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 KCeS2exhibits magnetic order belowTN= 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 belowTN, 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 ofab initiocalculations, and demonstrate that its magnetic ground state matches the experimental spin structure.
Keywords: f-electron systems; magnetic anisotropy; rare-earth delafossites; rare-earth magnetism; spin liquids; stripe-yz magnetic order; triangular-lattice antiferromagnets.
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