Static and dynamic properties of the frustrated spin-1/2 depleted-kagome antiferromagnet Cu$_7$(TeO$_3$)$_2$(SO$_4$)$_2$(OH)$_6$

TL;DR

The paper investigates the frustrated spin-1/2 depleted-kagome magnet CTSOH using X-ray diffraction, magnetization, heat capacity, and 1H NMR. It finds dominant antiferromagnetic exchange with a large negative Curie-Weiss temperature, estimates J/k_B ≈ 66 K, and identifies a highly frustrated system with f_r ≈ 12.5 and a possible canting-type long-range order at T^* ≈ 4 K, evidenced by a peak in 1/T1 and a bifurcation in DC susceptibility. Heat capacity shows a broad, defect-influenced feature at T^*, shifting with applied field due to Schottky effects, while magnetic entropy is substantially reduced, suggesting partial disorder. 1H NMR reveals three inequivalent H sites with transferred hyperfine couplings and a temperature- and field-dependent evolution of line shapes and relaxation, indicating static spin correlations and complex spin dynamics in the ordered state. Overall, CTSOH emerges as a promising platform to explore frustration-driven phenomena in depleted kagome systems, with neutron diffraction needed to pin down the exact magnetic structure.

Abstract

The structural and magnetic properties of the two-dimensional spin-$1/2$ depleted-kagome compound Cu$_7$(TeO$_3$)$_2$(SO$_4$)$_2$(OH)$_6$ are investigated using x-ray diffraction, magnetization, heat capacity, and $^1$H Nuclear Magnetic Resonance (NMR) measurements. From the analysis of magnetic susceptibility, we found a large Curie-Weiss temperature [$θ_{\rm CW} = -50(2)$ K] and the co-existence of antiferromagnetic and ferromagnetic interactions. The value of $θ_{\rm CW}$ gives an estimate of the average nearest-neighbour antiferromagnetic interaction of $J/k_{\rm B} \simeq 66$ K. The NMR relaxation rates ($1/T_1$ and $1/T_2$) exhibit a peak, providing evidence for a magnetic long-range order at $T^*\simeq 4$ K which appears to be canted antiferromagnetic type. Heat capacity also features a broad maximum at $T^*$ that moves towards higher temperatures with increasing magnetic field, reflecting defect induced Schottky anomaly. The frustration parameter $f_r = \lvert θ_{\rm CW} \lvert/{T^{*}}\simeq 12.5$ renders the compound a highly frustrated low-dimensional magnet.

Figures (10)

• Figure 1: (a) Three-dimensional view of the crystal structure of CTSOH featuring Cu$^{2+}$ polyhedras (blue) and other atoms. (b) A layer of the depleted-kagome lattice formed by Cu$^{2+}$ ions. The hollow circles represent the depleted sites. (c) The smallest repeating unit in the depleted kagome layer, highlighting the interaction pathways. The corresponding bond angles and bond distances are tabulated in Table \ref{['TableI']}.
• Figure 2: Room temperature powder XRD pattern of CTSOH. The open circles represent the experimental data and the black solid line is the Rietveld fit. Bragg peaks are shown as vertical bars and the difference between the experimental and Rietveld fit is shown as a solid line at the bottom.
• Figure 3: (a) $\chi$ vs $T$ in different fields. Upper inset: $\chi(T)$ measured in ZFC and FC conditions in a small field of $\mu_0H = 0.01$ T. Lower inset: $\chi T$ vs $T$ for $\mu_0 H = 0.1$ T. (b) $1/\chi$ vs $T$ for $\mu_0 H = 0.5$ T. The solid line is the Curie Weiss fit. Upper inset: A complete $M$ vs $H$ isotherm at $T = 0.4$ K in the low field regime. Lower inset: Magnetic isotherm at $T = 0.4$ K from 0 to 7 T.
• Figure 4: (a) Real part ($\chi^{\prime}$) and (b) imaginary part ($\chi^{\prime\prime}$) of the AC susceptibility as a function of $T$, measured at different frequencies from 99 Hz to 9999 Hz. The data sets in different frequencies are vertically offset for clarity.
• Figure 5: (a) Zero-field $C_{\rm p}$ vs $T$ along with the Debye-Einstein fit (solid line). The dashed line represents the magnetic heat capacity ($C_{\rm mag}$). Inset: $C_{\rm p}(T)$ around the low temperature anomaly, in different magnetic fields. (b) $C_{\rm mag}/T$ and normalized magnetic entropy ($S_{\rm mag}$/Rln2) vs $T$ on the left and right $y$-axes, respectively. Inset: Zero-field $C_{\rm mag}$ vs $T$ in the low temperature regime and the solid line is a power-law fit.