# Direct observation of two-dimensional magnons in atomically thin CrI3

## Abstract

Magnons are collective spin excitations in crystals with long-range magnetic order. The emergent van der Waals magnets1,2,3 provide a highly tunable platform to explore magnetic excitations in the two-dimensional limit with intriguing properties, manifesting from their honeycomb lattice structure and switchable magnetic configurations. Here, we report the direct observation of two-dimensional magnons through magneto-Raman spectroscopy with optical selection rules determined by the interplay between crystal symmetry, layer number and magnetic states in atomically thin CrI3. In monolayers, we observe an acoustic magnon mode at ~0.3 meV. It has strict cross-circularly polarized selection rules locked to the magnetization direction that originates from the conservation of angular momentum of photons and magnons dictated by three-fold rotational symmetry4. Additionally, we reveal optical magnon modes at ~17 meV. This mode is Raman silent in monolayers, but optically active in bilayers and bulk due to a relaxation of the parity criterion resulting from the layer index. In the layered antiferromagnetic states, we directly resolve two degenerate optical magnon modes with opposite angular momentum and conjugate optical selection rules. From these measurements, we quantitatively extract the spin-wave gap, magnetic anisotropy and intralayer and interlayer exchange constants, and establish two-dimensional magnets as a new platform for exploring magnon physics.

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## Data availability

The datasets generated during and/or analysed during this study are available from the corresponding author upon reasonable request.

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## Acknowledgements

We thank N. Wilson for helpful discussion. This work was mainly supported by the Department of Energy, Basic Energy Sciences, Materials Sciences and Engineering Division (DE-SC0012509). Device fabrication and understanding of magnon optical selection rules were partially supported by AFOSR MURI 2D MAGIC (FA9550-19-1-0390). Work at ORNL (M.A.M.) was supported by the US Department of Energy, Office of Science, Basic Energy Sciences, Materials Sciences and Engineering Division. K.W. and T.T. acknowledge support from the Elemental Strategy Initiative conducted by MEXT, Japan, and CREST (JPMJCR15F3), JST. B.H. acknowledges partial support from NW IMPACT. X.X. acknowledges support from the State of Washington funded Clean Energy Institute and from the Boeing Distinguished Professorship in Physics.

## Author information

Authors

### Contributions

X.X., J.C. and B.H. conceived the experiment. J.C. and B.H. fabricated and characterized the samples, assisted by A.M. and P.T. J.C. and B.H. performed the Raman and magnetic circular dichroism measurements, assisted by P.T. and T.S. J.C., B.H., N.S., D.X. and X.X. analysed and interpreted the results. T.T. and K.W. synthesized the hBN crystals. M.A.M. synthesized and characterized the bulk CrI3 crystals. J.C., B.H., X.X. and D.X. wrote the paper with input from all authors. All authors discussed the results.

### Corresponding authors

Correspondence to Di Xiao or Xiaodong Xu.

## Ethics declarations

### Competing interests

The authors declare no competing interests.

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## Supplementary information

### Supplementary Information

Supplementary Figs. 1–6 and Note 1.

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Cenker, J., Huang, B., Suri, N. et al. Direct observation of two-dimensional magnons in atomically thin CrI3. Nat. Phys. (2020). https://doi.org/10.1038/s41567-020-0999-1