Incommensurate magnetic order arising from frustrated interchain interactions in the spin-1/2 chain compound AgCuVO$_4$

TL;DR

This work addresses how frustrated interchain exchange affects magnetic order in a quasi-one-dimensional spin-1/2 chain material. Using muon spin spectroscopy and neutron diffraction, the authors determine the ground state of AgCuVO$_4$ and locate long-range order at $T_N = 2.0(1)$ K. The magnetic structure is incommensurate and amplitude-modulated with a very small moment of $0.13(2)\mu_\mathrm{B}$ and a propagation vector $\mathbf{k}_0=(0,0,0.45)$. The findings, consistent with frustrated interchain couplings and triangular motifs, position AgCuVO$_4$ as a model system to study frustration and quantum fluctuations in spin chains.

Abstract

Quantum spin chains with competing interactions offer a platform where low dimensionality and frustration--both acting to suppress magnetic order--intersect. We studied magnetic ordering in the spin-1/2 chain compound AgCuVO$_4$ using muon spin spectroscopy and neutron diffraction. Long-range magnetic order emerges at $T_N = 2.0(1)$ K, which is $\sim$1/200 of the dominant intrachain coupling $J$ and $\sim$1/15 of the interchain interactions. The collinear incommensurate amplitude-modulated magnetic structure features a reduced ordered moment of 0.13(3) $μ_\mathrm{B}$, confined to the $ab$ plane and modulated along the $c$ axis--perpendicular to the spin chains--indicating frustrated interchain couplings. The low $T_N$, small moment, and incommensurate order highlight strong frustration, positioning AgCuVO$_4$ as a model system for exploring frustration in quantum spin chains.

Figures (4)

• Figure 1: Muon polarization decay (a) as a function of magnetic field applied along the muon polarization at 5 K and (b) as a function of temperature in zero field (ZF). Lines in (a) correspond to the KT model for longitudinal field multiplied by an exponential relaxation, while in (b) they correspond to the sum of this KT model and a Gaussian line. (c) Extracted width of the Gaussian line as a function of temperature. Inset: The crystal structure of AgCuVO$_4$. The Cu$^{2+}$ spin-1/2 chains are composed of corner-sharing CuO$_4$ plaquettes (shown separately in the right corner) that run along the $b$ axis. The direction of the chains is highlighted by a thick semi-transparent black line drawn across the central chain.
• Figure 2: Powder neutron diffraction pattern at 10 K and the corresponding crystal structure refinement with impurity phases included.
• Figure 3: (a) Neutron powder-diffraction pattern measured at DMC instrument at 0.13 and 1.65 K. (b) The corresponding difference pattern (black line) and magnetic structure refinement for the three best magnetic-structure models (colored lines). The most prominent magnetic reflections are marked by arrows at the bottom. The magnetic-structure models corresponding to the (c) $\psi_1^1$, (d) $\psi_2^2$, and (e) $\psi_3^3$ basis vectors. The main intrachain interactions along the $b$ axis are marked by a dark gray line. The main interchain interactions within the $bc$ plane and those pointing out-of-plane are shown in red and blue tint, respectively. The light gray box denotes the crystal unit cell.
• Figure 4: The crystal structure of AgCuVO$_4$ for two different orientations (a) and (b), respectively. The atoms are represented by colored spheres, i.e., Cu (brown), V (blue), and Ag (light gray), while O atoms positioned at the corners of CuO$_4$ (brown) and VO$_3$ (green) units are skipped for clarity. The light yellow color represent the area, where the mean nuclear magnetic field equals 0.35 mT, while other colors represent areas, where magnetic field equals 1.1 mT for $\psi_1^1$ (green), for $\psi_2^2$ (blue), and for $\psi_3^3$ (red). The latter two are very narrow and almost too small to be seen.