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 K. Moreover, the helical structure is intrinsically unstable below , since the profile of the reflection at 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 . The spin-helix fluctuations are accompanied by intensive spin excitations observed in small-angle neutron scattering as -independent scattering at . The integral intensities of both the spin excitations and spin fluctuations obey exactly the same temperature law proportional to with the activation temperature K. These two contributions coexist above , being well distinguished up to K, where they are intermixed with and transform into diffuse scattering at K. Above the diffuse scattering is well described by a Gauss law, ascribed to static short-range ferromagnetic inhomogeneities with an inverse correlation length .
1 More- Received 10 September 2014
DOI:https://doi.org/10.1103/PhysRevB.90.174420
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