Magnetic and Structural Studies of the Quasi-Two-Dimensional Spin-Gap System (CuCl)LaNb2O7

TL;DR

This study investigates the magnetic and structural properties of the quasi-two-dimensional spin-gap system (CuCl)LaNb2O7 using various techniques, revealing the need to modify existing models and identifying a field-induced magnetic phase transition.

Contribution

The paper introduces alternative two-dimensional dimer models for (CuCl)LaNb2O7 based on NMR, NQR, and TEM findings, challenging the previously reported crystal structure and J1-J2 model.

Findings

Unique Cu and Cl sites confirmed by NQR.

Electric field gradient tensors inconsistent with original structure.

Field-induced magnetic phase transition at 10.3 T.

Abstract

We report magnetization, nuclear magnetic resonance (NMR), nuclear quadrupole resonance (NQR), and transmission electron microscopy (TEM) studies on the quasi-two-dimensional spin-gap system (CuCl)LaNb2O7, a possible candidate for the J1-J2 model on a square lattice. A sharp single NQR line is observed at the Cu and Cl sites, indicating that both Cu and Cl atoms occupy a unique site. However, the electric field gradient tensors at the Cu, Cl, and La sites do not have axial symmetry. This is incompatible with the reported crystal structure. Thus the J1-J2 model has to be modified. We propose alternative two-dimensional dimer models based on the NMR, NQR, and TEM results. The value of the hyperfine coupling constant at the Cu sites indicates that the spin density is mainly on the d(3z2-r2) orbital (z parallel c). At 1.5 K, Cu- and Nb-NMR signals disappear above the critical field Bc1 = 10.3 T determined from the onset of the magnetization, indicating a field-induced magnetic phase transition at Bc1.