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

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 behavior 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 Néel 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.

Magnetochemical effects on phase stability and vacancy formation in fcc Fe-Ni alloys

“…8, G mag f increase weakly with increasing temperature in the disordered structures with more than 30% Ni, while the trend is reversed in the disordered structures below 30% Ni. This can be correlated with our previous results which suggest that G mag f in fcc Fe and in Ni decreases and increases with increasing temperature, respectively [72].…”

“…The present EIM is based on our previously developed EIMs of fcc Fe and Ni (namely the previous model parameters are kept), whose accuracy has been demonstrated in a previous study [72]. In the following, we validate the EIM description of vacancy-containing Fe-Ni alloys, by comparing its predicted vacancy formation energies with DFT results in the ordered and disordered structures.…”

“…In our previous work, effective interaction models (EIMs) were parametrized for pure fcc Fe and Ni systems respectively [72]. In this work, they are unified and extended as a single EIM for the whole composition range of fcc Fe-Ni alloys.…”

“…The latter is a sign of the magnitude of the magnetic interaction energy, which is also the strongest around 65% Ni according to our model. Indeed, it is shown that the difference between G mag f in the paramagnetic state and the ground state is closely related to the magnetic interaction energy [72].…”

Defect modeling and control in structurally and compositionally complex materials

“…Similarly, magnetic disorder could also break the lattice symmetry, leading to different local magnetic environments around a defect. To investigate point defects such as vacancies in paramagnetic Fe-based alloys [121][122][123][124] , a spin-space averaging approach 123,125,126 was developed, which treats the disordered paramagnetic state as a quasirandom antiferromagnetic spin configuration (that is, magnetic SQS).…”