Pressure-induced large increase of Curie temperature of the van der Waals ferromagnet V I 3

Evolution of magnetism in single crystals of the van der Waals compound $\\mathrm{V}{\\mathrm{I}}_{3}$ in external pressure up to 7.3 GPa studied by measuring magnetization and ac magnetic susceptibility is reported. Four magnetic phase transitions, at ${T}_{1}=54.5\\phantom{\\rule{0.16em}{0ex}}\\mathrm{K}, {T}_{2}=53\\phantom{\\rule{0.16em}{0ex}}\\mathrm{K}, {T}_{\\mathrm{C}}=49.5\\phantom{\\rule{0.16em}{0ex}}\\mathrm{K}$, and ${T}_{\\mathrm{FM}}=26\\phantom{\\rule{0.16em}{0ex}}\\mathrm{K}$, respectively, have been observed at ambient pressure. The first two have been attributed to the onset of ferromagnetism in specific crystal-surface layers. The bulk ferromagnetism is characterized by the magnetic ordering transition at Curie temperature ${T}_{\\mathrm{C}}$ and the transition between two different ferromagnetic phases ${T}_{\\mathrm{FM}}$, accompanied by a structure transition from monoclinic to triclinic symmetry upon cooling. The pressure effects on magnetic parameters were studied with three independent techniques. ${T}_{\\mathrm{C}}$ was found to be almost unaffected by pressures up to 0.6 GPa whereas ${T}_{\\mathrm{FM}}$ increases rapidly with increasing pressure and reaches ${T}_{\\mathrm{C}}$ at a triple point at \\ensuremath{\\approx} 0.85 GPa. At higher pressures, only one magnetic phase transition is observed moving to higher temperatures with increasing pressure to reach 99 K at 7.3 GPa. In contrast, the low-temperature bulk magnetization is significantly reduced by applying pressure (by more than 50% at 2.5 GPa) suggesting a possible pressure-induced reduction of vanadium magnetic moment. First-principles calculations of $\\mathrm{V}{\\mathrm{I}}_{3}$ under pressure allow us to ascribe the evolution of ${T}_{\\mathrm{C}}$ with pressure to the reduction of interplanar distance, including the observed slope change at 0.6 GPa. These calculations also describe the associated band gap closing, showing that with a modest compression the material would become metallic. Overall, the large pressure range covered corresponds to a significant change of interplanar interactions. The obtained data thus allow us to shed light on how does the transition between the three-dimensional (3D) and quasi-2D system affect magnetic interactions in the system.