Bridging Atomistic to Continuum Scales – Multiscale Investigation of Self-Assembling Magnetic Dots in Heteroepitaxial Growth

Katsuyo Thornton, University of Michigan, DMR 0502737

Funded as a part of the NSF-EC Cooperative Activity in Computational Materials Research program, this grant aims to advance multiscale computational materials science through development of a suite of tools for modeling directed self-assembly of nanoscale magnetic dots. The grant is enabling extensive collaboration of researchers from the US and the EU, including M. Asta (UC Davis), J.S. Lowengrub (UC Irvine), P.W. Voorhees (Northwestern), A. Voigt (CAESAR, Germany), T. Ala-Nissilä (HUT, Finland), O. Fruchart (CNRS, France), M. Kotrla (ASCR, Czech Republic) and the PI, to achieve this goal.

The ongoing collaboration includes first-principles calculations of materials parameters, Monte Carlo calculations for stability of alloy structures in the presence of surfaces, kinetic Monte Carlo simulations of surface nanostructure self-assembly, step-flow dynamics, and continuum-level simulations of heteroepitaxy, as well as simulation-guided experiments and numerical method development.

Adaptive mesh refinement algorithm developed and implemented in 2D/3D multigrid simulation code

Experimental image of W/Mo/Al2O3 (left) and simulation result (right) of island formation.

Phase-field crystal simulation of island growth

Bridging Atomistic to Continuum Scales – Multiscale Investigation of Self-Assembling Magnetic Dots in Heteroepitaxial Growth

Katsuyo Thornton, University of Michigan, DMR0502737

A major challenge in modeling microstructural evolution is bridging atomistic and continuum length and time scales. The phase-field crystal (PFC) method developed recently has emerged as a powerful continuum simulation tool enabling simulations on a much larger time scale than atomistic simulations. The nature of the periodic density field of crystal in the PFC model gives rise to the elastic effects, dislocation dynamics and anisotropy due to crystallographic orientations. The PFC method has been shown to capture realistic aspects of elastic and plastic properties of solids and the anisotropy of interfacial free energies of solid-liquid systems.

We developed a modified phase-field crystal approach to simulate epitaxial growth in which the vapor phase is introduced by an additional phase field. This methodology extends the original phase-field crystal model for solid/liquid systems, and can be used to simulate vapor-liquid-solid systems. This model is used to investigate step flow dynamics, island formation and growth, and the effects of stress generated by a lattice misfit between the film and substrate on the dynamics of film growth. This work results from a close collaboration between Voorhees (co-PI), Thornton (PI), Ala-Nissilä (EU co-PI), and Elder (external collaborator).

Phase-field crystal simulation of island formation during epitaxial growth.

Phase-field crystal simulation of step flow dynamics.

Bridging Atomistic to Continuum Scales – Multiscale Investigation of Self-Assembling Magnetic Dots in Heteroepitaxial Growth

Katsuyo Thornton, University of Michigan, DMR 0502737

Education, Training, and OutreachTraining: The grant supports three graduate students and three postdoctoral fellows (of whom two are female) across four institutions in pursuing education and training in interdisciplinary science and technology. The PI also hosted a high school student and an undergraduate, one of whom is female, to provide them with research experience.High School: The funding enabled a four-week course taught as a part of the California State Summer School for Mathematics and Science (COSMOS). Modules for teaching crystal growth to youngsters were developed using Mathematica, Matlab, and other computational tools. Elementary School: The PI and her students have contributed to several local outreach activities. For example, they held two display stations at the Science Safari at the Ann Arbor Hands-On Museum, where the PI and three female group members reached over 150 Girl Scouts interested in learning about materials, including magnetic materials.

Top: One of the handouts used at Science Safari. Bottom: Thornton with the COSMOS high school students.