Tunable Magnetic Transition to a Singlet Ground State in a 2D van der Waals Layered Trimerized Kagomé Magnet

Incorporating magnetism
into two-dimensional (2D) van der Waals
(vdW) heterostructures is crucial for the development of functional
electronic and magnetic devices. Here, we show that Nb3X8 (X = Cl, Br) is a family of 2D layered trimerized kagomé
magnets that are paramagnetic at high temperatures and undergo a first-order
phase transition on cooling to a singlet magnetic state. X-ray diffraction
shows that a rearrangement of the vdW stacking accompanies the magnetic
transition, with high- and low-temperature phases consistent with
scanning transmission electron microscopy images of the end members
α-Nb3Cl8 and β-Nb3Br8. The temperature of this transition is systematically varied
across the solid solution Nb3Cl8–xBrx (x = 0–8),
with x = 6 having transitions near room temperature.
The solid solution also varies the optical properties, which are further
modulated by the phase transition. As such, they provide a platform
on which to understand and exploit the interplay between dimensionality,
magnetism, and optoelectronic behavior in vdW materials.