ANTIFERROMAGNETIC ORDERING AND PARAMAGNETIC BEHAVIOR OF FERROMAGNETICCU6 AND CU18 CLUSTERS IN BACUO2+X

Magnetization and neutron diffraction measurements on polycrystalline BaCuO2+x revealed a combination of magnetic behaviors. The Cu6 ring cl usters and Cu18 sphere clusters in this compound had ferromagnetic gro und states with large spins 3 and 9, respectively. The Cu6 rings order ed antiferromagnetically below the Neel temperature T(N) = 15 +/- 0.5 kelvin, whereas the Cu18 spheres remained paramagnetic down to 2 kelvi n. The ordered moment below T(N) was 0.89(5) Bohr magnetons per Cu in the Cu6 rings, demonstrating that quantum fluctuation effects are smal l in these atomic clusters. The Cu18 clusters are predicted to exhibit ferromagnetic intercluster order below about 1 kelvin.