Electronic structure, metamagnetism and thermopower of LaSiFe$_{12}$ and interstitially doped LaSiFe$_{12}$

TL;DR

This study uses density functional theory to analyze how interstitial doping with H, B, C, and N affects the electronic, magnetic, and thermoelectric properties of LaSiFe$_{12}$, revealing that hydrogen causes less electronic disturbance than other dopants.

Contribution

It provides a detailed microscopic understanding of how different interstitial dopants influence the electronic structure and magnetic transitions in LaSiFe$_{12}$, especially highlighting hydrogen's minimal impact.

Findings

Hydrogen doping minimally affects the double-well free energy structure.

B, C, and N cause strong s-p band hybridization and lattice expansion.

Hydrogen maintains the first-order magnetic transition, unlike other dopants.

Abstract

We present a systematic investigation of the effect of H, B, C, and N interstitials on the electronic, lattice and magnetic properties of La(Fe,Si)$_{13}$ using density functional theory. The parent LaSiFe$_{12}$ alloy has a shallow, double-well free energy function that is the basis of itinerant metamagnetism. On increasing the dopant concentration, the resulting lattice expansion causes an initial increase in magnetisation for all interstitials that is only maintained at higher levels of doping in the case of hydrogen. Strong s-p band hybridisation occurs at high B,C and N concentrations. We thus find that the electronic effects of hydrogen doping are much less pronounced than those of other interstitials, and result in the double-well structure of the free energy function being least sensitive to the amount of hydrogen. This microscopic picture accounts for the vanishing first order nature of the transition by B,C, and N dopants as observed experimentally. We use our calculated electronic density of states for LaSiFe$_{12}$ and the hydrogenated alloy to infer changes in magneto-elastic coupling and in phonon entropy on heating through $T_C$ by calculating the fermionic entropy due to the itinerant electrons. Lastly, we predict the electron thermopower in a spin-mixing, high temperature limit and compare our findings to recent literature data.