  Low temperature magnetic and structural behavior of the extended Kagome
system YBaCo4O7 has been studied by single crystal neutron diffraction and
high-resolution powder X-ray diffraction. Long-range magnetic ordering
associated with a structural transition from orthorhombic Pbn2_1 to monoclinic
P2_1 symmetry has been found at T_1 ~ 100 K. The interplay between the
structural and magnetic degrees of freedom testifies that the degeneracy of the
magnetic ground state, present in the orthorhombic phase, is lifted through a
strong magnetoelastic coupling, as observed in other frustrated systems. At T_2
~ 60 K, an additional magnetic transition is observed, though iso-symmetric.
Models for the magnetic structures below T_1 and T_2 are presented, based on
refinements using a large number of independent reflections. The results
obtained are compared with previous single crystal and powder diffraction
studies on this and related compositions.
