Nonthermal solid-solid phase transition in ferromagnetic iron

We posit the existence of a nonthermal phase transition in iron, driven by a loss of ferromagnetic ordering on ultrafast timescales with increasing electron temperature. The transition corresponds to a solid-solid bcc to fcc phase transformation and takes place at an electron temperature of 0.62 eV while the ion lattice remains near room temperature. The bcc structure initially undergoes phonon softening during the magnetic transformation, followed by a solid-solid phase transition to the fcc structure, and a subsequent hardening of phonon modes. We present a detailed physical picture of the process, supported by finite-temperature density functional theory simulations of the phonon dispersion curves, electronic density of states, and thermodynamic free energy.