Anisotropic Strain, Domain Structure And Charge Ordering In Epitaxial Pr_1-xSr_xMnO₃ Films

Since the discovery of colossal magnetroesistance(CMR) in perovskite manganites films,these materials have recently attracted much attentions for their potential application,furthermore,these materials are spin,charge,orbit and lattice degree of freedom strongly correlated systems,all kinds of interactions and the competition between them lead to very complex electronic,magnetic and structural phase diagrams,and abundant of physics phenomena such as the charge orbital orderings,phase separation,etc.,and it has become one of the most active research fields in condensed matter physics and material physics.The investigation in CMR films has great significance for fabrication of related prototype devices,especially for all oxides devices.In this thesis,through the study of manganites,the regulation and control the oxygen content of La_0.7Ca_0.3MnO_3-δ thin films and the diffusion of oxygen in atmosphere were systematic researched.The anti-ferromagnetic state in the films was induced by anisotropic strain.The thickness evolution of domain structures in films and the doping concentration of antiferromagnetic films on both magnetic and transport properties were carefully investigated.The main contents in the dissertation are presented as follows:Chapter 1:The general introduction of physical properties and research progress on perovskite manganites films are given.First,we make a brief introduction to the research history,the crystal structure and lattice distortion,the essential physical mechanism(such as double exchange interaction,the Jahn-Teller effect),the charge orbital order,phase separation,the anomalous physical properties associated with the charge orbital order transition,the facts that influence the stability of the charge orbital order state.Then discuss thickness effect induced by strain in manganites films, containing the influences of in plane lattice distortion induced by mismatch on crystal structure,transport and magnetic properties,and thickness effect on crystal parameters and physical properties. Chapter 2:The film preparation method and measurement were introduced. Especially,the pulsed laser deposition(PLD) method and the merit of this method were described in details.Also,the X-ray diffraction and the transport properties measurement were also introduced.Chapter 3:The control,diffusion,and stability of oxygen in the Single-crystalline La_0.7Ca_0.3MnO_3-δ films were carefully investigated.Excluding the relaxation of strain,the ex-situ vacuum-annealed films suffer from fast oxygen in-diffusion even at room temperature in ambient atmosphere,and the increased Mn⁴⁺/Mn³⁺ ratio in the films.With the time escaping,the out-of-plane lattice parameter was decreased,and the Curie temperature of the film was increased successively,however the in-plane lattice coherency maintains consistently at the same time.While,the oxygen deficiencies controlled in-situ by deposition at lower oxygen pressures were stable.The results indicate that the control of oxygen content via the in-situ deposition process is more favorable for manganite films.Chapter 4:In this chapter,the relationship between the thickness and crystal structure and electro-magnetic properties in Pr_0.5Sr_0.5MnO₃(PSMO) epitaxial films grown on(LaAlO₃)_0.3(Sr₂AlTaO₆)_0.7(110) substrates were systematically investigated. During cooling temperature,the PSMO/LSAT(110) films show the ferromagnetic state to the antiferromagnetic ones transition with a huge thermal hysteresis.With increasing film thickness,the paramagnetic-ferromagnetic transition temperature (Curie temperature) increases,the antiferromagnetic charge-orbital ordering temperature decreases,the stability of the antiferromagnetic state and the melting field decrease.We clearly show that these films have a monoclinic distortion due to the in-plane anisotropic strain.Ultrathin films(t＜12 nm) have a single-crystalline and a single-domain-like structure,with the unique axis b_m along[-110].Thickness larger than 12nm,the main spots in the vicinity of PSMO the diffraction spots,indicating the emergence of the periodic domain structure.The films increase disorder with another periodic domain structure.Transport properties of thin films and the crystal structure are closely related and the change in structure lead to appears thickness effect.The increase in degree of structural disorder in the sample reduced the antiferromagnetic state and the melting field.Chapter 5:The electronic and magnetic properties of Pr_1-xSr_xMnO₃(0.47≤x≤0.60) were investigated,and the influence of doping concentration and structure distortion on PSMO films properties was studied.During cooling from room temperature,the PSMO/LSAT(001) thin films exhibit only one transition from paramagnetic insulator to ferromagnetic metal state at Curie temperature,while the PSMO/LSAT(110) thin films show the coexistence of two magnetic transitions,from the paramagnetic state to the ferromagnetic state transition,and then from the ferromagnetic state to the antiferromagnetic ones transition with a huge thermal hysteresis.With the doping concentration increases,the antiferromagnetic states become more stabile.The origin for above results may be the attenuation of ferromagnetism correlation in high-doped samples,while the electron-phonon interaction enhanced by Jahn-Teller distortion,which prefers the localization of electrons.