MAGNETIC NEUTRON-DIFFRACTION FROM THE MAGNETIC CLUSTERS IN SINGLE-CRYSTAL BACUO2+X

Magnetic-neutron-diffraction studies of a single crystal of BaCuO2+x c onfirm the antiferromagnetic structure of the ringlike Cu-6 clusters w ith ground-state spin S=3 proposed by Wang et al. [Science 264, 402 (1 994)]. No indication of a spontaneous ferromagnetic (or simple antifer romagnetic) ordering of the spherelike Cu-18 S = 9 clusters were obser ved down to a temperature of 50 mK, in disagreement with the suggestio n based on magnetic-susceptibility measurements performed down to 2 K. However, diffraction scans under applied magnetic fields up to 3 T, a t temperatures of 0.05 and 4.2 K align the sphere clusters ferromagnet ically. The magnetic-field-induced ferromagnetic-diffraction pattern i s consistent with the form factor of the Cu-18 spherelike cluster. Fru stration in the coupling with nearest neighbors leads to a paramagneti c behavior or a complex magnetic structure (noncollinear and/or incomm ensurate) of the spherelike clusters, consistent with our findings.