Low-spin S=1/2 ground state of the Cu trimers in the paper-chain compound Ba3Cu3In4O12

Abstract

The magnetic ground states of the isostructural paper-chain compounds Ba 3Cu 3In 4O 12 and Ba 3Cu 3Sc 4O 12 were recently found to be ordered antiferromagnetically (AFM), while both quantum magnets reveal positive asymptotic Curie-Weiss temperatures suggestive of ferromagnetic (FM) interactions at high temperatures. Due to the AFM-FM competition the saturation magnetization in both compounds is reached already in modest magnetic fields with a nontrivial succession of two spin-flop and two spin-flip-like transitions in Ba 3Cu 3In 4O 12. We argue that the paper-chains in both compounds can be described as a system of CuI-2CuII trimers with a low-spin S=1/2 ground state which implies a nonmagnetic spin-singlet CuII dimer state and a spin-polarized CuI ion. To validate the model we performed extensive nuclear resonance-nuclear quadrupole resonance-nuclear magnetic resonance (NQR-NMR)-measurements in Ba 3Cu 3In 4O 12. Two different types of 63 ,65Cu nuclear resonance spectra were observed in the magnetically ordered state at zero external magnetic field: (i) a pure NQR spectrum in the frequency range of 24-30 MHz and (ii) a zero-field NMR spectrum in the frequency range of 50-65 MHz. This result unambiguously indicates that one of the two types of copper ions in Ba 3Cu 3In 4O 12 is in a nonmagnetic spin state below T N. It provides a unique criterion to test any theoretical model describing the ground-state spin structure in Ba 3Cu 3In 4O 12 . We attribute the pure NQR spectrum to CuII ions forming nonmagnetic spin-singlet 2CuII dimers and the zero-field NMR spectrum to spin-polarized CuI ions. Both NQR and zero-field NMR spectra point to an existence of at least two nonequivalent sets of the copper trimers, probably due to displacements of the CuI ions along the cavity in the c direction. © 2012 American Physical Society.