Coexisting Order and Disorder Hidden in a Quasi-Two-Dimensional Frustrated Magnet

Frustrated magnetic interactions in a quasi-two-dimensional < 111 > slab of pyrochlore lattice were studied. For uniform nearest neighbor (NN) interactions, we show that the complex magnetic problem can be mapped onto a model with two independent degrees of freedom, tricolor and binary sign. This provides a systematic way to construct the complex classical spin ground states with collinear and coplanar bipyramid spins. We also identify partial but extended zero-energy excitations amongst the ground states. For nonuniform NN interactions, the coplanar ground state can be obtained from the collinear bipyramid spin state by collectively rotating two spins of each tetrahedron with an angle alpha in an opposite direction. The latter model with alpha similar to 30 degrees fits the experimental neutron data from SrCr9pGa12-9pO19 well.