Abstract
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. However, we 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). Our result not only unveils the complex single-magnon behavior in 3D CrI3, but it also settles 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 opportunities for future 2D magnet-based magnetomechanical devices.Received 17 January 2020Revised 24 February 2020Accepted 27 February 2020DOI:https://doi.org/10.1103/PhysRevX.10.011075Published by the American Physical Society under the terms of the Creative Commons Attribution 4.0 International license. Further distribution of this work must maintain attribution to the author(s) and the published article’s title, journal citation, and DOI.Published by the American Physical SocietyPhysics Subject Headings (PhySH)Research AreasMagnetic orderPhase transitions by orderQuantum phase transitionsSpin dynamicsCondensed Matter, Materials & Applied Physics
Highlights
Our study here approaches the nature of CrI3 magnetism through exploring the magnetic field dependence of spin-wave excitations in CrI3 and interrogating the interplay between the magnetic order and the crystalline structure
In its thin-layer form, the CrI3 magnetism is known to survive below TAFM 1⁄4 45 K; in a unique layered AFM order in which spins align along the same out-of-plane direction within each layer and alternate to the opposite orientation between adjacent layers [3,4,5,6], while its crystal structure is much less studied experimentally
To provide a comprehensive picture of the magnetic field dependence of M0, we summarize in Fig. 2(a) the key experimental result of the M0 frequencies shifting as a function of the external magnetic field and the corresponding field-dependent spin-wave calculations in the left and right panels, respectively
Summary
Chromium triiodide (CrI3) stands out among the large family of van der Waals (VDW) ferromagnets (FMs) investigated so far, because its isolated atomic crystal is one of the first two to realize two-dimensional (2D) ferromagnetic long-range order [1,2] in the monolayer limit, and more importantly, it is the first-ever interlayer. In its thin-layer form, the CrI3 magnetism is known to survive below TAFM 1⁄4 45 K; in a unique layered AFM order in which spins align along the same out-of-plane direction within each layer and alternate to the opposite orientation between adjacent layers [3,4,5,6], while its crystal structure is much less studied experimentally The origin of this crossover from the FM in bulk to the layered AFM in thin films, as well as the role of the crystallographic structure in this transition, remains as outstanding open question in the field of 2D magnetism
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