Abstract
The magnetic structure of the cubic helimagnet MnGe was studied by small-angle neutron scattering in a wide temperature range from 10 to 300 K. We show that MnGe undergoes a complex order-disorder phase transition covering more than 100 K above the critical temperature T N =(130±2) K. Moreover, the helical structure is intrinsically unstable below T N , since the profile of the reflection at k h ∼2 nm −1 associated with the spin helix can be described by the sum of Gaussian and Lorentzian contributions, indicating a stable helix and spin-helix fluctuations, respectively. The Gaussian behavior determines the system at low temperature, whereas the Lorentzian is negligible. With increasing temperature, however, the Lorentzian starts to contribute and dominates at T N . The spin-helix fluctuations are accompanied by intensive spin excitations observed in small-angle neutron scattering as Q -independent scattering at Q
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