Ferromagnetic interlayer interaction in KCo2Se2−xSx (0 ≤ x ≤ 2) and its chemical origin

Abstract

Layered Co-based chalcogenides ACo₂Ch₂ (A = Tl, K, Rb and Cs; Ch = Se, S) are believed to adopt the ThCr₂Si₂-type structure and they all share a similar [CoCh] functional layer. However, their interlayer magnetic interactions show significant differences, with the underlying mechanism still unclear. In this work, the chemical origin of the magnetic interactions in layered KCo₂Se_2−xS_x (0 ≤ x ≤ 2) solid solution has been investigated by experiments and first-principles calculations. All the members of this solid solution are found to have the tetragonal ThCr₂Si₂-type structure with the space group I4/mmm. Both a and c lattice constants shrink monotonously with S doping. Ferromagnetic interactions are the same for all the members, even for a member with a similar layer distance to those of the antiferromagnetic TlCo₂Se_2−xS_x. The calculated band structure and density of electronic states of KCo₂Se_2−xS_x and TlCo₂Se_2−xS_x show dramatically different scenarios, which is mainly due to the nature of Tl–Ch and K–Ch chemical bonding. Compared with the Tl chalcogenides, the K counterparts with ionic K–Ch bonds present more localized electronic states in terms of energy, which leads to positive interaction constants in the RKKY model. Our results give the quantitative explanation of the different interlayer couplings in the ACo₂Ch₂ family, as well as the influence of the intercalating layer on the properties of the functional layers in the ThCr₂Si₂-type structure.