  Neutron diffraction studies have been carried out in the temperature(T) range
between 10 K and 130 K on a single crystal of YBaCo4O7, which has the stacking
of kagome and triangular lattices of CoO4 tetrahedra along the c-axis.
Structural transitions have been found at two temperatures Tc1~70 K and Tc2~105
K. With decreasing T, magnetic order appears along with the transition at Tc2,
but it does not grow to an ideal long-range order even at the lowest T studied
(~10 K). Two groups of magnetic diffuse reflections, which originate from the
Co-moments of the kagome and triangular lattices, are observed separately in
the reciprocal space. With further decreasing T, the growth rates of the
intensities of these two are enhanced by another transition at Tc1. These
magnetic behaviors seem to be related to the release of the geometrical
frustration. At 10 K, the patterns of the magnetic short-range order have been
determined for both sites of the kagome and triangular lattices.
