Multiple ferromagnetic transitions and structural distortion in the van der Waals ferromagnet VI3 at ambient and finite pressures

We present a combined study of zero-field V51 and I127 NMR at ambient pressure and specific heat and magnetization measurements under pressure up to 2.08 GPa on bulk single crystals of the van der Waals ferromagnet VI3. At ambient pressure, our results consistently demonstrate that VI3 undergoes a structural transition at Ts≈78K, followed by two subsequent ferromagnetic transitions at TFM1≈50K and TFM2≈36K upon cooling. At lowest temperature (T<TFM2), two magnetically ordered V sites exist, whereas only one magnetically ordered V site is observed for TFM1<T<TFM2. Whereas TFM1 is almost unaffected by external pressure, TFM2 is highly responsive to pressure and merges with the TFM1 line at p≈0.6GPa. At even higher pressures (p≈1.25GPa), the TFM2 line merges with the structural transition at Ts which becomes moderately suppressed with p for p<1.25GPa. Taken together, our data point toward a complex magnetic structure and an interesting interplay of magnetic and structural degrees of freedom in VI3.