Static magnetization and ac susceptibility measurements of the copper-oxygen cluster compoundBaCuO2+x

Static magnetization M and ac magnetic susceptibility measurements in applied magnetic fields H up to 55 kG are reported for a polycrystalline He-annealed sample of ${\mathrm{BaCuO}}_{2+\mathit{x}}$ with x=0.1±0.1. This compound has a large body-centered-cubic unit cell (${\mathit{a}}_0$=18.29 Å) containing 90 formula units. The cell contains six lone ${\mathrm{CuO}}_4$ units, two ${\mathrm{Cu}}_{18}$ ${\mathrm{O}}_{24}$ sphere clusters, and eight ${\mathrm{Cu}}_6$ ${\mathrm{O}}_{12}$ ring clusters. Clear evidence of long-range antiferromagnetic (AF) ordering at a Néel temperature ${\mathit{T}}_{\mathit{N}}$∼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 ${\mathrm{BaCuO}}_{2+\mathit{x}}$ emerged. The oxidation state of the copper is +2, and each ${\mathrm{Cu}}^{+2\mathrm{}}$ ion carries a spin ${\mathit{S}}_{\mathrm{Cu}}$=1/2. The ${\mathrm{Cu}}_{18}$ sphere clusters and ${\mathrm{Cu}}_6$ ring clusters both have ferromagnetic ground states with spins ${\mathit{S}}_{\mathit{s}}$=9 and ${\mathit{S}}_{\mathit{r}}$=3, respectively, with lower spin excited states. The ferromagnetic intracluster interactions arise from a Cu-O-Cu superexchange pathway with a nearly 90° bond angle; within the sphere and ring clusters, the Cu-Cu superexchange coupling constants are 2${\mathit{J}}_{\mathit{s}}$/${\mathit{k}}_{\mathit{B}}$=(80±16) K and 2${\mathit{J}}_{\mathit{r}}$/${\mathit{k}}_{\mathit{B}}$=(290±60) K, respectively.