Observing Altermagnetism using Polarized Neutrons

TL;DR

This paper proposes using polarized neutrons to detect and measure magnon chirality and domain populations in altermagnets, providing a new experimental approach to study their unique spin-split bands and multipolar order.

Contribution

It introduces a novel neutron scattering technique to directly observe magnon chirality and domain populations in altermagnets, expanding experimental tools for magnetic phase analysis.

Findings

Polarized neutrons can detect time-reversed domain populations.

Magnon chirality anisotropy can be measured in momentum space.

Chiral magnon features are stable under small symmetry-breaking perturbations.

Abstract

Altermagnets are colinear 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 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 analogue of spin polarized ARPES that has been used to discern altermagnetism in the electronic band structures of various candidate materials.