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

Abstract

${}^{63}\mathrm{Cu}$ nuclear magnetic resonance (NMR) has been carried out to investigate the complex magnetic behavior of antiferromagnetic (AF) rare earth cuprates ${\mathrm{LaCu}}_{2}{\mathrm{O}}_{4}$ ${(T}_{N1}=18\mathrm{K})$ and ${\mathrm{NdCu}}_{2}{\mathrm{O}}_{4}$ ${(T}_{N1}=23\mathrm{K},$ ${T}_{N2}=14\mathrm{K})$ 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(1) site and the diamagnetic Cu(2) site, respectively. The ordered moment of the Cu(1) ions in ${\mathrm{LaCu}}_{2}{\mathrm{O}}_{4}$ is estimated to be $1.09{\ensuremath{\mu}}_{B},$ consistent with the expected oxidation of ${\mathrm{Cu}}^{2+}$ $(\mathrm{spin}=\frac{1}{2}).$ For Cu(2), the zero-field 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 ${\mathrm{Cu}}^{3+}$ $(\mathrm{spin}=0)$ state. The increase in the magnetization below ${T}_{N1}$ is caused by canting of ordered ${\mathrm{Cu}}^{2+}$ moments. The second magnetic transition in ${\mathrm{NdCu}}_{2}{\mathrm{O}}_{4}$ has hardly any affect on the ${}^{63}\mathrm{Cu}$ NMR spectra. A large decrease in the electric quadrupole frequency of Cu(2) in ${\mathrm{LaCu}}_{2}{\mathrm{O}}_{4}$ below ${T}_{N1}$ is explained as due to the contraction of the unit cell volume, which aids the magnetic ordering in ${\mathrm{LaCu}}_{2}{\mathrm{O}}_{4}$ that has the larger cell volume.