Antiferromagnetic-ferromagnetic homostructures with Dirac magnons in van der Waals magnet CrI$_3$

TL;DR

This study investigates the magnetic and topological properties of CrI$_3$ in different stacking phases, revealing coexistence of antiferromagnetic and ferromagnetic orders and the persistence of Dirac magnon topology across phases.

Contribution

It provides the first neutron scattering evidence of AFM order in CrI$_3$ with monoclinic stacking and shows Dirac magnon features are preserved in this phase.

Findings

AFM transition observed below 50 K in M-type stacking

Magnon energy decreases in M phase compared to R phase

Dirac magnon gap persists in M phase

Abstract

Van der Waals (vdW) Dirac magnon system CrI$_3$, a potential host of topological edge magnons, orders ferromagnetically (FM) (T$_C=61$ K) in the bulk, but antiferromagnetic (AFM) order has been observed in nanometer thick flakes, attributed to monoclinic (M) type stacking. We report neutron scattering measurements on a powder sample where the usual transition to the rhombohedral (R) phase was inhibited for a majority of the structure. Elastic measurements (and the opening of a hysteresis in magnetization data on a pressed pellet) showed that an AFM transition is clearly present below $\sim$50 K, coexisting with the R-type FM order. Inelastic measurements showed a decrease in magnon energy compared to the R phase, consistent with a smaller interlayer magnetic coupling in M-type stacking. A gap remains at the Dirac point, suggesting that the same nontrivial magnon topology reported for the R phase may be present in the M phase as well.