Anomalous Hall conductivity and electronic structures of Si-substituted Mn$_{2}$CoAl epitaxial films

TL;DR

This study investigates how Si substitution affects the electronic structure and anomalous Hall conductivity in Mn$_{2}$CoAl films, revealing a Fermi level shift, sign reversal of conductivity, and implications for spin-gapless semiconductors.

Contribution

It demonstrates that Si substitution allows systematic tuning of electronic properties and Hall conductivity in Mn$_{2}$CoAl films without disrupting their spin-gapless nature.

Findings

Si substitution shifts the Fermi level without altering the electronic structure significantly.

The anomalous Hall conductivity changes sign around x=0.25 due to Fermi level shift.

Small anomalous Hall conductivity values are observed, unaffected by structural disorder.

Abstract

We study anomalous Hall conductivity ($\sigma$$_{\rm AHC}$) and electronic band structures of Si-substituted Mn$_{2}$CoAl (Mn$_{2}$CoAl$_{1-x}$Si$_{x}$). First-principles calculations reveal that the electronic band structure is like a spin-gapless system even after substituting a quaternary element of Si for Al up to $x = $0.2 in Mn$_{2}$CoAl$_{1-x}$Si$_{x}$. This means that the Si substitution enables the Fermi level shift without largely changing the electronic structures in Mn$_{2}$CoAl. By using molecular beam epitaxy (MBE) techniques, Mn$_{2}$CoAl$_{1-x}$Si$_{x}$ epitaxial films can be grown, leading to the systematic control of $x$ (0 $\le$ $x$ $\le$ 0.3). In addition to the electrical conductivity, the values of $\sigma$$_{\rm AHC}$ for the Mn$_{2}$CoAl$_{1-x}$Si$_{x}$ films are similar to those in Mn$_{2}$CoAl films shown in previous reports. We note that a very small $\sigma$$_{\rm AHC}$ of $\sim$ 1.1 S/cm is obtained for $x =$ 0.225 and the sign of $\sigma$$_{\rm AHC}$ is changed from positive to negative at around $x =$ 0.25. We discuss the origin of the sign reversal of $\sigma$$_{\rm AHC}$ as a consequence of the Fermi level shift in MCA. Considering the presence of the structural disorder in the Mn$_{2}$CoAl$_{1-x}$Si$_{x}$ films, we can conclude that the small value and sign reversal of $\sigma$$_{\rm AHC}$ are not related to the characteristics of spin-gapless semiconductors.