ISOLATED SPIN PAIRS AND 2-DIMENSIONAL MAGNETISM IN SRCR9PGA12-9PO19

We show by inelastic neutron scattering that the frustrated magnet SrC r9pGa12-9pO19 [p = 0.92(5)] has dispersionless magnetic excitations at energies 18.6(1) and 37.2(5) meV. The wave vector and temperature dep endence of the excitations as well as the ratio 1:2 of excitation ener gies is perfectly accounted for by isolated antiferromagnetically exch ange coupled pairs of spins s = 3/2. Consideration of the layered stru cture of magnetic chromium ions and the ligand environment indicates t hat these pairs are adjacent spins in neighboring triangular lattice p lanes which separate two-dimensional sublattices of interacting kagome -triangle-kagome layers.