A song of energy scales and technology: The elusive challenge of Heusler alloys

Fabrication of ternary intermetallic alloys with high atomic ordering is a critical step to realizing their predicted functional properties. For instance, Heusler alloys have long been predicted with high spin polarization, low spin damping, and high ferromagnetic ordering temperatures. Unfortunately, experimental realization and applications have proven stubbornly elusive: Models and rules-of-thumb have found limited success when expanded to a wider range of elemental choices, and atomic ordering has typically been lower than predicted, with deleterious effect on their material properties. In this talk, I will outline our theoretical and experimental efforts to understand the root causes behind the experimental disconnect with theory in Heusler systems, and our efforts to find and realize materials that do not have deleterious atomic disorder or melting temperatures too high to allow layered growth.

First, we use first principles calculations to screen for the convergence of useful material properties, alloy melting points as a fraction of our practical growth temperature maximum (1000 °C), and either an atomic ordering configuration robust against site swapping or a material system whose desired properties are robust against some amount of site swapping. We then grow thin films of material systems that meet the criteria using sputter beam epitaxy deposition, and characterize their atomic ordering and material properties for comparison to theory. Using this procedure, compositional series across elemental choices will determine the veracity and limits of hypotheses and lead to basic rules governing atomic ordering in complex alloys.