Tunable chiral and nematic states in the triple-Q antiferromagnet Co$_{1/3}$TaS$_2$

Complex spin configurations in magnetic materials, ranging from collinear single-Q to noncoplanar multi-Q states, exhibit rich symmetry and chiral properties. However, their detailed characterization is often hindered by the limited spatial resolution of neutron diffraction techniques. Here we employ magnetic circular dichroism (MCD) and magnetic linear dichroism (MLD) to investigate the triangular lattice antiferromagnet Co$_{1/3}$TaS$_2$, revealing three-state (Z3) nematicity and also spin chirality across its multi-Q magnetic phases. At intermediate temperatures, the presence of MLD identifies nematicity arising from a single-Q stripe phase, while at high magnetic fields and low temperatures, a phase characterized solely by MCD emerges, signifying a purely chiral non-coplanar triple-Q state. Notably, at low temperatures and small fields, we discover a unique phase where both chirality and nematicity coexist. A theoretical analysis based on a continuous multi-Q manifold captures the emergence of these distinct magnetic phases, as a result of the interplay between four-spin interactions and weak magnetic anisotropy. Additionally, MCD and MLD microscopy spatially resolves the chiral and nematic domains. Our findings establish Co$_{1/3}$TaS$_2$ as a rare platform hosting diverse multi-Q states with distinct combinations of spin chirality and nematicity while demonstrating the effectiveness of polarized optical techniques in characterizing complex magnetic textures.