NSF-EC Cooperative Activity in Computational Materials Research: Bridging Atomistic to Continuum - Multiscale Investigation of Self-Assembling Magnetic Dots During Epitaxial Growth

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This grant is a result of a proposal submitted to the Materials World Net program on NSF-EC Computational Materials Science.Intellectual Merits: In the fabrication of nanostructures for optoelectronic applications, tremendous advances have been realized in the development of self- assembly methods for semiconductor materials based on heteroepitaxial vapor-phase growth techniques. In particular, so-called directed self-assembly methods have been demonstrated recently based on the use of pre-patterned substrates or vertical alignment in growth of multilayers, allowing a high degree of control over size distributions and growth of complex patterned arrays. While tremendous progress has been made for semiconductor systems, self- assembly of metallic nanostructures has not been advanced to the same degree. Such developments offer the potential to revolutionize the development of future media for ultra-high-density magnetic recording. The primary goal of the project is to advance the state of the art in multiscale computational materials science through development of a suite of tools for predictive modeling of directed self- assembly. These tools will be utilized to build fundamental understanding of important microscopic and continuum-level phenomena governing nanoscale self-assembly. Although the computational tools to be developed are general and will have application beyond the systems studied, the feasibility of our approach in applications to specific systems will be demonstrated for the well-studied magnetic material systems Fe/Mo and Fe/W. We propose an integrated approach by joining forces of computational experts from the European Community and the United States, spanning the atomistic to the continuum scales. The work will involve ab initio calculations of surface energies, surface stress, and surface diffusion coefficients, as well as statistical mechanics, mesoscopic and continuum calculations of the evolution of nanostructural morphology and composition of dots during both deposition and annealing, and the resulting self-assembly. By parameter passing, each effort will feed into the other, as the information at the smaller scale will be employed in the larger scale calculations, enabling us to bridge a wide range of length and time scales. Using this approach, we can address questions such as how the interplay between kinetic and thermodynamic effects lead to nanostructural formation and what controls the spatial and size distributions of the dots. Answers to these questions are not only of fundamental interest but also allow us to provide insights into processes that produce large-scale self-organized arrays of magnetic dots. To fulfill these specific tasks, we combine modern computational tools, like phase-field models, homogenization techniques and asymptotic expansions with state- of-the-art computational methods, such as multigrid solvers, adaptive and composite finite elements and parallel kinetic Monte Carlo simulations. Furthermore, in collaboration with an experimental partner in EU, the models will be subjected to sharp and intensive tests and validation on the model systems.Broader Impacts: The major impacts on the community at large are two-fold: (1) New intensive summer courses on crystal and epitaxial growth for gifted high school students in both US and Europe, and (2) managed and conscientious effort by every one of the PIs to include undergraduate and graduate students with diverse backgrounds in this project. The US summer school will be offered in cooperation with the California State Summer School for Mathematics and Science (COSMOS) at the University of California, Irvine and will be planned and taught by all of the PIs in the US. The EC counterpart will also offer a similar program at CEASAR. Several of the PIs have experience with planning and executing such programs. To ensure diversity in recruitment, one of the PIs (Thornton) will be in charge of contacting high school teachers and researchersat targeted schools directly to identify talents in under-represented groups. In addition, PIs will attend and judge local science fairs at the high school level and utilize such forums as a ground for outreach activities.