William H. Butler, Ph.D. - US grants

0529369
Gupta

Intellectual merit of the proposed research
A fundamental study involving optimization of the CPP-GMR sensor stack is proposed. This will involve a fundamental theoretical study of the physics of the spintronic device, which will lead to an optimized device design, including selection of the materials, layer thicknesses, and device structure for optimized performance. The study in GMR stack deposition will emphasize optimization of thin-film deposition with particular attention being given to interfacial engineering. Furthermore, a thorough investigation of multilayer film nucleation and growth under various processing conditions, will lead to a better understanding of how to generate and control the properties of these films and their subsequent electrical and magnetic characteristics. Subsequent fabrication of the GMR sensor has sufficient complexity and will provide the student with substantial exposure to various state-of-the-art deposition, photolithography, e-beam lithography and dry and wet etch processing techniques and equipment. In-depth structural, magnetic and electrical characterization of these nanolaminates will be carried out using a wide range of advanced analytical equipment. Atomic scale characterization of the GMR stacks by transmission electron microscopy and atom probe tomography will greatly enhance the scientific merit of the proposed research. The technological motivation for this research is grounded in the desire to create novel materials, processes and structures for novel sensors for military and civilian applications. The research is interdisciplinary, involving the Metallurgical and Materials Engineering, Electrical and Computer Engineering and Physics departments.

The broader impacts resulting from the proposed activity
The proposed research is interdisciplinary, integrating faculty and students from three departments (Metallurgical and Materials Engineering, Electrical and Computer Engineering and Physics). Graduate students on this project, one a minority female will receive training in disciplines of materials selection, nanolayer processing, device fabrication, and atomic, microstructural, electrical and magnetic characterization, and will gain an understanding of physics and chemistry, metallurgical, materials and electrical engineering. The project will have collaboration with the Army Research Laboratories and Veeco Instruments, which will allow the students to be exposed to a broad range of technologies - from sensor design and fabrication to vacuum technology and cathode design and development, and have direct exposure to career options at the Army Research Laboratories as well as in industry. Recruitment of minority female undergraduate students from a neighboring HBCU is also planned. This, along with having a minority female graduate student as well as a female PI, will serve as a strong and positive role model for minority women entering science and engineering, both nationally and globally.

Technical: This project addresses growth and further understanding of rutile heterostructures for spin-electronic applications. Objectives are to gain understanding and control over the synthesis and processing of CrO2 and related rutile materials into heterostructures using a combined experiment/theory approach. Pulsed laser deposition and laser-assisted chemical vapor deposition (LCVD) will be used for the growth of the heterostructures. In LCVD, energy from a laser is used to photochemically decompose a gas precursor to provide epitaxial CrO2 growth at low pressure with in-situ electron diffraction crystal growth monitoring to obtain spin transport quality CrO2 films and interfaces. Both Meservey- Tedrow and inelastic tunneling spectroscopy will be performed, as well as basic magneto-transport to characterize prototype device structures.
Non-technical: The project addresses basic research issues in a topical area of electronic/photonic materials science with high technological relevance. Research and educational activities will be integrated with involvement of both graduate and undergraduates in the research program. Collaborations with Oak Ridge National Laboratory and international collaborations provide added benefits and special opportunities to assist integration of research and education. The project includes collaborations with minority faculty members from Historically Black Colleges and Universities (HBCUs).

Non-technical: The atom probe is a microscope that allows three dimensional rendering of individual atoms in a material. By being able to image how atoms cluster together, scientists are able to understand and thereby engineer materials for improved energy conversion, electrical conduction or magnetic data storage. The University of Alabama (UA) has key research programs in these and other areas that require this type of atomic level imaging. UA researchers are working on coatings that can improve the life cycle for turbine blades used in advanced power generators and aircraft engines. Additionally, faculty researchers have sponsored efforts in developing materials for fuel cells. UA houses a government and industrial sponsored magnetic recording research center. This center has active programs in developing materials for high storage densities, high sensitivity sensors and faster logic devices, such as transistors. Most of these materials for these new technologies use oxide-based materials, which are poor electrical conductors. Historically, atom probes required materials that were electrically conductive (metals). Recent advances in laser pulsing has allowed atom probes to image poor electrical conductors, such as semiconductors and insulators. The requested laser attachment to UA's atom probe will subsequently expand the range of materials that can be characterized in these strategic programs. The laser attachment provides a unique capability in fostering collaboration with several regional institutions, including historically black colleges and universities. Additionally, it serves in recruitment of students into the materials science discipline at UA.

Technical: The ability to pin-point an individual atom in a three-dimensional microstructure has become an essential need in materials characterization to link experimental observations to atomic scale modeling. The atom probe instrument field evaporates atoms from a specimen of interest which are collected on a position-sensitive, mass-spectrum detector. By reconstructing the trajectory path and impact position of each ion, a volumetric reconstructed rendering of the material is generated with near atomic precision for each individual atom. Historically, atom probe specimens needed to be conductive in order for the high voltage pulse to propagate to the apex of the specimen to field evaporate the surface atoms. The commercial advent of the laser now allows poor conductors (ceramics and semiconductors) to be thermally assisted in the evaporation process. The University of Alabama (UA) has several research programs that utilize dielectric materials. The ability to characterize these materials by atom probe microscopy would significantly advance these programs. For example, UA's efforts on high-k dielectric HfO2 for next-generation gate-values has shown that nitrogen-doping can significantly reduce intermixing between HfO2 and Si; however, an underlying understanding has been hampered by the inability to characterize subtle composition changes at the interface. UA has a track-record of being leaders in spintronic research for giant magnetoresistance sensors and tunneling magnetoresistance devices. The atom probe's ability to characterize buried oxide interfaces within these thin film stacks would further facilitate our linkage between measured properties and modeling. The laser would also allow us to field evaporate brittle intermetallics, like FePt, that are candidates for ultrahigh magnetic storage media. Finally, UA has active energy-based research programs. The laser attachment to our atom probe would allow us to characterize PtRu alloys on their catalytic support structures, such as graphite and alumina. Similarly, the laser will increase the capability to characterize oxide scale formation in thermal protective coatings used for power generation turbine blades. UA's supporting infrastructure and personnel is exceptionally well equipped to develop atom probe specimens and advance the usage of the laser to a wide range of materials. The increased capability will maintain UA as a national analytical facility and continue to foster our existing outreach research activities with HBCU institutions.

****Technical Abstract****
First-principles theory (DFT and beyond) will be used to screen thousands of half-metals and choose a set of experimentally-accessible starting materials. We will also develop models based on state-of-the-art Non-Equilibrium Green Functions to calculate their transport characteristics. Experimentally, we will synthesize the candidate materials, test their electrical, magnetic, and structural characteristics and compare to theoretical predictions. The results of this characterization will then be fed back to refine our theoretical methods. Promising materials will be tested with more advanced techniques (such as spin-polarized tunneling and local-electrode atom probe tomography), providing more detailed information for more advanced modeling. The most promising materials will be used in prototype TMR and (CIP/CPP)-GMR devices. A specific disruptive technology goal is the design, fabrication and demonstration of a low moment half-metal with perpendicular anisotropy and low magnetic damping ideally suited for STT-RAM. This will be accomplished through the tight circular work flow among rational design, computational verification, spin transport modeling, experimental characterization and device fabrication. Several interdisciplinary courses at UA and UVa will be developed to quickly incorporate lessons we have learned into the classroom and provide students with cutting-edge training. Software developed in the project will be deployed on the NSF NanoHUB.

****Non-Technical Abstract****
In today's electronic devices electrons are manipulated through their electrical charge. However, electrons have another property called "spin". Electrons behave as if they were spinning about an axis. According to quantum mechanics the spin axis of an electron can point in only one of two directions, i.e. either "up" or "down". In most materials there are equal numbers of up and down electrons and usually both types respond to an electric field in the same way. In magnetic materials, however, the number of up and down spin electrons may be different and the two types of electrons may respond to electric fields in different ways. The most extreme example of this phenomenon is a "half-metal" - meaning that one set of electrons is a metal and the other set is an insulator. A specific technology goal is the design, fabrication and demonstration of a half-metal with carefully controlled magnetic properties tailored to meet the requirements of non-volatile magnetic memories (which aim to replace traditional RAM). We aim to provide an improved understanding of half-metals and magnetic materials in general, in particular how one can relate 'first-principles' calculations to experimentally accessible and technologically relevant materials and device parameters. This project will help to increase the STEM workforce by providing research experiences for undergraduates and high school students and will enhance its diversity through its composition and collaboration with HBCU faculty. Several interdisciplinary courses at UA and UVa will be developed to quickly incorporate lessons learned into the classroom and provide students with cutting-edge training.