Jongjin Kim

Strain-mediated metal-insulator transition in epitaxial ultrathin films of NdNiO3

We have synthesized epitaxial NdNiO3 ultrathin films in a layer-by-layer growth mode under tensile and compressive strain on SrTiO3 (001) and LaAlO3 (001), respectively. A combination of x-ray diffraction, temperature dependent resistivity, and soft x-ray absorption spectroscopy has been applied to elucidate electronic and structural properties of the samples. In contrast to the bulk NdNiO3, the metal-insulator transition under compressive strain is found to be completely quenched, while the transition remains under the tensile strain albeit modified from the bulk behavior.

Orbital control in strained ultra-thin LaNiO3/LaAlO3 superlattices

In pursuit of rational control of orbital polarization, we present a combined experimental and theoretical study of single-unit-cell superlattices of the correlated metal LaNiO3 and the band insulator LaAlO3. Polarized X-ray absorption spectra show a distinct asymmetry in the orbital response under strain. A splitting of orbital energies consistent with octahedral distortions is found for the case of compressive strain. In sharp contrast, for tensile strain, no splitting is found although a strong orbital polarization is present. Density functional theory calculations including a Hubbard U-term reveal that this asymmetry is a result of the interplay of strain and confinement that induces octahedral rotations and distortions and altered covalency in the bonding across the interfacial Ni-O-Al apical oxygen, leading to a charge disproportionation at the Ni sites for tensile strain.

Strain-driven spin reorientation in magnetite/barium titanate heterostructures

We report spin reorientation transitions in a Fe3O4/BaTiO3 heterostructure driven by strain at the structural phase transitions of BaTiO3. These spin reorientations result from the emergence of an in-plane uniaxial magnetic anisotropy. The magnetoelastic response of Fe3O4 to the variations in epitaxial strain that occur at the BaTiO3 phase transitions gives rise to the uniaxial anisotropy. The anisotropy energies calculated from the in-plane strain are in quantitative agreement with a change in the Zeeman energy.

Surface influenced magnetostructural transition in FeRh films

Surface structural effects accompanying the antiferromagnetic-ferromagnetic magnetostructral transition of epitaxial FeRh thin films were investigated by grazing incidence x-ray scattering. Measurement of the film lattice parameters and variation of x-ray incident angles allow observation of the transition character on scales ranging from a few nm to the total through-thickness of the film. Out-of-plane lattice measurements confirm that the ferromagnetic phase nucleates from the surface during the heating process and is retained at the surface below the transition temperature during the cooling process. These results suggest that surface strain relief fosters nucleation of the ferromagnetic phase.

The Hydration Structure at Yttria-Stabilized Cubic Zirconia (110)-Water Interface with Sub-Ångström Resolution

The interfacial hydration structure of yttria-stabilized cubic zirconia (110) surface in contact with water was determined with ~0.5 Å resolution by high-resolution X-ray reflectivity measurement. The terminal layer shows a reduced electron density compared to the following substrate lattice layers, which indicates there are additional defects generated by metal depletion as well as intrinsic oxygen vacancies, both of which are apparently filled by water species. Above this top surface layer, two additional adsorbed layers are observed forming a characteristic interfacial hydration structure. The first adsorbed layer shows abnormally high density as pure water and likely includes metal species, whereas the second layer consists of pure water. The observed interfacial hydration structure seems responsible for local equilibration of the defective surface in water and eventually regulating the long-term degradation processes. The multitude of water interactions with the zirconia surface results in the complex but highly ordered interfacial structure constituting the reaction front.

Epitaxial growth of (1 1 1)-oriented spinel CoCr2O4/Al2O3 heterostructures

High quality (1 1 1)-oriented CoCr2O4/Al2O3 heterostructures were synthesized on the sapphire (0 0 0 1) single crystal substrates by pulsed laser deposition. The structural properties are demonstrated by in-situ reflection high energy electron diffraction, atomic force microscopy, X-ray reflectivity, and X-ray diffraction. X-ray photoemission spectroscopy confirms that the films possess the proper chemical stoichiometry. This work offers a pathway to fabricating spinel type artificial quasi-two-dimensional frustrated lattices by means of geometrical engineering.

Effect of defects on reaction of NiO surface with Pb-contained solution

In order to understand the role of defects in chemical reactions, we used two types of samples, which are molecular beam epitaxy (MBE) grown NiO(001) film on Mg(001) substrate as the defect free NiO prototype and NiO grown on Ni(110) single crystal as the one with defects. In-situ observations for oxide-liquid interfacial structure and surface morphology were performed for both samples in water and Pb-contained solution using high-resolution X-ray reflectivity and atomic force microscopy. For the MBE grown NiO, no significant changes were detected in the high-resolution X-ray reflectivity data with monotonic increase in roughness. Meanwhile, in the case of native grown NiO on Ni(110), significant changes in both the morphology and atomistic structure at the interface were observed when immersed in water and Pb-contained solution. Our results provide simple and direct experimental evidence of the role of the defects in chemical reaction of oxide surfaces with both water and Pb-contained solution.

Interfacial stability of ultrathin films of magnetite Fe3O4 (111) on Al2O3(001) grown by ozone-assisted molecular-beam epitaxy

The thin films of iron oxides including magnetite (Fe3O4) and hematite (α-Fe2O3) have many important applications. Both forms of oxide can occur naturally during film growth by iron deposition under various oxidation environments; an important issue is to understand and control the process resulting in a single-phase film. We have performed the in-situ -time studies using X-ray diffraction of such film growth on sapphire (001) under pure ozone by monitoring the (00L) rod. Stable magnetite growth can be maintained at growth temperatures below 600 °C up to a certain critical film thickness, beyond which the growth becomes hematite. The results demonstrate the importance of interfacial interaction in stabilizing the magnetite phase.

Epitaxial growth of (111)-oriented spinel CoCr2O4/Al2O3 heterostructures

High quality (1 1 1)-oriented CoCr2O4/Al2O3 heterostructures were synthesized on the sapphire (0 0 0 1) single crystal substrates by pulsed laser deposition. The structural properties are demonstrated by in-situ reflection high energy electron diffraction, atomic force microscopy, X-ray reflectivity, and X-ray diffraction. X-ray photoemission spectroscopy confirms that the films possess the proper chemical stoichiometry. This work offers a pathway to fabricating spinel type artificial quasi-two-dimensional frustrated lattices by means of geometrical engineering.