Charge-driven first-order magnetic transition in NiPS₃

Cross-coupling among the fundamental degrees of freedom in solids has been a long-standing problem in condensed matter physics. Despite its progress using predominantly three-dimensional materials, how the same physics plays out for two-dimensional materials is unknown. Here, we show that using³¹P nuclear magnetic resonance (NMR), the van der Waals antiferromagnet NiPS₃undergoes a first-order magnetic phase transition due to the strong charge-spin coupling in a honeycomb lattice. Our³¹P NMR spectrum near the N´eel ordering temperature T_N= 155 K exhibits the coexistence of paramagnetic and antiferromagnetic phases within a finite temperature range. Furthermore, we observed a discontinuity in the order parameter at T_Nand the complete absence of critical behavior of spin fluctuations above T_N, decisively establishing the first-order nature of the magnetic transition. We propose that a charge stripe instability arising from a Zhang-Rice triplet ground state triggers the first-order magnetic transition.