Structure And Physical Properties Of Cobaltate/Manganite Perovskite Films By Polymer Assisted Deposition Method

The strong correlation between spins,charges,lattices,orbits makes the perovskite cobalt and manganese oxides rich in magnetic and electric properties.Therefore,it has great development prospects in magnetoelectric sensors,magnetic random access memory components and spintronic devices.With the development of times,films with multi-functional integration and size miniaturization are more in line with people's needs.In addition,in the perovskite cobalt and manganese oxide film,substrate imposed strain also increases the effective way to manuscript its magnetoelectric properties,makes it more possible for meeting the needs of practical applications.However,the strain mechanism of the perovskite oxide film is very complicated.In addition to the effect of strain on the distortion and rotation of octahedron,it may also affect cationic ordering,the concentration of oxygen vacancies,the charge transfer between ions,the spin states of cations,etc.It is necessary for us to systematically study the strain mechanism to more effectively improve its magnetic and electrical properties.Therefore,in this paper,the Nd2CoMnO6 double perovskite film with cation order,LaBaCo2O5.5+? film with possible cation order and oxygen vacancy,and CaMnO3 film with possible oxygen vacancy were prepared by polymer assisted deposition method(PAD),and the effect of strain on their structure and magnetic and electric properties are discussed.The specific research content includes:In the first chapter,the structure and intrinsic interaction of perovskite oxide materials,as well as some basic concepts,such as electronegativity and cation order are introduced.In addition,the structure and basic physical properties of Ln2Co/NiMnO6,Ln2BaCoO5.5+? and CaMnO3 are mentioned,as well as some research hot issues in these perovskite oxide films.In the second chapter,the double-perovskite Nd2CoMnO6(NCMO)thin films were epitaxially grown on LaAlO3 and SrTiO3 substrates with different orientations by the polymer-assisted deposition(PAD)technique.A well crystallization quality of the films is confirmed by XRD,SEM and AFM.The magnetic results suggest that all the films have a single ferromagnetic(FM)transition,but its substrate-orientational dependence is different from those in normal perovskite ABO3.There are two aspects that competitively affect the magnetic properties of the NCMO film,i.e.,the variation of oxygen octahedron similar to that in the normal perovskite ABO3 film,and the degree of Co/Mn ionization with unchanged Co-O-Mn chain arrangement,resulting in the different orientation-dependent magnetism in NCMO films from that normally observed in ABO3 films.Our results give a systematic understanding of the substrate-induced-strain influence on the magnetic interaction in Nd2CoMnO6 thin films and widen its practical application.In Chapter 3,(001)-oriented La2BaCoO5.5+?(LBCO)films were prepared on different types of substrates using PAD technology.The electrical transport and magnetic measurement suggest that the conductivity and ferromagnetic transition temperature of the LBCO film did not monotonously change with the decreased lattice mismatch.It is suggested that three main factors essentially affects the physical properties of LBCO films,i.e.,the oxygen content,cationic ordering,and the cationic electronegativity.The tensile strain induced decrease of oxygen content and lowest nanoscale ordering of cations will result in the decrease of ferromagnetic interactions and conductivity,and the increase of antiferromagnetic interactions.Besides,the difference of the cationic electronegativities can lead to the different shrinkages of Co3+-O and Co4+-O bonds when strain is applied,especially under the compressive strain,resulting in the changes of the cobalt spin states and a charge transfer between Co3+and Co4+ions,altering the physical properties of LBCO films.The combination and competition of the functions of the oxygen content,cationic ordering,and the cationic electronegativity lead to an abnormal change of the conductivity and magnetization for LBCO films with substrates.This provides a new way to understand the function of electronegativity in the strain-controlled physical properties of the films.In Chapter 4,the PAD technology was used to epitaxially grow double perovskite LBCO films with different orientations on LaAlO3 substrates.The electric and magnetic measurement show that the conductivity and ferromagnetic transition temperature of the LBCO film are different from the normal single ABO3 perovskite on the substrate.Specifically,as oriention change from(100)to(111),and then to(110),the ferromagnetic interaction is weakened,while the antiferomagnetic interaction is enhanced.The changes in these properties are related to the spin state of the cobalt ion in the film.The more cobalt ions in the intermedia-spin state,the stronger the ferromagnetic interaction and the weaker the antiferromagnetic interatction.The change in the spin state is a result of competition between the cationic electronegativity and the orbital splitting energy.The change of the spin state of the cobalt ion is the result of the competition between the electronegativity of the cation and the splitting energy of the egand t2g orbitals in the CoO6 octahedron.This chapter demonstrates the feasibility of controlling the structure and physical properties of LBCO thin films by substrate orientation.At the same time,this chapter highlights the effect of substrate orientation on the regulation of cobalt ions,increases the potential physical mechanism of orientation regulation films,and improve the possibility of multifunctional perovskite film electronic devices being applied in practice.In Chapter 5,the PAD technology was used to grow single perovskite CaMnO3 films on different types of substrates.The electrical transport results show that the film with smaller the lattice mismatch has smaller resistivity.The measurement results of X-ray photoelectron spectroscopy show that tensile strain is beneficial to the formation of oxygen vacancies,and this brings about lattice expansion,which increases the lattice parameters of films,thus the bond length of Mn-O-Mn becomes longer.Therefore the magnetic and electric properties of CaMnO3 thin films are modulated,which lays a foundation for the subsequent research on the magnetic and electric properties of doped CaMnO3 thin films.In the last chapter,the results are summarized and further research directions are proposed.