Partial ordering and phase elasticity in the MnGe short-period helimagnet

Abstract : 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 $T_N$ = 170(5) 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 $T_{com}$ = 32(5) 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.