Magnetic Correlation Spectroscopy in CrSBr

TL;DR

CrSBr's layer-structured magnetism couples strongly to excitons, enabling optical access to magnetic order. The authors demonstrate layer-by-layer switching between AFM and FM states in multilayer CrSBr using a three-axis vector magnet at cryogenic temperature, tracking correlated changes in PL and differential reflectance spectra. A transfer-matrix analysis reveals that FM and AFM order can coexist within the same crystal near saturation, and reveals distinct behavior of X_B and X_D excitonic transitions under field. The work provides a noninvasive spectroscopic route to map magnetization configurations in layered magnets and informs exciton–magnon interactions in van der Waals materials.

Abstract

CrSBr is an air-stable magnetic van der Waals semiconductor with strong magnetic anisotropy, where the interaction of excitons with the magnetic order enables the optical identification of different magnetic phases. Here, we study the magnetic anisotropy of multi-layer CrSBr inside a three-axis vector magnet and correlate magnetic order and optical transitions in emission and absorption. We identify layer by layer switching of the magnetization through drastic changes of the optical emission and absorption energy and strength as a function of the applied magnetic field. We correlate optical transitions in reflection spectra with photoluminescence (PL) emission using a transfer-matrix analysis and find that ferromagnetic and antiferromagnetic order between layers can coexist in the same crystal. In the multi-peak PL emission the intensity of energetically lower lying transitions reduces monotonously with increasing field strength whereas energetically higher lying transitions around the bright exciton $X_B$ brighten close to the saturation field. Using this contrasting behavior we can therefore correlate transitions with each other.

Figures (7)

• Figure 1: Experiments on CrSBr in three-axis vector magnet a: CrSBr crystal structure. Crystallographic axes a,b,c are indicated by arrows. b: Sketch of a CrSBr sample inside the vector magnet and the orientation of Laser and PL polarization with respect to crystallographic axes. c-e: Exemplary PL magnetic field sweeps of the encapsulated sample for the same spatial spot along the crystal a-,b- and c-axis respectively displaying the magnetic anisotropy, where B$_{a}$$||$ a, B$_{b}$$||$ b and B$_{c}$$||$ c. Dashed white arrows are a guide for the eye for evolution of $X_D$ (at 1.324 eV at B=0) and $X_B$ (at 1.362 eV at B=0) transitions.
• Figure 2: Photoluminescence and differential reflectance contrast of encapsulated 14 layer CrSBr at T = 4.7 K. a: PL spectra at B = 0 T with detection polarization aligned with the crystal b- (black solid line) and a-axis (green solid line). b: Full polarization dependence of the emission. c: Power dependence of the PL emission at B = 0 T. d: PL magnetic field sweeps along the crystal b-axis for an encapsulated (14 L) sample. Magnetic field step size is 10 mT, sweep direction from positive to negative fields. e-f: DR/R and PL spectra of the 14 layer sample for the AFM-state (black) and the FM-state (red). Emissions indicated as in main text. Dashed gray and red lines indicate the energy of X$_{B}$ in the AFM and FM state respectively.
• Figure 3: Hysteresis of PL intensities in magnetic fields along $B_b$. a-b: Maximum intensity plots of $X_{D}$ (a) and $X_{B}$ (b) for the encapsulated 14L sample for up-sweeping (down-sweeping) magnetic field in red (black).
• Figure 4: Correlated emissions in magnetic fields. a: PL emission of the encapsulated CrSBr flake for selected magnetic field strengths $B_{b}$ along the crystal b-axis and spectral range displaying similar and consistent reduction of emission intensity with magnetic field. b: same as (a), but for negative magnetic fields. Gray arrow is a guide for the eye. c-d: Maximum intensity of the emissions marked in (a) for the magnetic field sweep in Figure \ref{['fig:general_specs']} d. e-f: Maximum emission intensity in c-d normalized by the intensity of $X_{D}$. Pointed parts of the graphs indicate regions with large error bars.
• Figure 5: Anti-correlated emissions and superimposed magnetic phases. a: PL emission for selected magnetic field strengths and spectral range along the crystal b-axis showing unexpected brightening of emissions. b: same as (a), but for negative magnetic fields. Grey arrows are guides for the eye. c: Maximum intensity of the $X_{B}$ and $X_{D}$ emissions for magnetic field sweep in Figure \ref{['fig:general_specs']} d. X$_{D}$ (X$_{B}$) reduces in intensity by 9.5 $\pm$ 2 $\%$ (4.1 $\pm$ 4) $\%$ for $B_{b}$ > 0.18 T and by 12.3 $\pm$ 2 $\%$ (8.1 $\pm$ 4.2) $\%$ for $B_{b}$ < -0.2 T. d: Maximum intensity of the remaining emissions marked in (a). Pointed parts of the graphs indicate regions with large error bars. e: Transfer-matrix-analysis of DR/R measurements for selected magnetic fields. Dashed grey lines indicate the energies of the respective oscillators.