Nuclear magnetic resonance investigation of complex magnetic behavior in rare-earth cuprates: RCu2O4 (R=La and Nd)

摘要

Cu-63 nuclear magnetic resonance (NMR) has been carried out to investigate the complex magnetic behavior of antiferromagnetic (AF) rare earth cuprates LaCu2O4 (T-N1 = 18 K) and NdCu2O4 (T-N1 = 23 K, T-N2= 14 K) with two inequivalent Cu sites. In the AF ordered state, two distinct zero-field Cu NMR spectra are observed in the high-frequency range (95-175 MHz) and the low-frequency range (10-32 MHz), corresponding to the magnetic Cu(I) site and the diamagnetic Cu(2) site, respectively. The ordered moment of the Cu(1) ions in LaCu2O, is estimated to be 1.09 mu (B), consistent with the expected oxidation of Cu2+ (spin=1/2). For Cu(2), the zero-held NMR in the low-frequency range is observed up to the paramagnetic state through T-N1 and the spectrum analysis indicates that the ground state is nonmagnetic, consistent with the expected Cu3+ (spin =0) state. The increase in the magnetization below TN1 is caused by canting of ordered Cu2+ moments. The second magnetic transition in NdCu2O4 has hardly any affect an the Cu-63 NMR spectra. A large decrease in the electric quadrupole frequency of Cu(2) in LaCu7O4 below T-N1 is explained as due to the contraction of the unit cell volume, which aids the magnetic ordering in LaCu2O4 that has the larger cell volume. [References: 11]