Approaching the ground state of the kagome antiferromagnet

Y(0.5)Ca(0.5)BaCo(4)O(7) contains kagome layers of Co ions, whose spins are strongly coupled, with a Curie-Weiss temperature of -2200 K. At low temperature, T=1.2 K, our diffuse neutron scattering study with polarization analysis reveals characteristic spin correlations close to a predicted two-dimensional coplanar ground state with staggered chirality. The absence of three-dimensional long-range antiferromagnetic order indicates negligible coupling between the kagome layers. The scattering intensities are consistent with high spin S=3/2 states of Co(2+) in the kagome layers and low spin S=0 states for Co(3+) ions on interlayer sites. Our observations agree with previous Monte Carlo simulations indicating a ground state of effectively short range, staggered chiral spin order.