Ligand-field helical luminescence in a 2D ferromagnetic insulator

Abstract: Bulk chromium triiodide (CrI$_3$) has long been known as a layered van der Waals ferromagnet. However, its monolayer form was only recently isolated and confirmed to be a truly two-dimensional (2D) ferromagnet, providing a new platform for investigating light-matter interactions and magneto-optical phenomena in the atomically thin limit. Here, we report spontaneous circularly polarized photoluminescence in monolayer CrI$_3$ under linearly polarized excitation, with helicity determined by the monolayer magnetization direction. In contrast, the bilayer CrI$_3$ photoluminescence exhibits vanishing circular polarization, supporting the recently uncovered anomalous antiferromagnetic interlayer coupling in CrI$_3$ bilayers. Distinct from the Wannier-Mott excitons that dominate the optical response in well-known 2D van der Waals semiconductors, our absorption and layer-dependent photoluminescence measurements reveal the importance of ligand-field and charge-transfer transitions to the optoelectronic response of atomically thin CrI$_3$. We attribute the photoluminescence to a parity-forbidden d-d transition characteristic of Cr$^{3+}$ complexes, which displays broad linewidth due to strong vibronic coupling and thickness-independent peak energy due to its localized molecular orbital nature.

• The paper demonstrates that circular photoluminescence in CrI₃ emerges below the Curie temperature, directly linking light emission to intrinsic ferromagnetism.
• It identifies distinct ligand-field and charge-transfer transitions that deviate from typical excitonic behavior in 2D semiconductors.
• The observed hysteresis in photoluminescence under varying magnetic fields offers new insights for developing spintronic and magnetic sensing devices.

Overview of Ligand-Field Helical Luminescence in a 2D Ferromagnetic Insulator

The research presented investigates the optoelectronic properties of monolayer chromium triiodide (CrI₃), a 2D ferromagnetic insulator. As a continuation of work exploring magneto-optical phenomena in atomically thin materials, this study reveals significant findings regarding light-matter interactions in this crystalline compound. The paper contributes to the understanding of ligand-field and charge-transfer transitions which diverge from the typical excitonic behavior observed in similar 2D van der Waals semiconductors.

The primary focus of the paper is on spontaneous circularly polarized photoluminescence (PL) in monolayer CrI₃ under linearly polarized excitation, indicating a linkage between PL helicity and the magnetization direction of the monolayer. The work provides evidence of an antiferromagnetic interlayer coupling in bilayer CrI₃, contrasting with the ferromagnetic order in single layers. These findings are supported by structural and optical analysis, reinforcing prior theoretical predictions about ligand-field effects in Cr-based materials.

Key Findings

1. Photoluminescence Dynamics: Upon cooling CrI₃ monolayers below their Curie temperature, the study demonstrates a shift from unpolarized to polar-dependent emission, influenced by intrinsic ferromagnetism. The correlation of PL helicity with magnetic order is particularly notable in the observed hysteresis loop under varying magnetic fields.
2. Ligand-Field and Charge-Transfer Transitions: The optical absorption features closely align with ligand-field transitions, diverging from the typical band-to-band or excitonic transitions observed in other 2D materials. This highlights the distinct molecular orbital nature of CrI₃, emphasized by localized d-d transitions attributable to Cr³⁺ complexes.
3. Hysteresis in Magneto-Optical Behavior: Experimental hysteresis in PL polarization as a function of applied magnetic field amplitude solidifies the claim of ferromagnetic order influencing light emission properties. This hysteresis underscores potential uses in data storage or magnetic sensing applications where switching behavior is crucial.

Implications and Future Research

This study opens pathways for integrating 2D ferromagnetic insulators like CrI₃ into advanced magneto-optic devices. The demarcation between ligand-field transitions and traditional excitonic effects marks an exciting opportunity to investigate novel photophysical phenomena inherent to quantum materials. Future research is invited to explore the tunable nature of these magnetic and optical parameters, possibly examining heterostructures that couple ferromagnetic layers with diverse 2D semiconductors.

Given this research's implications in spintronics, the development of future devices capitalizing on this unique luminescence could revolutionize information technologies with applications in optical communication, low-temperature spintronic devices, and sensors. Long-term quantification and understanding of the interlayer magnetic coupling in bilayers and further elucidation of the PL mechanisms are projected as fundamental endeavors that could expand our mastery over 2D materials and ferromagnetism at the atomic scale.