Evolution of Altermagnetism to Spin Density Waves in Co_xNbSe₂

Intercalated group-V transition metal dichalcogenides (TMD) exhibit a variety of magnetotransport phenomena arising from diverse nuclear and magnetic structures. However, a comprehensive mapping of the magnetic, thermodynamic, and structural phases is lacking. Here, we investigate Co_xNbSe₂ for x = 0.29-0.36, using quenching and controlled cooling to tune structural order and map the resulting magnetic phases. We find that the altermagnetic states (x = 1/4) and the spin density wave (x = 1/3) remain robust under both synthesis conditions. In contrast, intermediate compositions give rise to spin-glass-like behavior or magnetic phase coexistence, producing two distinct phase diagrams depending on the thermal history of the synthesis process. To identify the underlying drivers of each ground state, we applied reverse Monte Carlo modeling to diffuse neutron scattering data, revealing the presence of local $\sqrt{3}{a}_h\times \sqrt{3}{a}_h$ sublattice domains in all samples independently of the synthesis conditions. As the intercalant concentration decreases, the Co ions preferentially adopt the 2a_h× 2a_h sublattice, where dominant magnetic interactions emerge, stabilizing the altermagnetic phase. We evaluated these interactions through first-principles calculations for x = 1/4 and 1/3, where each domain interaction type is structurally and magnetically well-defined. The resulting exchange reveals spin-glass and spin density wave phases driven by frustration from tighter intercalant arrangement and altermagnetism from 2a_h× 2a_h domains. Overall, we present local structure-driven magnetic phases across Co_xNbSe₂ compositions, demonstrating how synthesis impacts local order of the Co atoms and thereby the overall magnetic properties.