Isolated Spin Pairs and Two-Dimensional Magnetism inSrCr9pGa12−9pO19

Abstract

We show by inelastic neutron scattering that the frustrated magnet ${\mathrm{SrCr}}_{9p}{\mathrm{Ga}}_{12-9p}{\mathrm{O}}_{19}$ $[p=0.92\mathrm{}(5\left)\right]$ 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 $s=3/2\mathrm{}$. 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 kagomé-triangle-kagomé layers.