Anisotropic Multi-Q Order in CoxTaS2

The cobalt-intercalated transition metal dichalcogenide CoₓTaS₂ hosts a rich landscape of magnetic phases that depend sensitively on x. While the stoichiometric compound with $x=1/3$ exhibits a single magnetic transition, samples with x≤ 0.325 display two transitions with an anomalous Hall effect (AHE) emerging in the lower temperature phase. Here, we resolve the spin structure in each phase by employing a suite of magneto-optical probes that include the discovery of anomalous magneto-birefringence -- a spontaneous time-reversal sensitive rotation of the principal optic axes. A symmetry-based analysis identifies the AHE-active phase as an anisotropic (2+1)Q state, in which magnetic modulation at one wavevector (Q) differs in symmetry from that at the remaining two. The (2+1)Q state naturally exhibits scalar spin chirality as a mechanism for the AHE and expands the classification of multi-Q magnetic phases.