Surface energies of stoichiometric FePt and CoPt alloys and their implications for nanoparticle morphologies

TL;DR

This study uses density functional theory to calculate surface energies of FePt, CoPt, and MnPt alloys, revealing how surface stability influences nanoparticle shapes and preferred structures at small scales.

Contribution

It provides detailed surface energy data for binary alloys and demonstrates how these energies affect nanoparticle morphology and stability, especially favoring the L11 structure in small particles.

Findings

Pt-covered surfaces are energetically preferred.

The (111) surface is the lowest energy orientation.

L11 structure is stabilized in small nanoparticles.

Abstract

We have calculated surface energies and surface magnetic order of various low-indexed surfaces of monoatomic Fe, Co, and Pt, and binary, ordered FePt, CoPt, and MnPt using density functional theory. Our results for the binary systems indicate that elemental, Pt-covered surfaces are preferred over Fe- and Co-covered and mixed surfaces of the same orientation. The lowest energy orientation for mixed surfaces is the highly coordinated (111) surface. We find Pt-covered (111) surfaces, which can be realized in the L11 structure only, to be lower in energy by about 400 meV/atom compared to the mixed L10 (111) surface. We conclude that this low surface energy stabilizes the L11 structure in small nanoparticles, which is suppressed in bulk alloys, but has been recently synthesized as thin film for CoPt. From the interplay of surface and bulk energies, equilibrium shapes of single-crystalline ordered nanoparticles and crossover sizes between the different orderings can be estimated.