Randomness in atomic disorder and consequent squandering of spin-polarization in a ferromagnetically fragile quaternary Heusler alloy FeRuCrSi

TL;DR

This study reveals that atomic disorder in FeRuCrSi disrupts its predicted half-metallic ferromagnetic state, showing the importance of local chemistry and disorder in determining magnetic properties of quaternary Heusler alloys.

Contribution

The paper provides an in-depth structural analysis demonstrating that atomic disorder between Fe and Ru sites challenges the assumed half-metallic ferromagnetic nature of FeRuCrSi.

Findings

Atomic disorder between Fe and Ru sites was observed.

Magnetic moments are primarily contributed by Fe atoms.

The material's properties are strongly influenced by local atomic disorder.

Abstract

Ru$_{2-x}$Fe$_x$CrSi ( 0 $<$ x $<$1) system is theoretically predicted to be one of the very few known examples of robust half-metallic ferromagnet with 100\% spin polarization. Since Cr is considered to be the main contributor to magnetism, the Fe/Ru substitution is not expected to disturb its magnetic properties any significantly, and hence all Fe-containing members of the series are predicted to follow Slater-Pauling rule with a saturation magnetic moment of 2 ${\mu_B}$/f.u. However, contrarily to the theoretical expectations, some experiments rather show a linear variation of the saturation magnetization and Curie temperature with Fe (\textit{x}) substitution. The equiatomic member FeRuCrSi of this family is also considered as a technologically important material, where the band structure calculations suggest the material to be spin gapless semiconductor. Through our in-depth structural analysis of FeRuCrSi using X-ray diffraction, extended X-ray absorption fine structure and $^{57}$Fe M\"{o}ssbauer spectrometry, we found a random disorder between Fe and Ru sites, while the magnetic moment in this system is actually contributed by Fe atoms, questioning the very basic foundation of the half-metallic character proposed by all theoretical calculations on Ru$_{2-x}$Fe$_x$CrSi series. Our M\"{o}ssbauer result also envisions a rather rare scenario where the main physical properties are intricately correlated to the chemistry of the material in the form of random atomic disorder on a localised scale.