Long-period helical structures and twist-grain boundary phases induced by chemical substitution in the Mn1−x(Co,Rh)xGe chiral magnet

We study the evolution of helical magnetism in MnGe chiral magnet upon partial substitution of Mn for $3d$-Co and $4d$-Rh ions. At high doping levels, we observe spin helices with very long periods---more than ten times larger than in the pure compound---and sizable ordered moments. This behavior calls for a change in the energy balance of interactions leading to the stabilization of the observed magnetic structures. Strikingly, neutron scattering unambiguously shows a double periodicity in the observed spectra at $x=0.5$ and g0.2 for Co- and Rh-doping, respectively. In analogy with observations made in smectic liquid crystals, we suggest that it may reveal the presence of magnetic ``twist grain boundary'' phases, involving a dense short-range correlated network of magnetic screw dislocations. The dislocation cores are here tentatively described as smooth textures, made of nonradial double-core skyrmions.