Magnetic ordering and excitation in triangulated anti-ferromagnet Cu9Cl2(cpa)6·nH2O

The Cu2+ ions in Cu9Cl2(cpa)6 · nH2O (cpa is the anion of 2-carboxy pentonic acid) form a slightly deformed Kagome lattice with a smaller triangle plaquette in each triangle of a Kagome lattice showing strong frustration. The anomalous features of magnetism are the reduction of the Curie constant below 50 K, fractional saturation even in a field of 38 T, and nonlinear magnetic susceptibility below 5 K. We performed inelastic neutron scattering measurements on powder samples and observed dispersionless scattering around 6.5 meV with a full width of about 8 meV indicating a single-site excitation with E/kB ~~70 K. The scattering is less conspicuous above 50 K. These results are interpreted in terms of the plaquette-ordering model that assumes a 120° arrangement of three moments on each triangular plaquette and a random freezing of the remaining paramagnetic moments below 5 K.