Van der Waals (vdW) magnetic materials are comprised of layers of atomically thin sheets, making them ideal platforms for studying magnetism at the two-dimensional (2D) limit. These materials are at the center of a host of novel types of experiments, however, there are notably few pathways to directly probe their magnetic structure. We confirm the magnetic order within a single crystal of NiPS₃ and show it can be accessed with resonant elastic X-ray diffraction along the edge of the vdW planes in a carefully grown crystal by detecting structurally forbidden resonant magnetic X-ray scattering. We find the magnetic order parameter has a critical exponent of β ~ 0.36, indicating that the magnetism of these vdW crystals is more adequately characterized by the three-dimensional (3D) Heisenberg universality class. We verify these findings with first-principles density functional theory, Monte-Carlo simulations, and density matrix renormalization group calculations.

范德华 （vdW） 磁性材料由原子薄片层组成，使其成为研究二维 （2D） 极限磁性的理想平台。这些材料是许多新型实验的核心，然而，直接探测其磁性结构的途径明显很少。我们确认了 NiPS₃ 单个晶体内的磁序，并表明通过检测结构上禁止的谐振磁 X 射线散射，可以通过沿着精心生长的晶体中 vdW 平面边缘的共振弹性 X 射线衍射来获得它。我们发现磁序参数的临界指数为 β ~ 0.36，这表明这些 vdW 晶体的磁性更充分地用三维 （3D） 海森堡普遍性类来表征。我们用第一性原理密度泛函理论、蒙特卡洛模拟和密度矩阵重整化组计算来验证这些发现。