Spin Configuration in the 1/3 Magnetization Plateau of Azurite Determined by NMR

TL;DR

This study uses high-field NMR to analyze the local spin polarization in azurite's 1/3 magnetization plateau, revealing a mostly singlet dimer state with some triplet admixture due to bond asymmetry.

Contribution

First NMR-based determination of local spin polarization in azurite's magnetization plateau, highlighting anisotropic hyperfine fields and triplet admixture effects.

Findings

Hyperfine field of Cu2 sites is field independent and anisotropic.

Approximately 10% of the d-orbital spins are polarized.

Triplet admixture is induced by asymmetry in exchange bonds.

Abstract

High magnetic field $^{63,65}$Cu NMR spectra were used to determine the local spin polarization in the 1/3 magnetization plateau of azurite, Cu$_3$(CO$_3$)$_2$(OH)$_2$, which is a model system for the distorted diamond antiferromagnetic spin-1/2 chain. The spin part of the hyperfine field of the Cu2 (dimer) sites is found to be field independent, negative and strongly anisotropic, corresponding to $\approx$10 % of fully polarized spin in a $d$-orbital. This is close to the expected configuration of the "quantum" plateau, where a singlet state is stabilized on the dimer. However, the observed non-zero spin polarization points to some triplet admixture, induced by strong asymmetry of the diamond bonds $J_1$ and $J_3$.