Magnetic ordering in VI3: a van der Waals material combining vastlydifferent magnetic anisotropies.

Among magnetic van der Waals materials, the vanadium trihalide family exhibits unique features. In particular, VI3 can contain two types of V atoms because two electronic occupations are energetically close and may coexist in real samples. These types have strikingly different magnetic anisotropies, predicted to differ by more than an order of magnitude. VI3 also shows an unusual thickness dependence: the reported monolayer Curie temperature, TC, is higher than that of the bulk, contrary to the usual expectation that interlayer coupling reinforces magnetic order.Here, we use atomistic spin-dynamics simulations to study critical temperatures in VI3 from the combined perspective of single-ion anisotropy and exchange interactions, aiming to identify the microscopic origin of the bulk-monolayer anomaly. We consider a system composed of two V types with properties predicted by first-principles calculations. The anisotropy contrast strongly affects thermal stability: increasing the fraction of high-anisotropy sites raises the energy cost of transverse spin fluctuations and increases TC. At the same time, the interlayer super-superexchange network is modified by the inhomogeneous V environment. It becomes spatially nonuniform and can contain competing exchange pathways, weakening coherent interlayer magnetic order while preserving robust intralayer ferromagnetic correlations.Our results show that changing the ratio of the two V types can cause a substantial shift in TC. The experimental bulk value is reproduced when the ratio is close to 1:1, consistent with two experimental indications of coexisting V configurations. Thus, the finite-temperature magnetization behavior provides further support for the coexistence of two V configurations in VI3. The experimentally observed monolayer TC is obtained for a slightly modified ratio, which may be related to the polaron concentration. The sensitivity of TC to the HO/LO ratio suggests that the ordering temperature of VI3 could be tuned over a broad range by controlling the relative occupation of the two vanadium configurations.