Overdamped magnons in the field-polarized phase of cubic helimagnet

Abstract Skyrmion-hosting cubic B20 alloys are considered a fruitful platform for various spintronic applications due to their pronounced tunability. However, disorder can limit their usability by pinning skyrmions and scattering magnons. Here, we probe the spin dynamics in the field-polarized phase of the skyrmion host Cr 0.82 Mn 0.18 Ge using spin wave small-angle neutron scattering (SWSANS). Unlike for typical B20 helimagnets, the corresponding SWSANS data do not possess a crucial cut-off feature but rather show smooth diffuse patterns. Our analytical calculations within the minimal model of a disordered cubic helimagnet reveal unusually strong damping of long-wavelength magnons in contrast with standard collinear spin structures. X-ray circular magnetic dichroism (XMCD) measurements indicate antiferromagnetic coupling between Cr and Mn ions, thus strongly supporting the considered model. Therefore, both experimental and theoretical evidence of the absence of well-defined helimagnon excitations in this material are provided.