  The chiral Fe-based langasites represent model systems of triangle-based
frustrated magnets with a strong potential for multiferroicity. We report
neutron scattering measurements for the multichiral Ba3MFe3Si2O14 (M = Nb, Ta)
langasites revealing new important features of the magnetic order of these
systems: the bunching of the helical modulation along the c-axis and the
in-plane distortion of the 120{\textdegree} Fe-spin arrangement. We discuss
these subtle features in terms of the microscopic spin Hamiltonian, and provide
the link to the magnetically-induced electric polarization observed in these
systems. Thus, our findings put the multiferroicity of this attractive family
of materials on solid ground.
