Doctor of Philosophy (PhD)
Physics and Astronomy
Perovskite oxides (ABO3) show wide range of functionalities originating from interplay of structural, spin, charge, and orbital degrees of freedoms. The bulk perovskite structure could be controlled via conventional chemical substitution, though exploiting heterostructure engineering novel ground states could be observed which otherwise are absent in bulk. In this thesis, the interest is to explore the electro-magnetic phenomena as complex oxides are confined in heterostructures.
I first investigate electromagnetic properties of ultrathin epitaxial ruthenate: SrRuO3 (SRO); spatially confined between SrTiO3 (STO) i.e., STO5-SROn-STO5 with n = 1- and 2-unit cells. It is shown that STO5-SRO2-STO5 heterostructure is nearly stoichiometric, metallic, and ferromagnetic with TC ~ 128 K, even though it lacks characteristic bulk-SRO octahedral tilts. In contrast, STO5-SRO1-STO5 is accompanied by a loss of metallicity and ferromagnetism, though octahedron remains non-tilted. However, STO5-SRO1-STO5 exhibits a substantial interface induced Ti-Ru intermixture, which is responsible for the loss of metallicity and ferromagnetism. The results highlight role of B-site non-stoichiometry and interface-induced intermixture.
Next, we report the emergent electro-magnetic properties in ultrathin CaRuO3 films by unique “d-doping” engineering i.e., atomically replacing A-site with single SrO layer, though keeping identical B-site (Ru). While bulk CaRuO3 is metallic and nonmagnetic, the films confined to thickness of ~15-unit cells (u.c.) are insulating and remain nonmagnetic. However, d-doping of SrO to middle of CaRuO3 films induces an insulator-to-metal transition and unusual ferromagnetism, while retaining bulk-orthorhombicity. Through, atomically resolved structural analysis, density-functional-theory calculations as well as transport and magneto-transport, we find that emergent magnetic ordering is coupled to subtle electronic and structural effects introduced via A-site cation.
To further investigate effect of A-site on ruthenates functionalities, we employ heterostructure engineering to stabilize Sr1-xBaxRuO3 (0 < x < 0.7) films. The strain-stabilized Ba substitution transmutes lattice symmetry from bulk-like orthorhombic (x = 0) to cubic phase (x = 0.2) without RuO6 rotations. The cubical film exhibits robust ferromagnetic ordering (Tc = 145 K) with a strong perpendicular magnetic anisotropy. Though, increased Ba-substitution significantly condenses the ferromagnetism, suggesting that besides determining octahedral distortion, the A-site introduces certain entangled electro-magnetic effects that greatly affect the ruthenates magnetic ordering.
Ali, Zeeshan, "Controlling Electro-Magnetic Functionality of Ruthenates by Heterostructure Design" (2022). LSU Doctoral Dissertations. 5972.