Research On Electrical Transport Of Strain-induced La_0.325Pr_0.3Ca_0.375MnO₃Films

There are several different interpretations for the striking electrical transport properties of perovskite manganite, such as metal-insulator transition (MIT) and colossal magnetoresistance (CMR). The most acceptable viewpoint is that there is an intrinsic tendency towards the coexistence of competing orders in manganite. Electronic phase separation (EPS) has been observed in different perovskite manganite systems. The shape and scale of electronic phase separation vary for different systems, and which can be controlled by strain, external magnetic field, and other factors. La5/8-0.3Pr0.3Ca3/8MnO3(LPCMO) film is a model system with EPS. In order to verify the modulation effect of epitaxial lattice strain on phase separation in LPCMO films, we first deposited LPCMO epitaxial films with different strain statse via using substrates with different film-substrate lattice mismatch. Then we studied the relationship between film strain and electrical transport property. This thesis contains four chapters. The first chapter is the introduction of background knowledge. The second chapter presents the details of deposition of LPCMO thin films by magnetron sputtering and the characterization of the structure and electrical transport properties. The third chapter describes the structure and electrical transport of (001)pc oriented LPCMO thin film. And the forth chapter describes the structure and electrical transport of (110) oriented LPCMO thin films.In chapter one, the background of perovskite manganite and the recent research progress were introduced. First, the structure of the perovskite manganite, the crystal field effect and double exchange effect, and metal-insulator transition mechanism were introduced. Then, we described the research of electronic phase separation. After that, recent research progresses of epitaxial LPCMO films were introduced.In chapter two, deposition method and characterization techniques of epitaxial LPCMO thin films were described. First, we explain how to fabricate the target of LPCMO and the points for attentions. Second, a brief introduction of the magnetron sputtering instrument was presented, followed by the details of deposition conditions of LPCMO film. At last, we introduced techniques used to characterize the thin films, such as XRD for structure analysis, AFM for surface topography, and PPMS for electrical measurement.In chapter three, the structure and electrical transport of (001)pc oriented LPCMO thin films were described. At first, LPCMO films were epitaxially grown on (001)pc SrTiO3(tensile strain), LaAlO3(compressive strain) and NdGaO3(near-zero strain) substrates. Before annealing, all the films are coherent-epitaxial and insulating through the measured temperature range. Under7T magnetic field, the metal-insulator (MI) transition appeared in LPCMO/LAO and LPCMO/NGO, except the LPCMO/STO. This suggests that magnetic field could enhance the EPS. Obvious change of film lattice is observed during the post-annealing:the in-plane strain in LPCMO/LAO varies from-1.5%to-0.1%while that in LPCMO/STO changes from1.6%to1.3%, and the lattice of LPCMO/NGO keeps constant because of the good lattice-match between LPCMO and NGO. Consequently, the varied film strain leads to the emergence of metal-insulator transitions in LPCMO/LAO film without magnetic field, and in all films with7T magnetic field. These results demonstrate that lattice-mismatch combined with post-annealing is an effective approach to tune strain in epitaxial LPCMO films, and thus to control the EPS and MI transition in the films.In chapter four, we describe the structure and electrical transport of (110) oriented LPCMO thin films. In our study LPCMO were grown on (110)-LAO,(110)-STO and (011)-NGO substrates. The EPS pattern and the electrical property of LPCMO film were modulated by biaxial anisotropic strain from different substrates. The maximum anisotropic resistivity was found in LPCMO/LAO film, and (ρ[001]-ρ[110])/ρ[110] reaches11300%at～108K in the cooling process without magnetic field. The maximum anisotropic resistivity is1400%and170%in LPCMO/STO and LPCMO/NGO, respectively. Intriguingly, we found the reemergent metal-insulator transitions in LPCMO/NGO film. Till now, reemergent of MI transition was only found in manganite nanowire but never in films. The investigation of reemergent MI transition in our LPCMO/NGO films is ongoing.