Systematic Mapping of Altermagnetic Magnons by Resonant Inelastic X-Ray Circular Dichroism

TL;DR

The paper demonstrates a domain-resolved approach to probe altermagnetic magnons in CrSb using resonant inelastic X-ray scattering with circular polarization and azimuthal scanning. It reveals pronounced circular dichroism in magnon peaks and an azimuthal dependence consistent with the predicted $E_\pm(\phi)=E_0\pm\Delta\cos(3\phi)$ splitting characteristic of altermagnetic chirality, though the spectral split is not resolved due to the instrumental resolution of $32.5\ \mathrm{meV}$. The study combines Cr L$_3$-edge RIXS data with DFT and linear spin-wave theory to map magnon dispersions up to $\sim$ $0.14\ \mathrm{eV}$ and shows opposite dichroic signs in different AFM domains, validating a robust protocol for investigating altermagnetic magnons. This work advances altermagnetic magnonics by providing a practical, domain-specific spectroscopic tool that could inform low-power spintronic and magnonic device applications.

Abstract

Altermagnets, a unique class of magnetic materials that combines features of both ferromagnets and antiferromagnets, have garnered attention for their potential in spintronics and magnonics. While the electronic properties of altermagnets have been well studied, characterizing their magnon excitations is essential for fully understanding their behavior and enabling practical device applications. In this work, we introduce a measurement protocol combining resonant inelastic X-ray scattering with circular polarization and azimuthal scanning to probe the chiral nature of the altermagnetic split magnon modes in CrSb. This approach circumvents the challenges posed by domain averaging in macroscopic samples, allowing for precise measurements of the polarization and energy of the magnons in individual antiferromagnetic domains. Our findings demonstrate a pronounced circular dichroism in the magnon peaks, with an azimuthal dependence that is consistent with the theoretical predictions and the $g$-wave symmetry. By establishing a reliable and accessible method for probing altermagnetic magnons, this work opens new avenues for fundamental studies of these collective excitations and for developing next-generation magnonic device applications.

Figures (3)

• Figure 1: Theoretical and experimental characterization of CrSb.a Crystal and magnetic structure of CrSb, indicating the nearest-neighbor intra- and intersublattice exchange couplings, $J_1^\mathrm{AA}$ and $J_1^\mathrm{AB}$, respectively, and the coupling responsible for the altermagnetic magnon splitting, $J_7^\mathrm{AA}$. b Computed magnon bands using the magnetic exchange interactions extracted from the DFT calculations at the theoretical Fermi energy and for a slightly smaller value. Previous INS measurements Radhakrishna1996 and our own RIXS measurements are also shown. c Scattering geometry for the RIXS experiment. The azimuthal angle $\phi$ is defined to be zero when the scattering plane contains the $a^*$-axis and $\mathbf{Q}\cdot\mathbf{a}^* > 0$. d - e RIXS spectra showing magnon and $d$-$d$ excitations at two different incident X-ray energies with $\mathbf{Q} = (0.27, 0, 0.25)$ [r.l.u.]. Inset in d: zoom into the low-energy region showing the loss feature due to magnon excitation. f Stack plot of representative RIXS spectra showing the evolution of the magnon peaks along the $\Gamma$ -- A direction.
• Figure 2: Dichroism in CrSb.a X-ray absorption and dichroic spectra from total fluorescence yield measurements using incident X-rays with circular-right (CR) and circular left (CL) polarization at normal incidence. The vertical dashed lines indicate the selected photon energies for RIXS. b RIXS spectra tuned to a magnon excitation with wave vector along the high-symmetry crystal axis showing no dependence on the circular polarization. c RIXS spectra tuned to the $d$-$d$ excitations showing no dependence on the circular polarization. d-e RIXS spectra tuned to a magnon excitation with wave vectors with opposite in-plane components show opposite dependence on the circular polarization. f Comparison between the theoretical azimuthal dependence of the two altermagnetic magnon bands, $E_\pm = E_0 \pm \Delta\cos(3\phi)$, and the magnon excitation energies from the RIXS spectra shown in d-e. In all panels error bars are standard deviations.
• Figure 3: Azimuthal dependence of the RIXS magnon dichroism in CrSb.a Comparison between RIXS spectra obtained with circularly polarized X-rays in two different AFM domains. Spectra were collected using circular right (CR) and circular left (CL) polarizations, with their intensity difference defining the absolute dichroism, $I_\mathrm{R} - I_\mathrm{L}$. b Azimuthal dependence of the relative circular dichroism, $R_\mathrm{CD} = 100 \times (I_\mathrm{R} - I_\mathrm{L})/(I_\mathrm{R} + I_\mathrm{L})$, at the magnon excitation peak for the two opposite antiferromagnetic domains, and fits to the predicted $\cos(3\phi)$ dependence. In all panels error bars are standard deviations.