  We have studied the frustrated system YBaCo4O7 generally described as an
alternating stacking of Kagome and triangular layers of magnetic ions on a
trigonal lattice, by single crystal neutron diffraction experiments above the
Neel ordering transition. Experimental data reveals pronounced magnetic diffuse
scattering, which is successfully modeled by direct Monte-Carlo simulations.
Long-range magnetic correlations are found along the c-axis, due to the
presence of corner-sharing bipyramids, creating quasi one-dimensional order at
finite temperature. In contrast, in the Kagome layers ab-plane, the spin-spin
correlation function -displaying a short-range 120 degrees configuration-
decays rapidly as typically found in spin-liquids. YBaCo4O7 experimentally
realizes a new class of two-dimensional frustrated systems where the strong
out-of-plane coupling does not lift the in-plane degeneracy, but instead act as
an external "field".
