Exploring half-metallic Co-based full Heusler alloys using a DFT+U method combined with linear response approach

A density functional theory (DFT)+U method based on linear response (LR) theory was applied to investigate the electronic structures of a Co-based ternary full Heusler alloy Co₂YSi to explore half-metallic (HM) ferromagnets with a wide HM gap. The LR-based DFT+U calculations tend to obtain a reasonable correlation parameter for the Y site, while the correlation of the Co site misdirects to the unphysical ground state due to the overestimated parameter value that arises from the delocalized electronic structure of Co. Furthermore, we found that the HM gap of Co₂MnSi originates from the Co orbital in the conduction state and the Co-Mn hybridizing t_2g orbital in the valence state around the Fermi energy. This means that the HM gap is a tunable property by selecting the Y element and/or mixing several elements into the Y site through t_2g atomic-orbital coupling. Our LR-based DFT+U method was extended to other ternary Co₂YSi and quaternary Co₂(Y,Mn)Si. We found that Co₂(Ti_0.25,Mn_0.75)Si and Co₂(Fe_0.25,Mn_0.75)Si show HM nature, with the Fermi energy being at almost the center of the minority band gap, which leads to high thermal stability.