Exploring new parameter regimes to realize and control novel phases of matter has been a main theme in modern condensed matter physics research. The recent discovery of two-dimensional (2D) magnetism in nearly freestanding monolayer atomic crystals has already led to observations of a number of novel magnetic phenomena absent in bulk counterparts. Such intricate interplays between magnetism and crystalline structures provide ample opportunities for exploring quantum phase transitions in this new 2D parameter regime. Here, using magnetic field- and temperature-dependent circularly polarized Raman spectroscopy of phonons and magnons, we map out the phase diagram of chromium triiodide (${\mathrm{CrI}}_3$) that has been known to be a layered antiferromagnet (AFM) in its 2D films and a ferromagnet (FM) in its three-dimensional (3D) bulk. However, we reveal a novel mixed state of layered AFM and FM in 3D ${\mathrm{CrI}}_3$ bulk crystals where the layered AFM survives in the surface layers, and the FM appears in deeper bulk layers. We then show that the surface-layered AFM transits into the FM at a critical magnetic field of 2 T, similar to what was found in the few-layer case. Interestingly, concurrent with this magnetic phase transition, we discover a first-order structural phase transition that alters the crystallographic point group from $C_{3i}$ (rhombohedral) to $C_{2h}$ (monoclinic). Our result not only unveils the complex single-magnon behavior in 3D ${\mathrm{CrI}}_3$, but it also settles the puzzle of how ${\mathrm{CrI}}_3$ transits from a bulk FM to a thin-layered AFM semiconductor, despite recent efforts in understanding the origin of layered AFM in ${\mathrm{CrI}}_3$ thin layers, and reveals the intimate relationship between the layered AFM-to-FM and the crystalline rhombohedral-to-monoclinic phase transitions. These findings further open opportunities for future 2D magnet-based magnetomechanical devices.

中文翻译：

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范德华磁铁中的磁场诱导量子相变

探索新的参数体系以实现和控制新的物质相已成为现代凝聚态物理研究的主要主题。最近在几乎独立的单层原子晶体中发现二维（2D）磁性的事实已经导致人们观察到了大量对应物中不存在的许多新颖的磁性现象。磁性和晶体结构之间如此复杂的相互作用为探索这种新的2D参数方案中的量子相变提供了充足的机会。在这里，使用声子和磁振子的磁场和温度相关的圆偏振拉曼光谱，我们绘制了三碘化铬的相图（${铬}_3$）在2D薄膜中是分层的反铁磁体（AFM），在三维（3D）体积中是铁磁体（FM）。但是，我们揭示了3D分层AFM和FM的新颖混合状态${铬}_3$层状AFM保留在表层中的块状晶体，而FM出现在较深的块状层中。然后，我们表明表面层AFM在2 T的临界磁场下过渡到FM，这与在多层情况下发现的情况相似。有趣的是，与此磁性相变同时发生，我们发现了一阶结构相变，该相变改变了$C_{3\mathrm{一世}}$ （菱形） $C_{2H}$（单斜）。我们的结果不仅揭示了3D中复杂的单磁振子行为${铬}_3$，但这也解决了如何 ${铬}_3$ 尽管最近在理解AFM的分层AFM的起源方面做出了努力，但从体FM过渡到了薄层AFM半导体。 ${铬}_3$薄层，并揭示了层状AFM-FM与结晶的菱形-单斜晶相变之间的密切关系。这些发现进一步为未来基于2D磁体的磁机械设备打开了机会。