Isolated spin pairs and two-dimensional magnetism in SrCr9pGa12-9pO19

We show by inelastic neutron scattering that the frustrated magnet SrCr9pGa12-9pO19 [p = 0.92(5)] has dispersionless magnetic excitations at energies 18.6(1) and 37.2(5) meV. The wave vector and temperature dependence of the excitations as well as the ratio 1:2 of excitation energies is perfectly accounted for by isolated antiferromagnetically exchange coupled pairs of spins s = 3/2. Consideration of the layered structure of magnetic chromium ions and the ligand environment indicates that these pairs are adjacent spins in neighboring triangular lattice planes which separate two-dimensional sublattices of interacting kagome-triangle-kagome layers.