Strain Mediated Magnetic And Electric Properties Of Transition Metal Oxide Films

In recent years, the perovskite-like manganite heterostructureshave been studied extensively. This is because on one hand, as the typical material of strong correlated system, the manganites have complex phase diagram and interesting physical phenomenon such as colossal magnetoresistance, charge/orbital ordering and various magnetic structures. On the other hand, due to the interplay of the charge, spin, orbital and lattice degrees of freedom,thephysical properties in manganites can be easily tuned by external stimuli such as theepitaxial strain. Therefore, investigating the strain-structure-properties in manganite-basedheterostructures is very important for the further understanding the emergentphenomena at perovskite interfaces and design of related multi-functional electronic devices. Based on this consideration, we focus our work on the manipulation of electric and magnetic properties of manganite heterostructure by static epitaxial strain and dynamic piezo-strain. The dissertion is arranged as follows:In chapter1, The progress of manganite especially the progress of manganite films recently are reviewed. Starting from the research history of manganite, we introduce the crystal structure, basic physical mechanism, phase separation of manganites. Then the recent progress of strain effect on epitaxial manganite films are highlighted. At last, three kinds of anisotropic effects in manganite films are discussed in detail.In chapter2, some important techniques for sample fabrications, structure characterization and various measurementfor physical properties are introduced, including three popularfabrication techniquesmagnetron sputtering, pulsed laser deposition and chemical vapor deposition;X-ray diffraction and atomic force microscope, electric and magnetic measurement as well as flexible combination of different instruments via Labview.In chapter3, we systematically investigate the anisotropic strain induced anisotropic effects such as magnetic anisotropy, anisotropic resistivity and anisotropic magnetoresistance in typical phase separation manganite La0.325Pr0.3Ca0.375MnO3film. In the thinner30nm film, the anisotropic strained inducedcolossal anisotropic resistivitycan reach105%around the metal-insulator transition temperature. However, in thestrain relaxed thicker120nm film, the anisotropic resistivity decrease significantly, about two orders of magnitude smaller than the thin film. In order to eluciated the relation of the anisotropy with anisotropic strain induced phase separation,the oriented magnetic domains are directly observed by magnetic force microscopic (MFM) imagingwhich is consistent with the transport and magnetic data. From the point of application, the large magnitude and tunability of anisotropic magnetic and electronic properties caused by the strain are believed to be potential for designing artificial materials and devices.In chapter4, dynamic strain manipulation of the electric property of manganite film/piezoelectric single substrate heterostructures are systematically investigated. The strain transformation is enhanced3times by inserting a buffer layer SrTiO3in La0.7Ca0.3MnO3/SrTiO3/PMN-PT heterostructure. Moreover, in this heterosturcture, the strain effect and ferroelectric field effect are quantitatively separated first time basing on the volatility of the strain effect and the non-volatility of the field effect. In addition, the piezostrain mediated phase separation in LPCMO/PMN-PT is also observed which leads to an increasement of the thermal hysteresis in the temperature dependence of resistivity curves with the polarization field kept on the substrate. However, the anisotropic resitivity is little affected. These findings are potential for designing new electronic devices.In chapter5, The magnetocaloric properties of the CrO2single crystal films are studied. From the isothermal magnetic data, the magnetic entropy change are calculated which could reache a value of8.46J/kg·K at385K for the field variesfrom0to5T.Moreover the calculated relative cooling power values of CrO2can reach410J/kg at5T,which is comparable to that for Gd and Gd5Si2Ge2. These results suggest that CrO2film is one of the promising candidates for magnetic refrigeration applications above room temperature.