Origin and Properties of the Gap in the Half-Ferromagnetic Heusler Alloys

TL;DR

This study investigates the origin of the electronic gap in half-ferromagnetic Heusler alloys, emphasizing the influence of chemical composition and lattice parameter changes on their half-metallicity.

Contribution

It provides a detailed analysis of how valence electron count and atomic substitutions affect the gap and magnetic properties in Heusler alloys using the full-potential screened KKR method.

Findings

Compounds with 18 valence electrons per unit cell are semiconductors.

Substituting $sp$ atoms shifts bands and destroys half-metallicity.

A 2% change in lattice parameter preserves the minority-spin gap.

Abstract

We study the origin of the gap and the role of chemical composition in the half-ferromagnetic Heusler alloys using the full-potential screened KKR method. In the paramagnetic phase the C1_b compounds, like NiMnSb, present a gap. Systems with 18 valence electrons, Z_t, per unit cell, like CoTiSb, are semiconductors, but when Z_t > 18 antibonding states are also populated, thus the paramagnetic phase becomes unstable and the half-ferromagnetic one is stabilized. The minority occupied bands accommodate a total of nine electrons and the total magnetic moment per unit cell in mu_B is just the difference between Z_t and $2 \times 9$. While the substitution of the transition metal atoms may preserve the half-ferromagnetic character, substituting the $sp$ atom results in a practically rigid shift of the bands and the loss of half-metallicity. Finally we show that expanding or contracting the lattice parameter by 2% preserves the minority-spin gap.