STATIC MAGNETIZATION AND AC SUSCEPTIBILITY MEASUREMENTS OF THE COPPER-OXYGEN CLUSTER COMPOUND BACUO2+X

Static magnetization M and ac magnetic susceptibility measurements in applied magnetic fields H up to 55 kG are reported for a polycrystalli ne He-annealed sample of BaCuO2+x with x = 0.1 +/- 0.1. This compound has a large body-centered-cubic unit cell (a(0) = 18.29 Angstrom) cont aining 90 formula units. The cell contains six lone CuO4 units, two Cu 18O24 Sphere clusters, and eight Cu6O12 ring clusters. Clear evidence of long-range antiferromagnetic (AF) ordering at a Neel temperature T- N similar to 15 K was found. From detailed analyses of isothermal M(H) data at 2 K and static magnetic susceptibility data from 2 to 400 K, the following model for the magnetic properties of BaCuO2+x, emerged. The oxidation state of the copper is +2, and each CU+2 ion carries a s pin S-Cu = 1/2. The Cu-18 sphere clusters and Cu-6 ring clusters both have ferromagnetic ground states with spins S-s = 9 and S-r = 3, respe ctively, with lower spin excited states. The ferromagnetic intracluste r interactions arise from a Cu-O-Cu superexchange pathway with a nearl y 90 degrees bond angle; within the sphere and ring clusters, the Cu-C u superexchange coupling constants are 2J(S)/k(B) = (80 +/- 16) K and 2J(r)/k(B) = (290 +/- 60) K, respectively. The clusters are in their g round states below similar to 40 K. The AF ordering at T-N arises from AF ordering of the Cu-6 spin 3 magnetic moments only; this aspect of our model has been confirmed by magnetic neutron-diffraction measureme nts. Remarkably, the Cuts clusters and lone Cu ions do not participate in this ordering and remain paramagnetic down to at least 2 K. Each C u-18 cluster is antiferromagnetically coupled to three lone Cu ions, a nd each lone Cu ion to one Cu-18 cluster, with AF coupling constant 2J (AF)/k(B) = -0.33 K. The Cuts clusters (with spin S-S = 9) are predict ed to undergo ferromagnetic intercluster ordering below similar to 1 K .