Intertwined Magnetic Orders and Structural Phase Transitions in Van der Waals Magnets VBr₃ and VI₃

Vanadium trihalides are van der Waals magnets that exhibit intriguing two-dimensional magnetism on the honeycomb lattice. Here, we present neutron diffraction studies on the crystal and magnetic structures of VBr₃ [1] and VI₃ [2] using polycrystalline and single-crystal samples. Both compounds show the structural phase transition that breaks the threefold symmetry before entering their respective magnetic ordering phases, indicating strong magnetoelastic interactions. VBr₃ exhibits antiferromagnetic ordering with a magnetic propagation vector of k = (0.5, 0, 1). Our analysis indicates a zigzag magnetic structure. VI₃ is ferromagnetic with its magnetic moment tilted approximately 35° from the c-axis. The magnetic order undergoes a spin reorientation accompanied by structural distortion within the honeycomb plane. Both materials exhibit strong spin-orbit couplings hinting the potential presence of the Kitaev interactions.