Résumé :
Altermagnets constitute a recently identified class of collinear, magnetically compensated materials in which oppositely oriented magnetic sublattices cannot be mapped into one another by a primitive lattice translation or inversion operation. These distinctive symmetry properties give rise to spin-split electronic bands and chirality-split magnon branches, both displaying characteristic anisotropic patterns in momentum space, even in the absence of relativistic spin-orbit coupling [1].
Experimental evidence for spin-split electronic bands has been reported in materials such as MnTe and CrSb using angle-resolved photoemission spectroscopy (ARPES) [2,3], while resonant inelastic x-ray scattering (RIXS) has revealed chiral magnon excitations in CrSb [4]. Polarized neutron scattering, however, provides a uniquely powerful probe of altermagnetism, as it enables a direct characterization of spin-wave excitations and allows both magnon-band splitting [5-7] and their associated chirality [8-10] to be measured simultaneously.
In this work, we investigated the chirality-split magnon spectrum of MnF₂ [9] using polarized inelastic neutron scattering. Our measurements reveal, for the first time in MnF₂, a small but clearly resolvable splitting of the magnon branches, primarily driven by long-range dipolar interactions. Furthermore, polarization analysis performed on a magnetically domain-biased sample uncovers a finite chiral contribution to the neutron scattering cross section, which reverses sign between the two split magnon modes. These observations provide direct spectroscopic evidence of altermagnetism in MnF₂.
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