# The phonon softening due to melting of the ferromagnetic order in elemental iron

## TL;DR

This study investigates how the melting of ferromagnetic order in iron leads to phonon softening, using advanced ab initio methods to distinguish magnetic effects from structural transitions.

## Contribution

It demonstrates that phonon softening in iron is caused by magnetic order melting, not structural phase changes, and predicts the dynamic stability of BCC iron across temperatures.

## Key findings

- Phonon softening is linked to magnetic order melting.
- BCC iron remains dynamically stable at all temperatures.
- The BCC phase is thermodynamically unstable between certain phases.

## Abstract

We study the fundamental question of the lattice dynamics of a metallic ferromagnet in the regime where the static long range magnetic order is replaced by the fluctuating local moments embedded in a metallic host. We use the \textit{ab initio} Density Functional Theory(DFT)+embedded Dynamical Mean-Field Theory(eDMFT) functional approach to address the dynamic stability of iron polymorphs and the phonon softening with increased temperature. We show that the non-harmonic and inhomogeneous phonon softening measured in iron is a result of the melting of the long range ferromagnetic order, and is unrelated to the first order structural transition from the BCC to the FCC phase, as is usually assumed. We predict that the BCC structure is dynamically stable at all temperatures at normal pressure, and is only thermodynamically unstable between the BCC-$\alpha$ and the BCC-$\delta$ phase of iron.