Magnetic properties of cluster-spins in BaCuO2.00 and BaCuO2.14, investigated by magnetization and NMR measurements

Abstract To elucidate the combined magnetic behavior of BaCuO2+x (x=0, 0.14), which contains in the BCC unit-cell two Cu18O24 spherical clusters, eight Cu6O12 ring clusters and six CuO4 lone units, we have carried out the magnetization, 63Cu NMR, and 63Cu PQR measurements. Quantitative analysis of present susceptibility data indicates that the spins within a Cu6 cluster of BaCuO2.00 are in the ferromagnetically ordered ground state with maximum spin Sr=3 even at room-T, and the spins within a Cu18 cluster are in a ground state below ∼20 K with net spin Ss∼5 less than the maximum spin. 63Cu NMR for BaCuO2.00 was observed below ∼15 K with a trapezoid-like shape spectrum and the relaxation rate T1−1 was roughly proportional to T3. These are consistent with the long-range antiferromagnetic ordering of the Cu6 cluster-spins below TN=15 K. The NMR spectrum for BaCuO2.14 has an electric-quadrupole split structure down to 1.7 K, which is indicative of no long-range magnetic ordering. We found, however, a few anomalies in the NMR and PQR data for BaCuO2.14, which could be understood by a short-range ordering of paramagnetic Cu18 cluster-spins.