We study the helimagnetic ground state of the MnGe cubic alloy using small-angle neutron scattering and a high-resolution method, the so-called MIEZE spectroscopy. Upon cooling below the Néel temperature $T_{\mathrm{N}}=170\left(5\right)\mathrm{K}$, we observe the proliferation of long-wavelength gapless spin fluctuations, concomitant with a continuous evolution of the helical correlation length. These fluctuations disappear at $T_{\mathrm{com}}=32\left(5\right)\mathrm{K}$ when the helical period becomes commensurate with the lattice. We propose to describe this intermediate phase as a soliton lattice, promoting nonlinear collective modes, or phasons, over a large temperature interval. We discuss the possible relevance of our results to the previously observed magnetotransport anomalies.

• Received 24 January 2019

DOI:https://doi.org/10.1103/PhysRevB.99.100402