Engineering topics
Oxley, Mark P
Publications and source records attributed to Oxley, Mark P.
Insulating ferromagnetic oxide films: the controlling role of oxygen vacancy ordering
The origin of ferromagnetism in strained epitaxial LaCoO3 films has been a long-standing mystery. Here, we combine atomically resolved Z-contrast imaging, electron-energy-loss spectroscopy, and density-functional calculations to demonstrate that, in epitaxial LaCoO3 films, oxygen-vacancy superstructures release strain, control the film s electronic properties, and produce the observed ferromagnetism via the excess electrons in the Co d states. Although oxygen vacancies typically dope a material n-type, we find that ordered vacancies induce Peierls-like minigaps which, combined with strain relaxation, trigger a nonlinear rupture of the energy bands, resulting in insulating behavior.
Electron transfer and ionic displacements as the origin of the 2D electron gas at the LAO/STO interface: Direct measurements with atomic-column spatial resolution.
We present direct, atomic-column-resolved scanning transmission electron microscopy and electron energy loss measurements of atomic displacements and Ti valence in abrupt, conductive LAO/STO interfaces. We find that two distinct but interrelated mechanisms are responsible for screening the diverging electric potential in the LAO film: 1) charge injection in the interfacial Ti planes, and 2) dielectric relaxation in both LAO and STO through ionic displacements. The injected charge density decays over a length of nearly 3 unit cells within the STO substrate. The total injected charge is lower than predicted by pure electronic reconstruction. The origin of this discrepancy is attributed to cation and oxygen displacements, which we observe in both LAO and STO, and generate a polarization opposite to the intrinsic polarization of the LAO film. Our data attribute a minor role to oxygen vacancies and cation intermixing.
Examining the structure and bonding in complex oxides using aberration-corrected imaging and spectroscopy
Our ability to directly characterize the atomic and electronic structures is crucial to developing a fundamental understanding of structure-property relationships in complex-oxide materials. Here, we examine one specific example, the misfit-layered thermoelectric material Ca3Co4O9, which exhibits a high Seebeck coefficient governed by spin-entropy transport as well as hopping-mediated electron transport. However, the role of oxygen and its bonding with cobalt in thermoelectric transport remains unclear. We use atomic-resolution annular bright-field imaging to directly image the oxygen sublattice and to combine our experimental data with multislice image calculations to find that the oxygen atoms in the CoO2 subsystem are highly ordered, while the oxygen-atomic columns are displaced in the Ca2CoO3 subsystem. Atomic-column-resolved electron energy-loss spectroscopy and spectrum image calculations are used to quantify the bonding in the different subsystems of incommensurate Ca3Co4O9. We find that the holes in the CoO2 subsystem are delocalized, which could be responsible for the p-type conductivity found in the CoO2 subsystem.