Flat band driven itinerant magnetism in the Co-pnictides (La,Ca)Co₂(As,P)₂

Flat bands can induce strong electron correlation effects that help stabilize both magnetic and superconducting states. Here, we carry out angle-resolved photoemission spectroscopy and density functional theory calculations to study the electronic structure of the Co-pnictides CaCo₂As₂ and LaCo₂P₂. We find that, while the kz Fermi topology of ferromagnetic LaCo₂P₂ is markedly 2-dimensional, antiferromagnetic CaCo₂As₂ develops a 3D Fermi surface described by a $zig-zag$-like band dispersion perpendicular to the Co-As plane. Furthermore, the magnetism is driven by the electronic correlations of the flat bands with dxy and dz² orbital character at the Fermi level. Our results link the electronic dimensionality and the magnetic order, and emphasize the critical role of the As-As and P-P bond strength along the c-direction to understand the electronic band structure and the rich phase diagram of transition metal pnictides.