Spin-ordering and magnetoelastic coupling in the extended kagome system YBaCo 4 O 7

Low-temperature magnetic and structural behavior of the extended Kagome system YBaCo{sub 4}O{sub 7} 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{sub 1} to monoclinic P2{sub 1} symmetry has been found at T{sub 1} {approx} 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{sub 2} {approx} 60 K, an additional magnetic transition is observed, though isosymmetric. Models for the magnetic structures below T1 and T2 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.