2021, Kulbakov, Anton A., Avdoshenko, Stanislav M., Puente-Orench, Inés, Deeb, Mahmoud, Doerr, Mathias, Schlender, Philipp, Doert, Thomas, Inosov, Dmytro S.

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.

2020, Bastien, Gaël, Rubrecht, Bastian, Haeussler, Ellen, Schlender, Philipp, Zangeneh, Ziba, Avdoshenko, Stanislav, Sarkar, Rajib, Alfonsov, Alexey, Luther, Sven, Onykiienko, Yevhen A., Walker, Helen C., Kühne, Hannes, Grinenko, Vadim, Guguchia, Zurab, Kataev, Vladislav, Klauss, Hans-Henning, Hozoi, Liviu, van den Brink, Jeroen, Inosov, Dmytro S., Büchner, Bernd, Wolter, Anja U.B., Doert, Thomas

Recently, several putative quantum spin liquid (QSL) states were discovered in ~S=1/2 rare-earth based triangular-lattice antiferromagnets (TLAF) with the delafossite structure. A way to clarify the origin of the QSL state in these systems is to identify ways to tune them from the putative QSL state towards long-range magnetic order. Here, we introduce the Ce-based TLAF KCeS2 and show via low-temperature specific heat and μSR investigations that it yields magnetic order below TN=0.38 K despite the same delafossite structure. We identify a well separated ~S=1/2 ground state for KCeS2 from inelastic neutron scattering and embedded-cluster quantum chemical calculations. Magnetization and electron spin resonance measurements on single crystals indicate a strong easy-plane g~factor anisotropy, in agreement with the ab initio calculations. Finally, our specific-heat studies reveal an in-plane anisotropy of the magnetic field-temperature phase diagram which may indicate anisotropic magnetic interactions in KCeS2.