Magnetic properties of cluster-spins in BaCuO 2.00 and BaCuO 2.14, investigated by magnetization and NMR measurements

Abstract

To elucidate the combined magnetic behavior of BaCuO 2+ x ( x=0, 0.14), which contains in the BCC unit-cell two Cu 18O 24 spherical clusters, eight Cu 6O 12 ring clusters and six CuO 4 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 Cu 6 cluster of BaCuO 2.00 are in the ferromagnetically ordered ground state with maximum spin S r=3 even at room- T, and the spins within a Cu 18 cluster are in a ground state below ∼20 K with net spin S s∼5 less than the maximum spin. 63Cu NMR for BaCuO 2.00 was observed below ∼15 K with a trapezoid-like shape spectrum and the relaxation rate T 1 −1 was roughly proportional to T 3. These are consistent with the long-range antiferromagnetic ordering of the Cu 6 cluster-spins below T N=15 K. The NMR spectrum for BaCuO 2.14 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 BaCuO 2.14, which could be understood by a short-range ordering of paramagnetic Cu 18 cluster-spins.