Topological stability of spin textures in Si/Co-doped helimagnet FeGe

Element substitutions with magnetic or non-magnetic atoms are known to significantly impact the magnetic structure and related transport properties of magnets. To clarify the change of magnetic structure of B20-type magnets with element doping, we conduct real-space observations of spin textures and their temperature (T)-magnetic field (H) phase diagrams of a helimagnet FeGe with partially substituting Fe and Ge with Co and Si, respectively. The helical period (lambda) changes dramatically by the element doping: lambda increases by 147% to 103 nm in 30% Co-doped FeGe, whereas it decreases by around 70% to 49 nm in 30% Si-doped FeGe, compared to the lambda =70 nm in FeGe. Upon applying the magnetic field normally to (001), (110), and (111) thin plates of both FeSi0.3Ge0.7 and Fe0.7Co0.3Ge, the hexagonal skyrmion crystal (SkX) state emerges. The magnetic phase diagrams observed through the real-space imaging reveal that (1) the SkX can extend to a larger T-H window by reducing the sample thickness or by cooling the sample under specific magnetic fields from temperatures above the transition temperature (TC ); (2) the stability of the SkX phase differs between Si-doped and Co-doped FeGe: the SkX phase is most unstable in the (111) FeSi0.3Ge0.7, while it remains robust in the (111) Fe0.7Co0.3Ge. These differences indicate distinct anisotropic behavior in FeGe with magnetic (Co) and non-magnetic-element (Si) dopants.