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 (CrI3) that has been known to be a layered antiferromagnet (AFM) in its 2D films and a ferromagnet (FM) in its three-dimensional (3D) bulk. We, however, reveal a novel mixed state of layered AFM and FM in 3D CrI3 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 C3i (rhombohedral) to C2h (monoclinic) and thus, from a symmetry perspective, this monoclinic structural phase belongs to the 3D nematic order universality class. Our result not only unveils the complex single magnon behavior in 3D CrI3, but also settles down the puzzle of how CrI3 transits from a bulk FM to a thin layered AFM semiconductor, despite recent efforts in understanding the origin of layered AFM in CrI3 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 up opportunities for future 2D magnet-based magneto-mechanical devices.

中文翻译：

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范德华磁体中磁场诱导的量子相变

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参数机制中的量子相变提供了充足的机会。在这里，我们使用声子和磁振子的磁场和温度相关的圆偏振拉曼光谱，绘制了三碘化铬（CrI3）的相图，该相在其二维薄膜和铁磁体（FM）中被称为分层反铁磁体（AFM）。 ）以其三维（3D）体。但是，我们 揭示了3D CrI3块状晶体中层状AFM和FM的新型混合状态，其中层状AFM保留在表层中，而FM出现在较深的块状层中。然后，我们表明表面分层的AFM在2 T的临界磁场下过渡到FM，类似于在少数情况下所发现的情况。有趣的是，与此磁性相变同时，我们发现了一阶结构相变，该结构将晶点组从C3i（菱面体）更改为C2h（单斜晶），因此，从对称的角度来看，此单斜晶相属于3D向列序通用性类。我们的结果不仅揭示了3D CrI3中复杂的单磁振子行为，而且还解决了CrI3如何从体FM过渡到薄层AFM半导体的难题，尽管最近在努力理解CrI3薄层中分层AFM的起源，但揭示了分层AFM到FM与晶体菱形到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。