Magnetic Order and Its Interplay with Structure Phase Transition in van der Waals Ferromagnet VI_3

Van der Waals magnet VI_3 demonstrates intriguing magnetic properties that render it great for use in various applications. However, its microscopic magnetic structure has not been determined yet. Here, we report neutron diffraction and susceptibility measurements in VI_3 that revealed a ferromagnetic order with the moment direction tilted from the c-axis by \sim 36^\circ at 4 K. A spin reorientation accompanied by a structure distortion within the honeycomb plane is observed, before the magnetic order completely disappears at T_\rm C = 50 K. The refined magnetic moment of \sim 1.3 \mu_\scriptscriptstyle \rm B at 4 K is much lower than the fully ordered spin moment of 2\mu_\scriptscriptstyle \rm B/V^3+, suggesting the presence of a considerable orbital moment antiparallel to the spin moment and strong spin–orbit coupling in VI_3. This results in strong magnetoelastic interactions that make the magnetic properties of VI_3 easily tunable via strain and pressure.