Magnetoelastic and structural properties of azurite ${\text{Cu}}_3{\left({\text{CO}}_3\right)}_2{\left(\text{OH}\right)}_2$ from neutron scattering and muon spin rotation

Azurite, ${\text{Cu}}_3{\left({\text{CO}}_3\right)}_2{\left(\text{OH}\right)}_2$, has been considered an ideal example of a one-dimensional diamond chain antiferromagnet. Early studies of this material imply the presence of an ordered antiferromagnetic phase below $T_N\sim 1.9\text{K}$ while magnetization measurements have revealed a 1/3 magnetization plateau. To the best of our knowledge, no corroborating neutron-scattering results have been published to confirm the ordered magnetic moment structure. We present recent neutron-diffraction results which reveal the presence of commensurate magnetic order in azurite which coexists with significant magnetoelastic strain. The latter of these effects may indicate the presence of spin frustration in zero applied magnetic field. Muon spin rotation reveals an onset of short-range order below 3 K and confirms long-range order below $T_N$.

Figure 1

(Color online) Powder neutron-diffraction spectra of azurite at (a) 5 K and in the AFM phase at (b) 1.28 K. Tics indicate structural Bragg-peak positions, $(+)$ represent experimental data. The solid line on the data shows the result of the refinement and the difference between refinement and fit is plotted at the bottom of each graph. In (c) the difference between the spectra taken at 1.28 and 5 K is plotted.

Figure 2

The temperature dependence of the integrated intensity of the (120) Bragg peak of ${\text{Cu}}_3{\left({\text{CO}}_3\right)}_2{\left(\text{OH}\right)}_2$, as obtained from single-crystal neutron-diffraction experiments. The inset displays the coefficient of uniaxial thermal expansion $\alpha$ along the $b$ axis as a function of temperature around $T_N=1.88\text{K}$.

Figure 3

Normalized magnetic fraction from 50 GTF (see text).

Figure 4

(Color online) Intensity distribution in reciprocal space taken from low-temperature single-crystal neutron diffraction of ${\text{Cu}}_3{\left({\text{CO}}_3\right)}_2{\left(\text{OH}\right)}_2$. These plots show a cut along the $ab$ plane with all information out of the plane binned. White stripes indicate regions where no data was collected. The circle indicates an additional Bragg peak occurring at $\left(\frac{1}{2}\frac{1}{2}\frac{1}{2}\right)$ at temperatures below $T_N=1.9\text{K}$. The two high-intensity concentric circles visible in both diffraction patterns are powder-diffraction lines from the sample mount.