Ligand-mediated Origin of Altermagnetic Spin-Splitting

Altermagnets host spin-split electronic bands despite zero net magnetization, opening new routes for spintronics beyond conventional ferromagnets. Going beyond symmetry-based classifications, which specify allowed terms but not their hierarchy, here we use first-principles calculations and Wannier Hamiltonian engineering to uncover the microscopic bonding contributions of altermagnetic spin splitting in the $g$-wave altermagnet Co$_{1/4}$NbSe$_2$. We show that the splitting is captured by a short-range tight-binding model, establishing its local origin. By selectively controlling hopping channels, we demonstrate that the dominant contribution arises not from direct magnetic-ion hopping, but from ligand-mediated hybridization that transfers anisotropy to itinerant states. This identifies ligand-assisted coupling as the key mechanism of altermagnetic spin splitting and provides a microscopic bridge between minimal models and symmetry guided first-principles material searches, enabling real-space design of altermagnetic functionality.