Effects of Structure, Crystallographic Orientation, and Dimensionality on Emergent Properties of Transition Metal Oxide Thin Films

Abstract

SrRuO3 is the only example of ferromagnetic perovskite oxide of a 4d transition metal, wherein the electron -electron correlation is still relevant while the heavier 4d ion (Ru) gives it a larger spin-orbit coupling strength which makes it an interesting material to study. In this thesis, we present our investigation of the structure and properties of SrRuO3 thin films of varying thickness grown on [001] and [111] crystallographic orientation of the SrTiO3 substrate. For SrRuO3(001), we present microscopically the presence of 90◦ in-plane rotated structural domains that are identified by the difference in octahedral rotations and tilts pattern. Our study of the structure in ultrathin SrRuO3(001) films show evidence of orthorhombic distortions down to a single unit-cell thickness. In thicker films, orthorhombic relaxation happens in 3-4 layers after which bulk-like structure is stabilized. We have found that unlike the interfaces with CaRuO3 or La2/3Sr1/3MnO3, the TiO6 octahedra of the SrTiO3 substrate at the interface is undistorted when interfaced with SrRuO3. Our study of SrRuO3(111) films show a deviation of the low temperature magnetization from the conventional Bloch law, an indication of suppressed spin-wave excitations. Instead a ∼ T2 suppression of magnetization corresponding to Stoner excitations is observed suggesting a strong orientation dependence of magnetism. An enhancement in the transition temperature TC and lower residual resistivity is also observed as compared to the SrRuO3(001) films. Thickness dependent investigation reveal a gradual decay of TC with thickness in SrRuO3(111) as compared to a rather abrupt loss of ferromagnetism in SrRuO3(001). We show the feasibility of layered Ruddlesden -Popper (RP) type materials to be used as a substrate for high quality thin film growth. We show the variation of symmetry perpendicular to the cleaving plane in the stacking of these materials can be exploited to induce systematic 2D defects (anti-phase boundaries (APB)) that runs somewhat perpendicular to the film surface. We observe that these APBs can merge when two steps are in close proximity. The ability to create a controllable distribution of 2D defects paves way for future studies to explore new physical phenomena and applications.