  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.
