The two-dimensional (2D) ferromagnetic materials and the related van der Waals homostructures have attracted considerable interest, while the 2D antiferromagnetic material has not yet been reported. Based on first-principles calculations, we investigate both electronic structures and magnetic orderings of bulk and monolayer of chromium diiodides (CrI₂). We demonstrate a counter-intuitive fact that the ground state of the free-standing monolayer of CrI₂ is antiferromagnetic though the bulk possesses macroscopic ferromagnetic ordering. The interlayer interaction remains antiferromagnetic up to few-layer scenarios. The unique feature of CrI₂ makes it an ideal workbench to investigate the relation between magnetic couplings and interlayer van der Waals interactions, and may offer an opportunity to 2D antiferromagnetic spintronic devices.

二维（2D）铁磁材料和相关的范德华同质结构引起了相当大的兴趣，而二维反铁磁材料尚未见报道。基于第一性原理计算，我们研究了二碘化铬（CrI ₂）块体和单层的电子结构和磁序。我们证明了一个违反直觉的事实，即 CrI ₂的独立单层的基态是反铁磁的，尽管块体具有宏观的铁磁有序性。层间相互作用在几层情况下仍然是反铁磁的。_{CrI 2}的独特之处使其成为研究磁耦合和层间范德华相互作用之间关系的理想工作台，并可能为二维反铁磁自旋电子器件提供机会。