare earth manganites microstructure can exert strong influence on its magnetic property,and the Mn³⁺-O-Mn⁴⁺network of manganites plays a crucial role in deciding the material's electrical and magnetic transport properties.Therefore,the electrical and magnetic properties of the system can be changed by changing the system's Mn³⁺-O-Mn⁴⁺network structure,which can be achieved by changing the proportion of Mn³⁺and Mn⁴⁺as well as the bond angle and bond length of Mn-O-Mn through doping.Up till now,there has been rare research on doping Pr,Eu,Y,Tb,Dy,Nd and Zn in La_2/3Sr_1/3MnO₃,and manganese oxide films doped with Nd or Zn prepared by magnetron sputtering.This thesis is a research on the impact of the structural changes of perovskite manganites on their properties,by treating LaMnO₃ as the parent compound,partially replacing La³⁺by doping alkaline earth metal ion,other rare earth ions or transition metal ions at the A site,and changing the A-site average ionic radius r _A,the magnetic moment as well as the valence state.Besides,the structure,XPS spectra and CMR of manganese oxide films are also studied by analyzing the doped manganese oxide films prepared by RF magnetron sputtering.The whole thesis contains seven chapters.Chapter One introduces the history and current situation of manganese oxide study,basic physical characteristics of the CMR rare earth manganese oxide,the physical mechanism of the CMR effect,as well as the significance of the topic and the main contents of this thesis.Chapter Two is about the preparation methods of the material and film,and the measuring of the material's properties.Chapter Three studies the structure,the infrared absorption property and magnetoelectric property of the material,with La_2/3Sr_1/3MnO₃as the parent material,La partially substituted by doped Pr,Eu,Y and Tb respectively,and block materials prepared by solid state reaction sintering.It is found that at the temperature of 200K and the magnetic field of 2.5T,the magnetoresistance?MR?of the doped sample of Tb³⁺(Tb⁴⁺),whose ionic radius is the smallest and magnetic moment the largest,can be up to 81%.Chapter Four explores the micro-structural features,the infrared absorption properties and the microwave absorption characteristics of(La_1-xDy_x)_2/3Sr_1/3MnO₃.The infrared absorption spectrum of the sample is found to change within the range of 591-629cm^-1,corresponding to the stretching vibration of the Mn-O bond.And the absorption mechanism of the sample in the microwave band is attributed to the co-existence of the magnetic and dielectric loss.Chapter Five analyzes the film structure and magnetic properties of(La_0.4Nd_0.1)Sr_0.5MnO₃,and finds that when the film's annealing temperature is within the range of 700⁹00?,a single phase perovskite structure is formed,and particles on the surface form a regular conical distribution structure,with stable epitaxial relationship with the substrate.At the annealing temperature of 850?,the(La_0.4Nd_0.1)Sr_0.5MnO₃film obtains the best crystallinity,and at the temperature of 300K and magnetic field of 1.5T,the MR of the sample reaches25.1%.Meanwhile,the annealing temperature also has an impact on the oxygen vacancy concentration and the proportion of Mn⁴⁺/Mn³⁺.Chapter Six examines the film structure and its magnetic properties of La_1-x-x Zn_xMnO_3-??x=0.3,0.5,0.7?and La_0.7(Sr_1-xZn_x)_0.3MnO_3-??x=0.3,0.5,0.7?.At the annealing temperature of 850?,the film forms a single phase perovskite structure and maintains a stable epitaxial relationship with the substrate.And the magnetic resistance of the annealed film La_0.7(Sr_1-xZn_x)_0.3MnO_3-?,at the room temperature of 300K and the magnetic field of 1.5T,MR=30.9%.Chapter Seven offers a conclusion of the whole thesis.