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 () 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 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 (rhombohedral) to (monoclinic). Our result not only unveils the complex single-magnon behavior in 3D , but it also settles the puzzle of how transits from a bulk FM to a thin-layered AFM semiconductor, despite recent efforts in understanding the origin of layered AFM in 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 2020
- Revised 24 February 2020
- Accepted 27 February 2020
DOI:https://doi.org/10.1103/PhysRevX.10.011075
Published 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 Society
Physics Subject Headings (PhySH)
synopsis
Solving a Magnetic Puzzle
Published 31 March 2020
Spectroscopic measurements explain why a van der Waals ferromagnet displays different magnetic behavior in its layered and bulk forms.
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Popular Summary
The recent discovery of 2D magnetism in freestanding monolayer atomic crystals has led to observations of a number of novel magnetic phenomena absent in 3D counterparts. Such intricate interplays between magnetism and crystalline structures provide ample opportunities for exploring quantum phase transitions in two dimensions. To further those goals, we map out the phase diagram of chromium triiodide (), an extraordinary material known to be a layer antiferromagnet in 2D films and a ferromagnet in its 3D form. We find a novel mixed state of layer antiferromagnetism and ferromagnetism along with several intriguing phase transitions, all of which shed light on the unusual properties of this material.
Our team used polarized Raman spectroscopy to probe the collective excitations of both spin precessions (magnons) and lattice vibrations (phonons) in a sample of . By tracking the evolution of magnons and phonons as functions of temperature and magnetic field, we map out the phase diagram.
Among several findings, we find a novel mixed state where the layered antiferromagnetism survives in the surface layers and the ferromagnetism appears in deeper layers. We also find that the surface layer creeps into the interior at a critical magnetic field of 2 T. This magnetic phase transition coincides with a structural phase transition that alters the material’s crystalline architecture.
Our results address the crossover from 3D ferromagnetism to 2D layered antiferromagnetism and show the intimate relationship between the spin and lattice degrees of freedom in . Using as an example, we believe that our work represents a major milestone in the rapidly developing field of correlated physics in 2D atomic crystals.