Stripe-yz magnetic order in the triangular-lattice antiferromagnet KCeS2

TL;DR

This study uncovers the magnetic structure of KCeS2, a triangular-lattice antiferromagnet, revealing a stripe-yz order through neutron diffraction and modeling its ground state with ab initio calculations.

Contribution

It provides the first detailed magnetic structure of KCeS2 and links experimental results with an effective Hamiltonian based on ab initio calculations.

Findings

KCeS2 exhibits stripe-yz antiferromagnetic order.

No structural distortions are observed below TN.

The effective Hamiltonian accurately predicts the magnetic ground state.

Abstract

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.