Flat band driven itinerant magnetism in the Co-pnictides <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:msub> <mml:mi mathvariant="normal">CaCo</mml:mi> <mml:mn>2</mml:mn> </mml:msub> <mml:msub> <mml:mi mathvariant="normal">As</mml:mi

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 ${\mathrm{CaCo}}_{2}{\mathrm{As}}_{2}$ and ${\mathrm{LaCo}}_{2}{\mathrm{P}}_{2}$. We find that the ${k}_{z}$ Fermi topology of ferromagnetic ${\mathrm{LaCo}}_{2}{\mathrm{P}}_{2}$ is markedly two-dimensional, while the antiferromagnetic ${\mathrm{CaCo}}_{2}{\mathrm{As}}_{2}$ develops a 3D Fermi surface described by a zigzaglike band dispersion perpendicular to the Co-As plane. Furthermore, the magnetism is driven by the electronic correlations of the flat bands with ${d}_{xy}$ and ${d}_{{z}^{2}}$ 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.