%0 journal article
%@ 1098-0121
%A Altynbayev, E.V, Siegfried, S.-A., Dyadkin, V., Moskvin, E.V., Menzel, D., Heinemann, A., Dewhurst, C., Fomicheva, L.N., Tsvyashchenko, A.V., Grigoriev, S.V.

%D 2014
%J Physical Review  B
%N 17
%P 174420 
%R doi:10.1103/PhysRevB.90.174420
%T Intrinsic instability of the helix spin structure in MnGe and order-disorder phase transition
%U https://doi.org/10.1103/PhysRevB.90.174420
17 
%X 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<k h  . The integral intensities of both the spin excitations and spin fluctuations obey exactly the same temperature law proportional to exp(T/T a )  with the activation temperature T a =(80±1)  K. These two contributions coexist above T N  , being well distinguished up to T DM =150  K, where they are intermixed with and transform into diffuse scattering at T SR =(175±5)  K. Above T SR   the diffuse scattering is well described by a Gauss law, ascribed to static short-range ferromagnetic inhomogeneities with an inverse correlation length κ≈2  nm −1  .