Transition between long and short-range helical orders in mixed B20 cubic helimagnets

A model of B20 helimagnet with strong (frustrating) defect bonds is considered. Single defect provides dipolar field-like distortion of the helical magnetic ordering. At finite defect bond concentrations, the spiral vector acquires correction to its value in the pure system. Moreover, it is shown that the strong defects of sufficient but small concentration are able to destroy long-range order driving the system to the phase with short-range helical ordering. It is argued that the developed approach allows describing main experimental observations in Mn$_{1-x}$Fe$_x$Si and Mn$_{1-x}$Co$_x$Si compounds.