Antiferromagnetic Nuclear Resonance in the Quasi-Two-Dimensional (CuBr)LaNb2O7

TL;DR

This study uses nuclear magnetic resonance to investigate the antiferromagnetic order in (CuBr)LaNb2O7, revealing significant atomic displacements and challenging previous magnetic models, thus providing new insights into its magnetic structure.

Contribution

The paper presents NMR evidence of atomic displacements and magnetic order in (CuBr)LaNb2O7, invalidating the frustrated J1-J2 model and supporting an alternative structural model.

Findings

NMR spectra show a large internal field at Br sites confirming magnetic order.

Br atoms are displaced off the Cu square center, affecting magnetic coupling.

Results invalidate the C4-symmetric J1-J2 model for this compound.

Abstract

We report nuclear magnetic resonance (NMR) studies in the antiferromagnetic state of the quasi-two-dimensional (CuBr)LaNb2O7. The NMR spectra at zero magnetic field and 4.2 K indicate a unique Cu and Br sites with an internal field of 5.7 T (at Cu) and 16.4 T (at Br), confirming a magnetic order. For the large internal field at the Br sites to be compatible with the collinear antiferromagnetic order observed by neutron diffraction experiments (N. Oba et al., J. Phys. Soc. Jpn. 75, (2006) 113601), the Br atoms must move significantly off the center of the square of the Cu sublattice so that the Br nuclei couple predominantly to two parallel Cu moments. While invalidating the frustrated J1-J2 model defined on a C4-symmetric square lattice, our results are compatible with the structural model proposed for (CuCl)LaNb2O7 by Yoshida et al. (J. Phys. Soc. Jpn. 76 (2007) 104703).