Chemical ordering and composition fluctuations at the (001) surface of the Fe-Ni Invar alloy

TL;DR

This study combines first-principles calculations and STM experiments to analyze chemical ordering and composition fluctuations on the (001) surface of Fe-Ni Invar alloy, revealing surface buckling, chemical contrast, and subsurface effects.

Contribution

It provides new insights into the surface atomic structure, chemical order, and electronic properties of Fe-Ni Invar alloy surfaces through combined computational and experimental approaches.

Findings

Surface atoms are buckled with Fe outward and Ni inward.

Chemical contrast in STM images reveals c2x2 order with Fe-rich defects.

Subsurface composition fluctuations influence surface atom apparent height.

Abstract

We report on a study of (001) oriented fcc Fe-Ni alloy surfaces which combines first-principles calculations and low-temperature STM experiments. Density functional theory calculations show that Fe-Ni alloy surfaces are buckled with the Fe atoms slightly shifted outwards and the Ni atoms inwards. This is consistent with the observation that the atoms in the surface layer can be chemically distinguished in the STM image: brighter spots (corrugation maxima with increased apparent height) indicate iron atoms, darker ones nickel atoms. This chemical contrast reveals a c2x2 chemical order (50% Fe) with frequent Fe-rich defects on Invar alloy surface. The calculations also indicate that subsurface composition fluctuations may additionally modulate the apparent height of the surface atoms. The STM images show that this effect is pronounced compared to the surfaces of other disordered alloys, which suggests that some chemical order and corresponding concentration fluctuations exist also in the subsurface layers of Invar alloy. In addition, detailed electronic structure calculations allow us to identify the nature of a distinct peak below the Fermi level observed in the tunneling spectra. This peak corresponds to a surface resonance band which is particularly pronounced in iron-rich surface regions and provides a second type of chemical contrast with less spatial resolution but one that is essentially independent of the subsurface composition.