Moment-volume coupling in La(Fe$_{1-x}$Si$_x$)$_{13}$

TL;DR

This study uses first-principles calculations to explore how local Fe magnetic moments influence volume changes and phase transitions in La(Fe$_{1-x}$Si$_x$)$_{13}$, revealing a key moment-volume coupling mechanism.

Contribution

It demonstrates that the volume variation is proportional to the square of the local Fe moments and links this to the nature of magnetic phase transitions in the system.

Findings

Volume varies with the square of local Fe moments.

Anomalous softening at transition depends on moment-volume coupling.

Si content influences the order of magnetic phase transitions.

Abstract

We investigate the origin of the volume change and magnetoelastic interaction observed at the magnetic first-order transition in the magnetocaloric system La(Fe$_{1-x}$Si$_x$)$_{13}$ by means of first-principles calculations combined with the fixed-spin moment approach. We find that the volume of the system varies with the square of the average local Fe moment, which is significantly smaller in the spin disordered configurations compared to the ferromagnetic ground state. The vibrational density of states obtained for a hypothetical ferromagnetic state with artificially reduced spin-moments compared to a nuclear inelastic X-ray scattering measurement directly above the phase transition reveals that the anomalous softening at the transition essentially depends on the same moment-volume coupling mechanism. In the same spirit, the dependence of average local Fe moment on the Si content can account for the occurence of first- and second-order transitions in the system.