FeMn3Ge2Sn7O16: a perfectly isotropic 2-D kagomé lattice that breaks magnetic symmetry with partial spin order

Abstract

FeMn3Ge2Sn7O16 is a fully ordered stoichiometric phase containing an undistorted hexagonal kagomé lattice of Mn2+ cations. It represents not only an important expansion of the chemistry of the complex composite FeFe3Si2Sn7O16 structure type, by replacing silicon with germanium, but also an improvement on the perfection of the kagomé lattice by replacing anisotropic high-spin Fe2+ (d6, L = 2) with isotropic high-spin Mn2+ (d5, L = 0), controlled by the size-matched replacement of SiO44– with GeO44– bridging units. This anisotropy was suspected of playing a role in the unique “striped” magnetic structure of FeFe3Si2Sn7O16 at low temperatures, which breaks hexagonal symmetry and leaves one-third of the magnetic moments geometrically frustrated and fluctuating down to at least 0.1 K. We observe the same striped magnetic structure in FeMn3Ge2Sn7O16, ruling out single-ion anisotropy as the driving force and deepening the intrigue around the apparent “partial spin-liquid” nature of these compounds.