Investigation On Strain And Ambient Annealing Effects Of La_0.7Ba_0.3MnO₃ Thin Films

Since the discovery of colossal magnetoresistance (CMR) effect in doped manganese oxides R_1-xA_xMnO₃ (where R is trivalent rare earth ions, such as Pr, La, Sm, etc; A is divalent alkaline earth ions,such as Ca, Sr, Ba, etc), they have been extensively investigated. This is not only because of their potential applications in devices, but also that the interesting interactions among charge, spin, lattice and obital degrees of freedom make them very arrractive for theoretical and experimental investigation. CMR thin films are of special interest due to their advantages in practial applications. In addition to the dominant parameters as in bulk materials, including A ion size and doping level x, the biaxial strain due to lattice mismatch between film and substrate plays a very important role in controlling the properties of thin films, especially Curie temperature, magnetic anisotropy and transport properties. Strained CMR thin films usually show properities much different from those of bulk compounds. In most cases, tensile strain suppresses ferromagnetism and reduces ferromagnetic (FM) Curie temperature TC in CMR thin films, which is generally interpreted by considering a strain-induced distortion of MnO6 octahedra. With the progress of investigation, some anomalous results have also been reported, showing ferromagnetism enhanced by tensile strain.Although La_1-xBa_xMnO₃ show room-temperature CMR effect and higher TC in low doping level than other CMR systems, little research has focused on this CMR material, especially the LBMO thin films. Kanki et al. found that both TC and CMR effect are enhanced in tensile strained La_0.8Ba_0.2MnO₃/SrTiO₃ thin films. This result is different from the usual trend in other well studied CMR thin films, and suggests that there is an anomalous strain effect, which may induce somen novel and interesting properies in LBMO thin films. Chosing La_0.7Ba₍0.3_MnO₃ as the target, we have carried out an entire investigation on the properties of epitaxial LBMO thin films on LaAlO₃ substrates with a very large lattice mismatch, and SrTiO3 substrates with a subtle lattice mismatch were also used for comparison. In this thesis, we studied the strain effect of LBMO film and made a further research on the annealing mechanism in this highly strained LBMO film. Furthermore, a phase separation tendency was observed in thinner LBMO/LAO film which could be confirmed under the assistance of microfabrication techniques and the reason was analyzed.There are five chapters in this thesis, which can be presented as follows:In chapter one, we first present a brief review of the history of CMR manganite. Next, we introduce some physical properties such as crystal structure,J-T distortion,electronic structure,phase diagram, etc. Then, we make a detailed discussion about the properties of manganite films, indicating that strain effects and oxygen contents have a crucial influence on the properties and structure of manganite thin films. At last, we present several main models which are usually used to explain the phenomena emerged in manganite thin films.In chapter two, we first introduce some methods about film preparation and characterization. Then these LBMO films prepared by DC magnetron sputtering technique are characterized. At last, the strain effects are discussed in details and we find that the metal-insulator transition temperature T_P and Curie temperature T_C decrease with the reduction of film thickness for oxygen annealed LBMO/LAO films under a higher temperature, indicating that a compressive strain suppresses ferromagnetism and reduces ferromagnetic (FM) Curie temperature T_C in the films which can be interpreted by two-orbital model.In chapter there, a detailed discussion of the mechanism of annealing on LBMO/LAO thin films is made firstly. We find that the oxygen atoms in ab-plane can transfer to c-axis by annealing and the oxygen diffusion of oxygen ambience plays a minor role. A relationship between the strain and oxygen content is observed, which can be represented that a larger in-plane compressive strain will capture more oxygen in this plane. Next, the evolution of oxygen distribution in LBMO films after an annealing treatment is studied. Oxygen transfer rate from ab plane to c-axis becomes slow under an intermediate temperature annealing treatment. At last, a phase separation tendency is observed in the thinner LBMO/films annealed under an intermediate temperature, which can attributed to cooperative interaction of strain and oxygen distribution.In chapter four, we have a research on the CMR effect and find that the films annealed under at high temperature in Ar ambience or intermediate temperature in O₂ ambience have a larger low temperature CMR effect with no signature of saturation even under a large external magnetic field. We think this is evidence of phase separation in these films. Then ,by the fitting of resistivity data of these films, we find that the transport mechanism at low temperatures is dominated by the electron-electron scattering and electron-magneton scattering ,while that for the high temperatures is dominated by the variable range hopping of small polarons.In chapter five, we first make a brief introduction to the microfabrication technique. In the micro-structure with a submicron width for high temperature annealed LBMO/LAO film, an obvious upturn of resistivity curve emerges at low temperatures, indicating a phase separation area with a size of submicrometer. For intermediate temperature annealed LBMO/LAO films, it shows an insulator behavior for the micro-structure with channel width in micron size, indicating an increase of the size of phase seperation area in the films.