# Dynamic Simulation of Structural Phase Transitions in Magnetic Iron

## TL;DR

This study uses advanced spin-lattice dynamic simulations based on ab initio data to directly demonstrate the finite-temperature phase transitions between bcc and fcc structures in magnetic iron, highlighting the role of magnetic and lattice interactions.

## Contribution

It introduces a novel dynamic simulation approach that couples magnetic fluctuations and atomic vibrations to accurately model phase transitions in iron.

## Key findings

- Identified two phase transition points between bcc and fcc phases.
- Quantified the free energy difference, maxing at 2 meV, consistent with experimental data.
- Demonstrated the importance of magnetic-lattice coupling in phase stability.

## Abstract

The occurrence of bcc-fcc ($\alpha$-$\gamma$) and fcc-bcc ($\gamma$-$\delta$) phase transitions in magnetic iron stems from the interplay between magnetic excitations and lattice vibrations. However, this fact has never been proven by a direct dynamic simulation, treating non-collinear magnetic fluctuations and dynamics of atoms, and their coupling at a finite temperature. Starting from a large set of data generated by ab initio simulations, we derive non-collinear magnetic many-body potentials for bcc and fcc iron describing fluctuations in the vicinity of near perfect lattice positions. We then use spin-lattice dynamics simulations to evaluate the difference between free energies of bcc and fcc phases, assessing their relative stability within a unified dynamic picture. We find two intersections between the bcc and fcc free energy curves, which correspond to $\alpha$-$\gamma$ bcc-fcc and $\gamma$-$\delta$ fcc-bcc phase transitions. The maximum fcc-bcc free energy difference over the temperature interval between the two phase transition points is 2 meV, in agreement with other experimental and theoretical estimates.