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Spin-Forbidden Excitations in the Magneto-optical Spectra of CrI3 Tuned by Covalency

Connor A. Occhialini1,*,†, Luca Nessi1, Luiz G. P. Martins1,2, Ahmet Kemal Demir1, Qian Song1, Vicky Hasse3, Chandra Shekhar3, Claudia Felser3, Kenji Watanabe4 et al.

Takashi Taniguchi5, Valentina Bisogni6, Jonathan Pelliciari6, and Riccardo Comin1,‡

  • *Contact author: co2625@columbia.edu
  • †Present address: Department of Physics, Columbia University, New York, New York 10027, USA.
  • ‡Contact author: rcomin@mit.edu
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Phys. Rev. X 15, 031005 – Published 2 July, 2025

DOI: https://doi.org/10.1103/4km4-4rvn

Abstract

Spin-forbidden (ΔS≠0) multiplet excitations and their coupling to magnetic properties are of increasing importance for magneto-optical studies of correlated materials. Nonetheless, the mechanisms for optically brightening these transitions and their generality remain poorly understood. Here, we report magnetic circular dichroism (MCD) spectroscopy on the van der Waals ferromagnet (FM) CrI3. Previously unreported spin-forbidden (ΔS=1) A42g→E2g/T21g Cr3+ dd excitations are observed near the ligand-to-metal charge-transfer excitation threshold. The assignment of these excitations and their Cr3+ multiplet character is established through complementary Cr L3-edge resonant inelastic x-ray scattering measurements along with charge-transfer multiplet calculations and chemical trends in the chromium trihalide series (CrX3, X= Cl, Br, I). We utilize the high sensitivity of MCD spectroscopy to study the thickness-dependent optical response. The spin-forbidden excitations remain robust down to the monolayer limit, and we observe a significant magnetic-field dependence of the MCD spectrum across the antiferromagnetic-to-FM transition in few-layer samples. We preliminarily attribute this behavior to changes in the metal-ligand covalency with magnetic state. Our results clarify the magneto-optical response of CrI3 and identify covalency as a central mechanism for the brightening and field tunability of spin-forbidden multiplet excitations.

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Popular Summary

Light interacting with magnetism offers a powerful way to detect and control magnetic states, especially in ultrathin materials called van der Waals (vdW) magnets. These materials are so thin that traditional methods struggle to measure their magnetic properties. In insulating magnets, the interaction with visible light mainly involves excitations of magnetic ions—changes in their electronic states related to their spins. Although these excitations usually produce weak optical signals, finding ways to enhance them is important for better magnetic sensing. In this study, we discover unusually strong optical signals in the vdW ferromagnet CrI3, revealing new insights into its magnetic behavior.

To investigate this, we use two complementary techniques: magnetic circular dichroism (MCD) spectroscopy and resonant inelastic x-ray scattering (RIXS). MCD helps us identify electronic excitations linked to magnetism, while RIXS measures local excitations of chromium ions in the material. We find that strong optical signals come from spin-forbidden transitions—transitions between different spin states of Cr3+ ions that usually do not show up strongly in optical measurements. The surprising brightness of these signals is due to mixing, or hybridization, between the chromium ion orbitals and the surrounding iodine atoms.

Our findings provide clues on how to make these normally weak excitations more visible and how to adjust their energies by controlling this hybridization. Future work will focus on developing detailed theoretical models to explain these effects fully, helping to find or design new materials with bright, tunable optical signals for advanced magnetic sensing and device applications.

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Phys. Rev. X 15, 031005– Published 2 July, 2025
  • Received 28 January 2025
  • Revised 5 May 2025
  • Accepted 20 May 2025
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