Vacancy-mediated fcc/bcc phase separation in Fe1-xNix ultrathin films

TL;DR

This study investigates phase separation in Fe-Ni ultrathin films near the Invar composition, revealing vacancy-mediated formation of bcc and fcc domains through combined experimental and theoretical approaches.

Contribution

It demonstrates that vacancy formation drives phase separation in Fe-Ni thin films, supported by spectromicroscopy and density functional theory calculations.

Findings

Fe0.70Ni0.30 films break into bcc and fcc domains after annealing.

Ni diffuses to form chemical heterogeneity.

Vacancy formation energy is approximately 1.59 eV.

Abstract

The phase separation occurring in Fe-Ni thin films near the Invar composition is studied by using high-resolution spectromicroscopy techniques and density functional theory calculations. Annealed at temperatures around 300 C, Fe0.70Ni0.30 films on W(110) break into micron-sized bcc and fcc domains with compositions in agreement with the bulk Fe-Ni phase diagram. Ni is found to be the diffusing species in forming the chemical heterogeneity. The experimentally determined energy barrier of 1.59 +- 0.09 eV is identified as the vacancy formation energy via density functional theory calculations. Thus, the principal role of the surface in the phase separation process is attributed to vacancy creation without interstitials.