Universality of the helimagnetic transition in cubic chiral magnets: Small angle neutron scattering and neutron spin echo spectroscopy studies of FeCoSi

We present a comprehensive Small Angle Neutron Scattering (SANS) and Neutron Spin Echo Spectroscopy (NSE) study of the structural and dynamical aspects of the helimagnetic transition in Fe_1-xCo_xSi with x = 0.30. In contrast to the sharp transition observed in the archetype chiral magnet MnSi, the transition in Fe_1-xCo_xSi is gradual and long-range helimagnetic ordering coexists with short-range correlations over a wide temperature range. The dynamics are more complex than in MnSi and involve long relaxation times with a stretched exponential relaxation which persists even under magnetic field. These results in conjunction with an analysis of the hierarchy of the relevant length scales show that the helimagnetic transition in Fe_1-xCo_xSi differs substantially from the transition in MnSi and question the validity of a universal approach to the helimagnetic transition in chiral magnets.