Spin-wave dynamics in Mn-doped FeGe helimagnet: Small-angle neutron scattering study

We report on the measurements of the spin-wave stiffness A and its temperature dependence close to TC for two compounds: FeGe and the solid solution Fe0.8Mn0.2Ge. The small-angle neutron scattering was used to detect the inelastic scattering in the magnetic field above the transition from the conical to the field-polarized state. This inelastic contribution is shaped as two circles on a neutron scattering map. Their radius is directly related to the spin wave stiffness A that decreases with a temperature but has a finite value at Tc for both compounds. Since the softening of spin waves near Tc, which is expected for a second-order phase transition, is not observed, we make a conclusion about the transition as having a first-order nature.