Origin and consequence of an unpinned helical magnet: application to partial order in MnSi under pressure

Abstract

We study a classical ferromagnetic Heisenberg model in the presence of Dzyaloshinskii-Moriya interactions on the corner-shared triangle lattice formed by the Mn sites of MnSi. We show that a sizable spin helicity can be obtained only when the microscopic Moriya vectors lie parallel to the Mn-Mn bonds. Further, such vectors are shown to produce an unpinned helical order characterized by a particular ordering wavevector magnitude but unpinned direction, dubbed partial order, at physically realizable temperatures. A consequence of such an unpinned helical ordering is that the neutron scattering intensity is sharply peaked at this wavevector magnitude. The surface formed by connecting these wavevectors is a sphere, around which the neutron scattering weight is spread. We further show that the observed neutron scattering intensity can be anisotropic along this surface and that this anisotropy is dependent on the experimentalist's choice of lattice Bragg peak through a geometric factor. A neutron scattering measurement near the Bragg point ($\frac{2\pi}{a}$,$\frac{2\pi}{a}$,0) naturally leads to a highest intensity along the (1,1,0) direction consistent with the observed anisotropy in MnSi [Pfleiderer {\it{et al.}} Nature {\bf{427}} 227 (2004)]. A possible mechanism for pinning the helical order, and a way to distinguish an ordered and a partially ordered state in the context of neutron scattering are discussed.