  Yb- and Ce-based delafossites were recently identified as effective spin-1/2
antiferromagnets on the triangular lattice. Several Yb-based systems, such as
NaYbO2, NaYbS2, and NaYbSe2, exhibit no long-range order down to the lowest
measured temperatures and therefore serve as putative candidates for the
realization of a quantum spin liquid. However, their isostructural Ce-based
counterpart KCeS2 exhibits magnetic order below TN = 400 mK, which was so far
identified only in thermodynamic measurements. Here we reveal the magnetic
structure of this long-range ordered phase using magnetic neutron diffraction.
We show that it represents the so-called "stripe-yz" type of antiferromagnetic
order with spins lying approximately in the triangular-lattice planes
orthogonal to the nearest-neighbor Ce-Ce bonds. No structural lattice
distortions are revealed below TN, indicating that the triangular lattice of
Ce3+ ions remains geometrically perfect down to the lowest temperatures. We
propose an effective Hamiltonian for KCeS2, based on a fit to the results of ab
initio calculations, and demonstrate that its magnetic ground state matches the
experimental spin structure.
