Spin-wave dynamics in the helimagnet FeGe studied by small-angle neutron scattering

We have studied the spin-wave stiffness of the Dzyaloshinskii-Moriya helimagnet FeGe in a temperature range from 225~K up to $T_C \approx$~278.7~K by small-angle neutron scattering. The method we have used is based on [S. V. Grigoriev et al. Phys. Rev. B \textbf{92} 220415(R) (2015)] and was extended here for the application in polycrystalline samples. We confirm the validity of the anisotropic spin-wave dispersion for FeGe caused by the Dzyaloshinskii-Moriya interaction. We have shown that the spin-wave stiffness $A$ for FeGe helimagnet decreases with a temperature as $A(T) = 194(1-0.7(T/T_C)^{4.2})$ meV\AA$^2$. The finite value of the spin-wave stiffness $A = 58$ meV\AA$^2$ at $T_C$ classifies the order-disorder phase transition in FeGe as being the first order one.