A. James Clark School of Engineering
Permanent URI for this communityhttp://hdl.handle.net/1903/1654
The collections in this community comprise faculty research works, as well as graduate theses and dissertations.
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Item On Mapping Electron Clouds with Force Microscopy(2012) Wright, Charles Alan; Solares, Santiago D.; Mechanical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)At its core, this is a story about electrons. Electrons drive the interactions of matter at the nanoscale, so an understanding of electron behavior offers significant insight into the behavior of nanoscale materials. Atomic force microscopy (AFM) has demonstrated great success as a tool for probing matter at the nanoscale, and recent reports suggest that it may even be capable of mapping electron clouds on atomic surfaces. The most recent of these claims came in 2004, when Hembacher et al. [Science 305] observed subatomic features while imaging a graphite surface with a tungsten tip using higher-harmonics frequency modulation AFM (FM-AFM). The authors' interpretation of these features as the footprint of the electron density at the tungsten tip's apex atom has been met with much skepticism. But despite the potential significance of the results, a detailed theoretical study has not been performed. In this work, a computational method based in density functional theory (DFT) is developed in order to simulate the imaging process and draw fundamental conclusions regarding the feasibility of subatomic imaging with higher harmonics FM-AFM. The application of this method to the tungsten/graphite system reveals that the bonding lobes of increased charge density are in fact present at the tungsten tip's apex atom and that the corresponding higher harmonics images can exhibit subatomic features similar to those observed experimentally. We further show that the filtering process used to experimentally measure the harmonics does not introduce imaging artifacts but that harmonics averaging is not an appropriate method for enhancing contrast. We then suggest an alternate approach: the individual mapping of the first two harmonics, which are expected to dominate the contrast under the experimental conditions studied. Finally, we demonstrate the important role played by the surface atom used to probe the AFM tip. We find that a small, non-reactive atom is necessary for resolving subatomic features. Most importantly, we show that the observed features are not a direct reflection of the electron density at the AFM tip's front atom. Instead, they represent a measure of the bonding stiffness between the tip's front atom and the atoms in the layer above.Item Atomic-level Characterization of Fe(001)/MgO(001)/Fe(001) Tunneling Magnetoresistance Structures and Spin-polarized Scanning Tunneling Microscopy(2010) Lee, Jookyung; Gomez, Romel D.; Electrical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)This thesis seeks to understand the Fe-MgO-Fe system through a series of atomic level studies of the topographic, electronic, and magnetic properties of these epitaxial films. This multilayer system is uniquely important because of its huge tunneling magnetoresistance (TMR) arising from spin coherence and strong spin filtering through the structure. MgO-based magnetic tunnel junctions have been actively investigated and are now successfully applied to commercial products such as non-volatile magnetic random access memories and read-write heads for hard disk. However, despite its popularity most work has been done on macroscopic samples and has focused on the device-level performance. Yet very little effort has been devoted towards the understanding at the atomic length scales including the effects of atomic steps and local variation in stoichiometry. The primary goal of this work is to elucidate the interplay between morphology, stoichiometry, local magnetism, and local electronic properties. To this end a multifaceted approach was used involving atomic/magnetic force microscopy (AFM/MFM), scanning tunneling microscopy (STM), scanning tunneling spectroscopy (STS), Auger electron spectroscopy, and low energy electron diffraction (LEED), which were operated in the cleanest possible conditions under an ultra-high vacuum. I linked the morphology directly to the formation of different magnetic domain configurations as a function of growth temperature and film thickness. I also correlated these atomic-level properties to the device-level performance. By investigating the topography and the surface electronic density of states with length scales in the nanometer regime, I found that the films had extremely inhomogeneous surface states. Because the structural defects such as surface steps, deep trenches and grain boundaries, as well as the existence of chemical impurities can perturb the spin-coherent tunneling, our observation of the electronic inhomogeneity can provide a direct clue for explaining the diminished TMR phenomenon on real systems compared to the theoretical expectation, which is one of longstanding problems to achieve high TMR in actual devices. In addition to the Fe/MgO/Fe work, I also demonstrated spin polarized STM which revealed the anti-ferromagnetic spin-structure of single crystal chromium and the magnetic domains structure of permalloy film on silicon oxide.