First-principles modeling of the Invar effect in Fe65Ni35 by the spin-wave method

TL;DR

This paper uses first-principles calculations with an advanced spin-wave method to model the thermal expansion of Fe65Ni35 alloy, highlighting the importance of magnetic short-range order and entropy effects for accurate predictions.

Contribution

It introduces a generalized spin-wave method for ferromagnetic states with short-range order to improve ab initio modeling of thermal expansion in Fe-Ni alloys.

Findings

Magnetic short-range order significantly affects lattice constants.

Magnetic entropy contributes notably at high temperatures.

Theoretical results align semi-quantitatively with experimental data.

Abstract

Thermal lattice expansion of the Invar Fe65Ni35 alloy is investigated in first-principles calculations using the spin-wave method, which is generalized here for the ferromagnetic state with short range order. It is shown that magnetic short-range order effects make substantial contribution to the equilibrium lattice constant and cannot be neglected in the accurate ab initio modeling of the thermal expansion in Fe-Ni alloys. We also demonstrate that at high temperatures, close and above the magnetic transition, magnetic entropy associated with transverse and longitudinal spin fluctuations yields a noticeable contribution to the equilibrium lattice constant. The obtained theoretical results for the temperature dependent lattice constant are in semiquantitative agreement with the experimental data apart from the region close the magnetic transition.