Growth and Emergent Functionalities of Oxide Thin Films Utilizing Interface Engineering

Complex oxide interfaces have offered intriguing novel emergent phenomena and multiple functionalities through interfacial reconstructions of spin, orbital, charge, and lattice degrees of freedom. Interface engineering via manipulating interfacial interaction, defects and multiple interfacial quantum charges and orders constitutes the essential method and technique to achieve desired functionalities in oxide heterostructures. In this thesis, shown are two examples of utilizing interfacial reconstruction and interfacial strain engineering to achieve intrinsic exchange bias and realize epitaxial growth of mixed-valence hexagonal manganite thin films, respectively. Firstly, we demonstrated intrinsic exchange bias induced by interfacial reconstruction in NixCoyFe3-x-yO4(111)/α-Al2O3(0001) (0≤x+y≤3) tunable beyond the coercivity. The “hidden” antiferromagnet is proposed to be the rock-salt CoO in the interfacial layer, supported by the data of reflection high energy electron diffraction (RHEED), magnetization characterization, X-ray photoemission spectroscopy, X-ray reflectometry and polarized neutron reflectometry. Such interfacial reconstruction and intrinsic exchange bias can be tuned by Co concentration y and the growth oxygen pressure. This study establishes new material platforms to study novel interfacial structural and magnetic states supporting potential applications in magnetic storage and spintronics and highlight the powerful interface engineering strategy in manipulating material functionalities. Secondly, we have managed to stabilize h-Lu1-xCaxMnO3 (x=0.1,0.2,0.3,0.4,0.5) epitaxial thin films over sapphire and YSZ substrates via pre-depositing a 1 nm h-ReFeO3 or h-ReMnO3 (Re= Y, Ho-Lu, Sc) buffer layer. By selecting varying combinations of buffer layers and substrates, epitaxial h-Lu1- xCaxMnO3 films with both compressive and tensile strain were achieved to certain maximum thicknesses. In h-Lu1-xCaxMnO3/h-ScFeO3/α-Al2O3 samples, thickness-resolved and temperature-resolved RHEED patterns reveal that the geometric polar distortion could be boosted by the doping of Ca, when the doping concentration x ≥ 0.2, the common interface clamping effect that suppresses ferroelectric distortion in the first unit cell of h-ReMnO3 can be eliminated. This work establishes a potential quasi-2D ferroelectric system and suggests a general strain engineering method to strengthen improper ferroelectricity in the ultra-thin regime