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

A density functional theory (DFT)+ method based on linear response (LR) theory was applied to investigate the electronic structures of a Co-based ternary full Heusler alloy CoYSi to explore half-metallic (HM) ferromagnets with a wide HM gap. The LR-based DFT+ 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 CoMnSi originates from the Co orbital in the conduction state and the Co-Mn hybridizing t 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 atomic-orbital coupling. Our LR-based DFT+ method was extended to other ternary CoYSi and quaternary Co(Y,Mn)Si. We found that Co(Ti,Mn)Si and Co(Fe,Mn)Si show HM nature, with the Fermi energy being at almost the center of the minority band gap, which leads to high thermal stability.