Spinning order from chaos in metal alloys

Electron spin can bring order out of chaos, that's a prediction regarding the ordering of Cr in an equiatomic fcc NiFeCrCo high entropy alloy (HEA) according to research by a team of scientists at North Carolina State University and Furman University, South Carolina, USA. The team tested these predictions through the synthesis of three samples by casting/annealing or milling. The samples exhibited low temperature magnetic moments consistent with trends from the first principles simulations and advanced scanning transmission electron microscopy identified ordered nano-domains. [Niu et al., Appl Phys Lett, 2015, 106, 161906; DOI: 10.1063/1.4918996]

HEAs, with their four or more metals present in approximately equal amounts, have some intriguing magnetic and mechanical properties and have become the focus of much research during the last decade or so, according to NC State materials scientist Doug Irving. "For example, NiFeCrCo-based HEAs have a good combination of hardness, tensile strength, ductility, and fracture resistance at extremely low temperatures," Irving explains. "If you look at NiFeCrCo, it has a fixed structure, but which atoms fill each site is seemingly random - it seems impossible to predict which element might be in any given site." That impression of chaos is why they're called high entropy alloys.

Irving and his team have now demonstrated that there is greater order in this chaos than was previously imagined and it is the spins of the electrons on chromium atoms that drive this order. It is well known that electron spins can be aligned in the same direction in ferromagnetic materials - cobalt, iron and nickel. Conversely in antiferromagnetic materials, such as chromium, spins align in the opposite direction to their neighbors.

To complicate matters, in an HEA such as NiFeCrCo, spins on the chromium electrons can align against their neighbors if they are surrounded by iron, nickel or cobalt. Those three metals can display all spins up while chromium has spin down. However, if two chromium atoms are themselves neighbors they cannot of course align their spins differently from all of their neighbors because they themselves are neighbors. The result is that the spin properties of chromium force the chromium atoms to reside in the HEA with the greatest separation possible. This results in nanoscopic domains of order within the overall chaos of the HEA.

"Showing that this material has order at the nanoscale will likely lead to new work on how to expand these ordered domains, and potentially manipulate the material's mechanical properties," Irving explains.

Irving told Materials Today about a few future directions. "The first would be the impact of this local ordering on properties of the alloy," he says. "What role does local order play in the remarkable mechanical properties? Additionally, it would be interesting to analyze how this extends to systems with more components where multiple elements are anti-ferromagnetic."

David Bradley blogs at Sciencebase Science Blog and tweets @sciencebase, he is author of the popular science book "Deceived Wisdom".