Stress-induced magnetic domain selection reveals a conical ground state for the multiferroic phase of Mn 2 GeO 4

At ambient pressure $P$ and below 5.5 K, olivine-type ${\mathrm{Mn}}_{2}{\mathrm{GeO}}_{4}$ hosts a multiferroic (MF) phase where a multicomponent, i.e., multi-$k$ magnetic order generates spontaneous ferromagnetism and ferroelectricity (FE) along the c axis. Under high $P$ the FE disappears above 6 GPa, yet the $P$ evolution of the magnetic structure remained unclear based on available data. Here we report high-$P$ single crystal neutron diffraction experiments in the MF phase at $T=4.5$ K. We observe clearly that the incommensurate spiral component of the magnetic order responsible for FE varies little with $P$ up to 5.1 GPa. With support from high $P$ synchrotron x-ray diffraction measurements at room temperature $(T)$, the $P$-driven suppression of FE is proposed to occur as a consequence of a crystal structure transition away from the olivine structure. In addition, in the low $T$ neutron scattering experiments an emergent nonhydrostatic $P$ component, i.e., a uniaxial stress, leads to the selection of certain multi-$k$ domains. We use this observation to deduce a double-$k$ conical magnetic structure for the ambient $P$ ground state, this being a key ingredient for a model description of the MF phase.