Magnetic neutron diffraction from the magnetic clusters in single crystalBaCuO2+x

Magnetic-neutron-diffraction studies of a single crystal of ${\mathrm{BaCuO}}_{2+x}$ confirm the antiferromagnetic structure of the ringlike ${\mathrm{Cu}}_6$ clusters with ground-state spin $S=3\mathrm{}$ proposed by Wang et al. [Science 264, 402 (1994)]. No indication of a spontaneous ferromagnetic (or simple antiferromagnetic) ordering of the spherelike ${\mathrm{Cu}}_{18}$ $S=9\mathrm{}$ clusters were observed down to a temperature of 50 mK, in disagreement with the suggestion based on magnetic-susceptibility measurements performed down to 2 K. However, diffraction scans under applied magnetic fields up to 3 T, at temperatures of 0.05 and 4.2 K align the sphere clusters ferromagnetically. The magnetic-field-induced ferromagnetic-diffraction pattern is consistent with the form factor of the ${\mathrm{Cu}}_{18}$ spherelike cluster. Frustration in the coupling with nearest neighbors leads to a paramagnetic behavior or a complex magnetic structure (noncollinear and/or incommensurate) of the spherelike clusters, consistent with our findings.