Multiple ferromagnetic transitions and structural distortion in the van-der-Waals ferromagnet VI${}_3$ at ambient and finite pressures

Abstract

We present a combined study of zero-field ${}^{51}$V and ${}^{127}$I 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 VI${}_3$. At ambient pressure, our results consistently demonstrate that VI${}_3$ undergoes a structural transition at $T_s\approx$78 K, followed by two subsequent ferromagnetic transitions at $T_{FM1}\approx$50 K and $T_{FM2}\approx$36 K upon cooling. At lowest temperature ($T<T_{FM2}$), two magnetically-ordered V sites exist, whereas only one magnetically-ordered V site is observed for $T_{FM1}<T<T_{FM2}$. Whereas $T_{FM1}$ is almost unaffected by external pressure, $T_{FM2}$ is highly responsive to pressure and merges with the $T_{FM1}$ line at $p\approx 0.6$GPa. At even higher pressures ($p\approx$1.25\,GPa), the $T_{FM2}$ line merges with the structural transition at $T_s$ which becomes moderately suppressed with $p$ for $p<1.25$ GPa. Taken together, our data point towards a complex magnetic structure and an interesting interplay of magnetic and structural degrees of freedom in VI${}_3$.

14 pages, 9 figures (+1 figure in Appendix)