Spin-forbidden excitations in the magneto-optical spectra of CrI$_3$ tuned by covalency

TL;DR

This study uncovers spin-forbidden excitations in CrI$_3$ using magneto-optical spectroscopy, revealing covalency as key to their brightness and tunability, with implications for magnetic and optical properties in 2D materials.

Contribution

It demonstrates the presence of spin-forbidden excitations in CrI$_3$ and links their optical brightness and tunability to covalency effects, supported by experimental and theoretical analysis.

Findings

Spin-forbidden excitations are observed near the charge transfer threshold.

These excitations are robust down to monolayer thickness.

Magnetic field can tune these excitations across magnetic phase transitions.

Abstract

Spin-forbidden ($\Delta S \neq 0$) 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 (vdW) ferromagnet (FM) CrI$_3$. Previously unreported spin-forbidden ($\Delta S = 1$) ${}^4A_{2\mathrm{g}} \to{}^2E_\mathrm{g}/{}^2T_{1\mathrm{g}}$ Cr${}^{3+}$ $dd$ excitations are observed near the ligand-to-metal charge transfer (LMCT) excitation threshold. The assignment of these excitations and their Cr$^{3+}$ multiplet character is established through complementary Cr $L_3$-edge resonant inelastic X-ray scattering (RIXS) measurements along with charge transfer multiplet (CTM) calculations and chemical trends in the chromium trihalide series (CrX$_3$, 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 exhibit a significant magnetic field tunability across the antiferromagnetic to FM transition in few-layer samples. This behavior is associated to changes in the metal-ligand covalency with magnetic state, as supported by our CTM analysis. Our results clarify the magneto-optical response of CrI$_3$ and identify covalency as a central mechanism for the brightening and field-tunability of spin-forbidden multiplet excitations.