Surface segregation in nanoparticles from first principles

TL;DR

This study uses first-principles calculations and Monte Carlo simulations to analyze surface segregation in FePt nanoparticles, revealing that surface Pt segregation is balanced by depletion in subsurface layers, suggesting kinetic effects limit ordering.

Contribution

It introduces a local cluster expansion fitted to ab initio data to accurately model surface segregation thermodynamics in nanoparticles.

Findings

Surface Pt segregation is balanced by subsurface Pt depletion.

Core ordered state remains largely unaffected by surface thermodynamics.

Weak ordering is likely due to kinetic effects, not thermodynamic limitations.

Abstract

FePt nanoparticles are known to exhibit reduced L1$_0$ order with decreasing particle size. The reduction in order reduces the magnetic anisotropy and the thermal stability of the direction of magnetization of the particle. The phenomenon is addressed by investigating the thermodynamic driving forces for surface segregation using a local (inhomogeneous) cluster expansion fitted to ab initio data which accurately represents interatomic interactions in both the bulk and surface regions. Subsequent Monte Carlo simulations reveal that first surface layer Pt segregation is compensated by Pt depletion in the second subsurface layer. This indicates that the core's ordered state is not affected by surface thermodynamics as much as previously thought. Thus, the weak ordering experimentally observed is likely not due to fundamental thermodynamic limitations but rather to kinetic effects.