Evidence for an internal-field-induced spin-flop configuration in the extended kagome YBaCo4O7

The spin structure and spin dynamics in the extended kagome frustrated antiferromagnet YBaCo 4 O 7 have been investigated using zero field and low applied field 59 Co NMR. The YBaCo 4 O 7 lattice is made up of bipyramid Co-ion units that form alternating planes of edge-sharing spin triangles and corner-sharing kagome spin triangles in an unusual exchange topology. Our low-temperature spin configuration results, based on hyperfine field orientations, are consistent with those from neutron scattering for the triangle spins which order antiferromagnetically below 106 K. For the kagome spins at low temperatures the static hyperfine fields are found to be oriented orthogonal to those of the triangle spins in a spin-flop configuration that is in disagreement with the neutron findings. Nuclear relaxation rate measurements made as a function of temperature show that inhomogeneous dynamic spin disorder occurs in kagome planes well below the Néel point