CAREER: Atomic-Resolution Study of Electron-Spin Interaction in Strongly-Correlated Mixed-Valence Cobalt Oxide Nano-Structures

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NON-TECHNICAL DESCRIPTION: The performance and reliability of many electronic devices is frequently governed by the stability and durability of the device?s active component materials. For example, atomic level structural defects, or interfaces between different materials often cause decreases in device performance, or total failure at a macro level. At the same time, however, some future applications will actually rely on the presence of these atomic level defects and interfaces to improve the device?s performance. These defects and interfaces are unavoidable, but by understanding how to control and manipulate their influence on a material?s properties, we can improve the functionality of the material on the macro level. Accordingly, a fundamental understanding of how atomic-scale defects or interfaces influence a material?s macroscopic behavior is essential to continuing the development of reliable, next generation devices. Cobalt oxides are one class of ceramic materials that has recently attracted scientific attention as an inexpensive, non-toxic, and highly stabile group of compounds with properties such as superconductivity, thermo-electric behavior, or magneto-resistive behavior. These properties make cobalt oxides attractive for use in a wide range of new device applications. For example, the magnetic properties of cobalt oxides have the potential to revolutionize magnetic storage in high capacity solid-state computer hard-drives, while their thermo-electric properties may lead to the development of coatings that will allow heat from automotive tailpipes or furnace exhaust pipes to be converted into electricity. At present, however, cobalt oxides are neither reliable nor efficient enough for viable macroscopic applications, because there is a lack a fundamental understanding of what is happening with the material on the atomic-level. This CAREER grant utilizes atomic-resolution scanning transmission electron microscopy combined with electron energy-loss spectroscopy (EELS) to investigate the fundamental mechanisms governing the magnetic and electrical properties of cobalt-oxide ceramics and the effects of defects and interfaces on these properties. The educational aspect of this proposal involves training of undergraduate and graduate students, in particular underrepresented minorities, in state-of-the-art materials characterization. Summer programs for undergraduate students will be developed, and the participation of undergraduate students in research projects will be fostered further through the PI?s Journal of Undergraduate Research at the University of Illinois at Chicago.TECHNICAL DESCRIPTION: The objective of this CAREER proposal is to examine how charge, orbital and spin-interactions influence the magnetic and electronic structures of mixed-valence cobalt-oxide ceramics. Two systems will be examined: LaCoO3 and Ca3Co4O9. These systems were chosen for their intriguing, potentially widely useful properties, and for their status as model structures for other strongly correlated cobalt-oxide ceramics. LaCoO3 exhibits magneto-transport properties as well as two spin-state transitions that result in magnetic phase transitions without any structural transition, while the misfit-layered Ca3Co4O9 shows an exceptionally high thermo-power. Atomic-resolution Z-contrast imaging in a scanning transmission electron microscope (STEM) with (EELS) and in-situ heating/cooling experiments (10 T 1000 K) is being used to study the local atomic and electronic structures of these materials. The PI?s laboratory setup includes an aberration-corrected STEM, which will allow for sub-Å spatial-resolution and sub-eV energy-resolution, as well as a conventional TEM/STEM for atomic-resolution insitu heating and cooling experiments. The insitu STEM analysis will be supported by MBE thinfilm synthesis, macroscopic magnetization, transport measurements, and surface characterization, as well as first principles modeling. This combination of experimental and theoretical techniques enables the fundamental structure-properties relationship of charge, orbital and spin-interactions in mixed-valence cobalt-oxide ceramics to be unraveled, which could advance the field of electro-ceramics and lead to the discovery of emergent phenomena that can be used as innovative devices and sensors. An important feature of this program is the integration of research and education through the training of undergraduate and graduate students in cutting-edge transmission electron microscopy and theoretical materials science.