Spin-ordering and magnetoelastic coupling in the extended kagome system YBaCo4O7

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 similar to 100 K. The interplay between the structural and magnetic degrees of freedom indicates 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 similar to 60 K, an additional magnetic transition is observed, though isosymmetric. 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.