Noncentrosymmetric cubic helical ferromagnets [Mn.sub.1-y][Fe.sub.y]Si and [Fe.sub.1-x][Co.sub.x]Si

Two systems of noncentrosymmetric cubic helical magnets [Mn.sub.1-y][Fe.sub.y]Si (y = 0.06, 0.08, 0.10) and [Fe.sub.1-x][Co.sub.x]Si (x = 0.10, 0.15, 0.20, 0.25, 0.30, 0.35, 0.50) have been compared. The concentration dependences of the critical temperature and magnetic field have been obtained using small-angle polarized-neutron scattering and analyzed in the framework of the Bak--Jensen model. It has been established that, among the two interactions that play the main role in these systems, i.e., the isotropic symmetric ferromagnetic exchange and the Dzyaloshinskii--Moriya isotropic antisymmetric interaction, the former interaction determines the critical temperature in the [Mn.sub.1-y][Fe.sub.y]Si system and the latter interaction determines this temperature in the [Fe.sub.1-x][Co.sub.x]Si system.