  We have experimentally identified a new magnetic ground state for the kagome
lattice, in the perfectly hexagonal Fe2+ (3d6, S = 2) compound Fe4Si2Sn7O16.
Representational symmetry analysis of neutron diffraction data shows that below
T_N = 3.5 K, the spins on 2/3 of the magnetic ions order into canted
antiferromagnetic chains, separated by the remaining 1/3 which are
geometrically frustrated and show no long-range order down to at least T = 0.1
K. Moessbauer spectroscopy confirms that there is no static order on the latter
1/3 of the magnetic ions - i.e., they are in a liquid-like rather than a frozen
state - down to at least 1.65 K. A heavily Mn-doped sample
Fe1.45Mn2.55Si2Sn7O16 has the same magnetic structure. Although the propagation
vector q = (0, 1/2 , 1/2 ) breaks hexagonal symmetry, we see no evidence for
magnetostriction in the form of a lattice distortion within the resolution of
our data. We discuss the relationship to partially frustrated magnetic order on
the pyrochlore lattice of Gd2Ti2O7, and to theoretical models that predict
symmetry breaking ground states for perfect kagome lattices.
