Magnetic properties of copper cluster-spins inBaCuO2+x (x = 0 and 0.14), investigated by magnetization and nuclear magnetic resonance measurements

Abstract

To clarify the combined magnetic behaviour ofBaCuO2+x, which contains in the body-centred-cubic unit cell twoCu18O24 spherical clusters,eight Cu6O12 ringclusters, and six CuO4 lone units, we made measurements of magnetization,63,65Cu nuclear magnetic resonance (NMR) near 6 T, and pure quadrupoleresonance (PQR) in zero field on samples with well-defined oxygen contentsx = 0 and 0.14. Quantitative analyses of Curie–Weiss-like susceptibility data forBaCuO2.00 indicate that Cuspins within the Cu6 clusters go into a ferromagnetically (FM) ordered ground state with a maximum spinSr = 3 below K, and those within the Cu18 clusters into an FM-like ground state below K with net spin that is less than the maximum possible spinSs = 9. ForBaCuO2.14 the Cu spinswithin the Cu6 clusters go into an FM ordered ground state at a lower temperature of K, and these clusters exhibit no signature of long-range magnetic ordering down to 2 K. The NMR spectrafor BaCuO2.00 observed below K have a trapezoidal structure that is known to be a characteristic of long-rangemagnetic ordering, and is consistent with the antiferromagnetic ordering of theCu6 clustersbelow TN = 15 K predicted previously. On the other hand, the spectrum forBaCuO2.14 observed in a temperature range between 1.2 and K has an electric quadrupole-split powder pattern structure. This providesmicroscopic evidence that there is no long-range magnetic ordering inBaCuO2.14. The addition of 0.14 moles of oxygen atoms results in a large suppression of somemagnetic entities. We find, however, some anomalies in the NMR and PQR data forBaCuO2.14 near K, which could be associated with short-range ordering of the paramagneticCu18 clusters.