Magnetic properties of Tm3+ in layered triangular lattices

Rare-earth chalcogenides, ARCh2 (A = alkali metal or monovalent ion; R = rare-earth elements; Ch = O, S, and Se), have been identified as promising candidates for exploring a variety of novel frustrated quantum magnetic phenomena. Tm-based series, ATmCh2, where Tm3+ ions are arranged on various frustrated geometric lattices, provide a platform for investigating the competitions among spin–orbit coupling, crystal field effects, and magnetic exchange interactions in the context of geometric frustration. In this study, we present how the crystallographic structures of newly synthesized ATmSe2 (A = Li and Na) influence their frustrated magnetic behaviors. Both NaTmSe2 and LiTmSe2 adopt a delafossite-type structure with a two-dimensional (2D) triangular lattice, but LiTmSe2 has a much shorter c-axis parameter compared to this in NaTmSe2. Both heat capacity and magnetic susceptibility measurements confirm the absence of long-range magnetic order in either compound. However, magnetic measurements data reveals the differences in magnetic interactions: NaTmSe2 exhibits features of low-dimensional magnetism, likely driven by its layered crystal structure, compared to LiTmSe2.