Effects of magnetic excitations and transitions on vacancy formation: cases of fcc Fe and Ni compared to bcc Fe

TL;DR

This study investigates how magnetic excitations influence vacancy formation energies in different iron and nickel crystal structures using DFT and Monte Carlo simulations, revealing system-specific magnetic effects on defect properties.

Contribution

It provides new insights into the role of longitudinal and transversal spin excitations on vacancy formation energies across fcc Ni, fcc Fe, and bcc Fe, highlighting the importance of magnetic transitions.

Findings

Magnetic transitions significantly affect vacancy formation energies in bcc Fe.

Longitudinal spin excitations impact vacancy free energy in fcc Fe and Ni.

Vacancy formation energies align well with experimental data at high temperatures.

Abstract

Vacancy is one of the most frequent defects in metals. We study the impacts of magnetism on vacancy formation properties in fcc Ni, and in bcc and fcc Fe, via density functional theory (DFT) and effective interaction models combined with Monte Carlo simulations. Overall, the predicted vacancy formation energies and equilibrium vacancy concentrations are in good agreement with experimental data, available only at the high-temperature paramagnetic regime. Effects of magnetic transitions on vacancy formation energies are found to be more important in bcc Fe than in fcc Fe and Ni. The distinct behaviour is correlated to the relative roles of longitudinal and transversal spin excitations. At variance with the bcc-Fe case, we note a clear effect of longitudinal spin excitations on the magnetic free energy of vacancy formation in fcc Fe and Ni, leading to its steady variation above the respective magnetic transition temperature. Below the N\'{e}el point, such effect in fcc Fe is comparable but opposite to the one of the transversal excitations. Regarding fcc Ni, although neglecting the longitudinal spin excitations induces an overestimation of the Curie temperature by 220 K, no additional effect is visible below the Curie point. The distinct effects on the three systems are closely linked to DFT predictions of the dependence of vacancy formation energy on the variation of local magnetic-moment magnitudes and orientations.