Influence of spin fluctuations on structural phase transitions of iron

TL;DR

This study uses a tight-binding model to show that spin fluctuations significantly influence iron's structural phase transitions, accurately predicting transition temperatures and elucidating different mechanisms for each transition.

Contribution

It introduces a combined computational approach that captures the role of spin fluctuations in iron's phase transitions, aligning theoretical predictions with experimental data.

Findings

Spin fluctuations are essential for both phase transitions.

The model accurately predicts transition temperatures.

Different mechanisms govern each phase transition.

Abstract

The effect of spin fluctuations on the $\alpha$ (bcc) - $\gamma$ (fcc) - $\delta$ (bcc) structural phase transitions in iron is investigated with a tight-binding (TB) model. The orthogonal $d$-valent TB model is combined with thermodynamic integration, spin-space averaging and Hamiltonian Monte Carlo to compute the temperature-dependent free-energy difference between bcc and fcc iron. We demonstrate that the TB model captures experimentally observed phonon spectra of bcc iron at elevated temperatures. Our calculations show that spin fluctuations are crucial for both, the $\alpha$ - $\gamma$ and the $\gamma$ - $\delta$ phase transitions but they enter through different mechanisms. Spin fluctuations impact the $\alpha$ - $\gamma$ phase transition mainly via the magnetic/electronic free-energy difference between bcc and fcc iron. The $\gamma$ - $\delta$ phase transition, in contrast, is influenced by spin fluctuations only indirectly via the spin-lattice coupling. Combining the two mechanisms, we obtain both, the $\alpha$ - $\gamma$ and the $\gamma$ - $\delta$ phase transitions with our TB model. The calculated transition temperatures are in very good agreement with experimental values.