Experimental confirmation of the magnetic ordering transition induced by an electronic structure change in the metallic triangular antiferromagnet <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:mrow> <mml:msub> <mml:mi>Co</mml:mi> <mm

We report ARPES studies combined with DFT+DMFT calculations to confirm that the magnetic ordering vector transition from \textbf{Q}=(1/2,0,0) to \textbf{Q}=(1/3,0,0) in the metallic triangular antiferromagnets Co$_{1/3\pmε}$TaS$_2$ ($ε\approx$0.007) is induced by the electronic structure change in the system. The ARPES-measured Fermi surface (FS) maps of Co$_{0.325}$TaS$_2$ show two hexagonal and one circular hole-like FSs around $Γ$, which matches well with the triple-\textbf{Q} state by taking into account the contribution of nesting vectors occurring between Co 3$d$ and Ta 5$d$ orbitals. In the case of Co$_{0.340}$TaS$_2$, a new electron pocket around K appears and the FS geometry changes as a result of the correlation effect of Co$_4$S$_{18}$ tripods forming in the system. The magnetic susceptibility calculations based on the charge-self-consistent DFT+DMFT band structures and the random phase approximation indicate that the most stable magnetic ordering vector (1/2,0,0) split into (1/6,0,0) and (1/2,0,0), which is consistent with the magnetic phase transition around $x$=1/3 in Co$_{x}$TaS$_2$.