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

Abstract

We present a combined study of zero-field 51V and 127I 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 𝑇FM⁢1≈50K and 𝑇FM⁢2≈36K upon cooling. At lowest temperature (𝑇<𝑇FM⁢2), two magnetically ordered V sites exist, whereas only one magnetically ordered V site is observed for 𝑇FM⁢1<𝑇<𝑇FM⁢2. Whereas 𝑇FM⁢1 is almost unaffected by external pressure, 𝑇FM⁢2 is highly responsive to pressure and merges with the 𝑇FM⁢1 line at 𝑝≈0.6GPa. At even higher pressures (𝑝≈1.25GPa), the 𝑇FM⁢2 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.