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.

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• Electronic structure
• Fermi surface
• Flat bands
• Magnetism
• Density functional theory
• Photoemission spectroscopy