Observing altermagnetism using polarized neutrons

Altermagnets are collinear compensated magnets whose magnetic symmetries at zero spin-orbit coupling break spin degeneracy leading to spin-split electronic and magnonic bands that reflect an underlying multipolar order. When there is an approximate $U(1)$ symmetry the magnons in altermagnets are split into equal and opposite chiral pairs. We show that in altermagnets polarized neutrons provide a means to detect the population of time-reversed domains and allow direct measurement of the magnon chirality anisotropy in momentum space---the central signature of the altermagnetic phase. We demonstrate this response to polarized neutrons in two candidate materials ${\mathrm{MnF}}_{2}$ and MnTe and show that the presence of these chiralities is stable to small perturbations that break spin-rotation symmetry. This provides a magnonic analog of spin-polarized ARPES that has been used to discern altermagnetism in the electronic band structures of various candidate materials.