Strong Spin-Lattice Interaction in Layered Antiferromagnetic CrCl$_\textrm{3}$

TL;DR

This work demonstrates strong spin–lattice coupling in CrCl$_3$ by combining polarization-resolved Raman spectroscopy with PL, PLE, and absorption measurements, supported by DFT phonon calculations. All eight Raman-active phonons ($4\,A_g$ and $4\,E_g$) are experimentally assigned, confirming theoretical predictions and revealing interference-dominated excitation-energy effects rather than purely resonant Raman processes. Temperature-dependent Raman data uncover pronounced spin–phonon coupling across a sequence of magnetic regimes, from fully antiferromagnetic to a domain-like ferromagnetic state, and document a rhombohedral-to-monoclinic structural transition. The results highlight the intricate coupling among lattice, electronic, and magnetic degrees of freedom in CrCl$_3$ and establish Raman spectroscopy as a powerful probe of spin–lattice interactions in layered vdW magnets.

Abstract

Understanding the coupling between lattice vibrations and magnetic order is crucial for controlling properties of two-dimensional magnetic materials. Here, we investigate the vibrational properties of bulk and thick-flake CrCl$_\textrm{3}$ using polarization-resolved Raman spectroscopy, complemented by photoluminescence, photoluminescence excitation, and optical absorption measurements. Symmetry analysis, supported by first-principles phonon calculations, enables the unambiguous assignment of all eight Raman-active modes, four $\textrm{A}_\textrm{g}$ and four $\textrm{E}_\textrm{g}$, previously predicted only theoretically. Excitation-energy-dependent measurements reveal that the strong enhancement of selected phonon modes originates primarily from interference effects rather than resonant Raman scattering. Temperature-dependent Raman spectroscopy further reveals pronounced signatures of spin-phonon coupling across the transition from a fully antiferromagnetic phase, through an intermediate regime with local, domain-like ferromagnetic order, to the paramagnetic phase, accompanied by a clear rhombohedral-to-monoclinic structural transition. Together, these results demonstrate how lattice, electronic, and magnetic degrees of freedom collectively govern the Raman response of CrCl$_\textrm{3}$.

Figures (12)

• Figure 1: Schematic representation of the rhombohedral crystal structure of CrCl$_\textrm{3}$. (a) Top view of a monolayer, with the unit cell indicated by a black rectangle and shown in a simplified form for clarity. (b) Perspective view of the bulk structure, illustrating the stacking of CrCl$_\textrm{3}$ layers along the c-axis and the arrangement of multiple layers within the unit cell. (c) Phonon dispersion of bulk CrCl$_\textrm{3}$ calculated for the rhombohedral primitive cell with ferromagnetic (bluFe) and antiferromagnetic (red) spin configurations. (d) and (e) Low-temperature ($T$=5 K) Raman scattering spectra of bulk CrCl$_\textrm{3}$ measured in co-linear and cross-linear polarization configurations using 3.06 eV and 1.96 eV laser excitation, respectively, with a laser power of 75 $\mu$W. Panel (e) is limited to two $\textrm{A}_\textrm{g}$ modes exhibiting doublet structures.
• Figure 2: Resonant Raman scattering investigation in the CrCl$_\textrm{3}$ crystal. (a) Raman scattering spectra of the CrCl$_\textrm{3}$ crystal measured at 5 K using different excitation energies: 1.96 eV, 2.21 eV, 2.41 eV, 2.54 eV, 3.06 eV, with an excitation power of 50 $\mu$W. The spectra have been vertically shifted for better visual clarity. (b) Left axis: graph shows points representing the intensity of individual Raman modes present in every spectra collected with different excitation lasers (in logarithmic scale). Right axis: normalized PL (black), PLE (grey) and Abs (blue) spectra measured on the CrCl$_{3}$ crystal. The PL spectrum was measured with 2.21 eV laser with power around 0.5 $\mu$W. The PLE spectrum was detected via supercontinuum laser with power around 2 $\mu$W. (c) Simulated enhancement of the A$^{3/4}_\mathrm{g}$ intensity using the transfer-matrix method. The colored vertical dashed lines indicate the excitation energies used in the experiment.
• Figure 3: Temperature evolution of the Raman modes. (a) The top panel shows a false-color map of the Raman spectra of a CrCl$_\textrm{3}$ crystal, while the bottom panel presents the Raman spectrum measured at $T$=5 K. The intensities are plotted on a logarithmic scale to better visualize the temperature dependence of the Raman shifts for all observed modes. (b)–(g) Temperature dependence of the Raman shift for all observed phonon modes. Solid lines represent fits using Eq. \ref{['eq;balkanski']}. The insets show the low-temperature range of the frequency evolution of the modes, as indicated by the gray rectangular regions.
• Figure S1: (a) Optical image of the investigated CrCl$_\textrm{3}$ crystal. The green rectangle indicates the scanning area profiled by the stylus (center of rectangle), while the red square marks the spot selected for Raman measurements. (b) Topographical profile corresponding to the region marked in (a). The green curve shows the height variation across the sample. The dashed red line indicates the thickness at the Raman measurement spot (marked by the red square), which is 92.0 $\mu$m. Note that the distance axis has been offset to align the scan range with the optical image's field of view.
• Figure S2: (a) Optical image of the investigated CrCl$_\textrm{3}$ flake. The red square indicates the probed area. (b) Atomic force microscopy (AFM) topography of the investigated flake. The red line (A--B) marks the location of the cross-section used for thickness determination. (c) Height profile extracted along the line (A-B) revealing a flake thickness of 227.35$\pm$0.63 nm.