Determining Chemical Order Parameter from Small Volumes

1. Determining Chemical Order Parameter from Small Volumes Gregory B. Thompson, University of Alabama Tuscaloosa, DMR 0547445 Intellectual Merit Research motivation: The most readily available technique for the determination of long-range order parameter, S, is X-ray diffraction. Experimentally, S is determined by measuring the total integrated peak intensities of the superlattice and fundamental reflections according to kinematical scattering theory. However, X-ray scattering from thin films can be very small and difficult to measure with laboratory diffractometers. In contrast, electron scattering can be more amenable for diffraction studies of small volumes, though strong interaction of electrons with the material results in multiple scattering events. As a result, the scaling of the ratio of integrated intensities is no longer valid and S determination becomes more complex. To correctly account for multiple electron scattering events, a multislice simulation can be used to predict the convergent beam electron diffraction (CBED) intensities for a given order parameter, orientation and thickness. Results: A series of FePt films with various orientations, compositions, thicknesses and S were fabricated. We determined the long-range order parameter using convergent electron diffraction along with multislice simulations. The results of this study were validated at the National Synchrotron Light Source at Brookhaven National Laboratory. We successfully determined the long-range order parameter for all specimen utilizing this technique. Future work will extended this approach to determining S of individual grains and nanoparticles. (a) (b) Ta enrichment at a triple junction grain boundary (c) Figure 1. An 11.7 nm Fe50Pt50 film with {001} orientation. (a) The intensity ratio vs. S curve. (b) A simulated CBED pattern at S=0.5. (c) The experimental CBED pattern. SCBED=0.48±0.02 and SXRD=0.46±0.03. Karen Torres, PhD graduate student supported on this grant, received the Microscopy Society of America’s 2010 Presidential Student Award for her research using CBED and simulations to determine order parameter in FePt.

2. Utilizing Research for Educational and International Collaborations Gregory B. Thompson, University of Alabama Tuscaloosa, DMR 0547445 Broader Impact The CAREER grant has supports the Ph.D. studies of Ms. Karen Torres (Hispanic American) and Mr. Bianzhu Fu (Asian). In May 2010, Ms. Diondra Means (African American), partially supported by this grant ,received her M.S. degree which involved Ta clustering studies in FePt thin films. She is now a Texas Instrument engineer. Professor Thompson continues to strengthen our national scientific workforce by mentoring and graduating students from historically underrepresented science and engineering demographics. While at UA, these students have ample multi-disciplinary interaction in UA’s Center for Materials for Information Technology (www.mint.ua.edu). The research results continue to be disseminated to the scientific community through annual MINT Fall reviews with magnetic hard-drive industrial sponsors as well as traditional publications and professional meeting conferences (MRS, TMS, M&M and IFES). During the 2009 summer, Ms. Torres collaborated with Drs. Simon Ringer and Michael Moody for two weeks at the University of Sydney in New South Wales, Australia. The research involved quantifying the aberrations in atom probe reconstructions of FePt. This collaboration provided Ms. Torres with an international experience in preparation for her post-doctoral career. Over the academic year, Professor Thompson and Ms. Torres continued this collaboration and the results were presented at the 2010 International Field Emission Symposium (IFES) in Sydney, Australia. A publication of the research is currently under peer-review. In 2008, Professor Thompson launched a research homepage providing up-to-date highlights from this and other research programs . The website also contains his outreach activities. Please visit http://www.bama.ua.edu/~gthomps/ Ms. Karen Torres (right) with Dr. Michael Moody (left) working on atom probe data reconstruction of an FePt alloy at the University of Sydney.