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Growth and Emergent Functionalities of Oxide Thin Films Utilizing Interface Engineering

Thesis Defense

9:30 am–11:00 am
Jorgensen Hall
Target Audiences:
Physics Department, (402) 472-2770,
Detian Yang will present his thesis defense topic, “Growth and Emergent Functionalities of Oxide Thin Films Utilizing Interface Engineering” via Zoom.

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Abstract: Complex oxide interfaces have offered intriguing novel emergent phenomena and multiple functionalities through interfacial reconstructions of spin, orbital, charge, and lattice degrees of freedoms. 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)/-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, 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, with pulsed laser deposition, 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 buffer layer. By choosing different combinations of the buffer layers and the substrates, epitaxial h-Lu1-xCaxMnO3 films with both compressive and tensile strain were achieved to some maximum thicknesses. In h-Lu1-xCaxMnO3 h-ScFeO3/ ?-Al2O3 samples, ferroelectric distortion can be boosted by the doping of Ca and when the doping concentration x >=0.2, the common interface clamping effect that suppresses improper ferroelectricity of h-ReMnO3 can be eliminated and a potential quasi-2D ferroelectric system is established. Such enhancement in improper ferroelectric distortion originates from the competition between the rigid compressive strain of h-ScFeO3 and the doping effects of Ca. In this part, we established a potential quasi-2D ferroelectric system and suggest a general strain engineering method to strengthen improper ferroelectricity in the ultra-thin regime.

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