Quantum spin fluctuations are studied in chiral helicoidal Fe_ x Mn_1 – _ x Si ferromagnets on the basis of the direct calculations of the electronic structure. The disappearance of the helicoidal long-range order is shown to be accompanied by the appearance of the crossover of the thermodynamic and quantum transitions with a sharp decrease in the local magnetization and the zero fluctuation amplitude. Suppression of the local magnetization by quantum spin fluctuations leads to the concentration–temperature magnetic transition with the disappearance of the helicoidal short-range order. The calculations of the magnetization of the Fe_ x Mn_1 – _ x Si compositions show that at 0.12 < x < 0.20 the magnetic state in the thermodynamic limit is characterized by the short-range (not long-range) helicoidal order.