Single domain magnetic helicity and triangular chirality in structurally enantiopure Ba3NbFe3Si2O14

A novel doubly chiral magnetic order is found out in the structurally chiral langasite compound Ba$_3$NbFe$_3$Si$_2$O$_{14}$. The magnetic moments are distributed over planar frustrated triangular lattices of triangle units. On each of these they form the same triangular configuration. This ferro-chiral arrangement is helically modulated from plane to plane. Unpolarized neutron scattering on a single crystal associated with spherical neutron polarimetry proved that a single triangular chirality together with a single helicity is stabilized in an enantiopure crystal. A mean field analysis allows discerning the relevance on this selection of a twist in the plane to plane supersuperexchange paths.