Antiferromagnetic-ferromagnetic homostructures with Dirac magnons in the van der Waals magnet CrI3

The Dirac magnon system ${\mathrm{CrI}}_{3}$ with a honeycomb lattice is a potential host of topological edge magnons. It ideally orders ferromagnetically (FM) $({T}_{c}=61 \mathrm{K})$ on cooling from a monoclinic (M) to a rhombohedral (R) phase, but antiferromagnetic (AFM) order has been detected in nanometer thin flakes, attributed to M-type layer stacking. There remains confusion, however, as to the extent to which such behavior is present in bulk samples. Using a powder sample in which the sliding transition to the R phase was largely inhibited (2:1 M:R ratio), clear evidence for M-type AFM order $({T}_{N}\ensuremath{\sim}50 \mathrm{K})$ coexisting with R-type FM order is observed in the bulk. From inelastic neutron scattering, a lower magnon energy is observed compared to the R phase, consistent with smaller interlayer interactions expected in the M phase. While a gap at the Dirac points has been reported in the R phase, the gap is clearly observed even when the majority is M type, as in our sample, suggesting that the same nontrivial magnon topology of the R phase is present in the M phase as well.