On The Strain State And Thickness Effects Of Epitaxial (La, Nd)_0.7Sr_0.3MnO₃ Thin Films

The studies of perovskite manganese oxides have attracted much renewed attention in strongly correlated electron system since the discovery of colossal magnetoresistance (CMR) effect in the mid-1990s. Because CMR effects are great valuable in industrial demand, such as the magnetic memory, magnetic random access memories, magnetic sensors and the spin-polarized dependent of all oxide devices; however, these demands mainly depend on the properties of the thin-films. As a fundamental physics research, the manganite system exhibits many intriguing physical behaviors; e.g. paramagnetic-ferromagnetic phase transition together with insulator-metal transition, charge-orbital ordering, phase separation, Jahn-Teller distortion and the coupling between them; especially, the investigation in the manganite films not only have important significance for the spintronic application, but also stimulate the great progress in the growth of epitaxial thin films for all oxide or oxide-metal device.In this thesis, the influence of the lattice-misfit strain and oxygen contents on the thickness effects of CMR manganite thin films was carefully investigated. Specially, the in-situ deposition oxygen pressure affects on the structural and transport properties of the ultra-thin films, and effects of the angle-distortion induced strain on the transport behavior of epitaxial thin films is also studied in details. It is discussed that the domain width induced from the angle relaxation is closely correlated to the thickness of the coherent epitaxial films.The whole thesis consists of five chapters.Chapter 1: The general review of the history and present research situation of the perovskite manganite physics and thin films is given. Some related properties, such as the crystal and electronic structure, exchange interactions, charge-ordering, electronic transport, CMR effect, magnetic anisotropy, phase separation, effects of doping level and theoretical model, are introduced. In the end, we sum up the current research in the manganite thin films (such as the influence of oxygen stoichiometry, lattice-misfit strain and angle distortion induced strain on the thickness effects) and some existent issues.Chapter 2: The oxygen content effects on the structural and physical properties of the CMR films are discussed. Based on the relation between the in-situ deposition oxygen pressures and thickness effects in epitaxial Nd_0.7Sr_0.3MnO₃ (NSMO) thin films, it is indicated that the structural and transport properties of ultra-thin films strongly depend on the deposition oxygen pressures; but for the partially relaxed thicker films, the structure and transport behaviors is affected by both in-situ deposition and the post-annealing process, due to the column boundaries in thicker films could take-up a atomic oxygen from the ex-situ oxygen annealed. To get a higher electrical conductivity in the ultra-thin films, a higher deposition oxygen pressure is crucial. For the thicker NSMO films, a single cell volume expansion along c* axis induced from the vacuum processing is observed.Chapter 3: Effects of the lattice-misfit strain induced from the substrate on the electro-magnetic properties of the NSMO thin films was studied. It was found that the strain state of the NSMO films is closely related to the transport behaviors. The structural and transport properties of the thin strained films depend strongly on the Jahn-Teller term of lattice distortion and the thickness. For the thicker NSMO films, the large is the lattice misfit between the film and substrate, the small is the strain relaxation thickness, and the metal-insulator transition temperature T_P rapidly approaches that T_P of the bulk materials with increasing of the film thickness. So, the biaxial strain is an important factor for the thickness effect in epitaxial manganite films, and it should be considered for fabricating a high quality of CMR films.Chapter 4: Influence of the angle distortion induced from rhombohedral La_0.7Sr_0.3MnO₃ (LSMO) grown on the cubic substrates on the structure and transport properties of CMR thin films is carefully studied. The La_0.7Sr_0.3MnO₃ (LSMO) bulk is rhombohedral at room temperature with the pseudocubic parameter a of 3.873 A and a large distorted pseudocubic angle α of 90.26 °; when it grown at the cubic substrates, besides the elastic normal strains due to the lattice mismatch between the film and substrate, there is a further type of distortion in coherent LSMO films, namely, elastic shear. This angle distortion results from the rhombohedral symmetry of LSMO, when it is matched onto a cubic substrate. As the distortion angle is partially recovered in coherently LSMO films, the transition temperature T_P of the films almost approaches to that value of the LSMO bulk. It indicates that the angle relaxation is independent of the lattice-mismatch strain relaxation with the increasing of the thickness.Chapter 5: The correlation between the thickness and domain width in coherent epitaxial LSMO thin films is also studied. It is found that the periodic structural domain only occurred in the LSMO thin films on (LaAlO₃)_0.3(Sr₂AlTaO₆)₍0.7 (LSAT) substrates. As the film thickness was changed from 22 nm to 100 nm, a monotonic increasing of domain width is observed, this relation of the thickness and the domain width is consistent with the twin modeling of epitaxial La_0.67Sr_0.33MnO₃ thin films. The structural domain is not observed in coherently LSMO films on SrTiO₃ (STO) substrate. It is explained as a larger lattice-mismatch strain between the LSMO and STO substrate inducing the elastic strain energy is mainly relieved by the formation of misfit dislocation in the thicker LSMO/STO films.