Magnetic properties of non-Kramers thulium trihalides TmX₃ (X=Cl, Br, I)

Rare-earth halides display exciting crystal structures for quantum magnetism. For instance, a 2D honeycomb lattice is realized in the quantum spin liquid candidate YbCl₃, and 1D chains with dominating intra-chain interactions are realized in other structures. Although this material series has been well studied, there is a gap in the study of non-Kramers ions such as thulium; therefore, little is known about the magnetic behavior of TmX₃ (X=Cl, Br, I) compounds. Here, we investigate the behavior of thulium trihalide materials as thulium's crystal field results in a spin system with the possible realization of unique magnetic models. Specifically, TmCl₃ and TmBr₃ both form 1D chains while TmI₃ realizes the 2D honeycomb structure of YbCl₃. Heat capacity measurements and magnetization indicate non-trivial magnetic behavior with a magnetic ordering of TmI₃ around 1.0 K under zero field. All compounds deviate from the Curie-Weiss law at low temperatures. Neutron diffraction data further elucidates this magnetic behavior and the role the crystal field plays on thulium as a non-Kramers ion. We will also discuss the role of thulium in the related rare-earth oxyhalide which hosts structures similar to YbOCl.