Low temperature magnetization of the $S=\frac{1}{2}$ kagome antiferromagnet $\mathrm{Zn}{\mathrm{Cu}}_3{\left(\mathrm{O}\mathrm{H}\right)}_6{\mathrm{Cl}}_2$

The dc magnetization of the unique $S=\frac{1}{2}$ kagome antiferromagnet Herbertsmithite has been measured down to $0.1\mathrm{K}$. No sign of spin freezing is observed in agreement with former $\mu \mathit{SR}$ and ac-susceptibility results. The low temperature magnetic response is dominated by a defect contribution which exhibits an energy scale $\simeq 1\mathrm{K}$, likely reflecting the coupling of the defects. The defect component is saturated at low temperature by $H\gtrsim 8\mathrm{T}$ applied magnetic fields which enables us to estimate an upper bound for the nonsaturated intrinsic kagome susceptibility at $T=1.7\mathrm{K}$.

Figure 1

Molar dc susceptibility of Herbertsmithite versus temperature in a $0.1\mathrm{T}$ external field on a log-log plot. The local susceptibility measured by a ${}^{35}\mathrm{Cl}$ NMR line shift falls in the shaded area (Ref. 15). The dashed line is a Curie Weiss fit for $1.5\mathrm{K}<T<3\mathrm{K}$ (see text). Inset: $T$-dependence of the inverse of the susceptibility at low temperature.

Figure 2

(Color online) $M∕H$ as a function of temperature for different $H$. Inset: $M$ normalized by the saturated magnetization of $1\mathrm{mol}$ of $S=\frac{1}{2}$ spins $\left({\mathrm{Cu}}^{2+}\right)$ $M_{\mathit{sat}}=5583\mathrm{emu}$, at $0.2\mathrm{K}$ as a function of $H$. The data are extracted from the temperature scans of the main panel. The straight line is a plot of $\chi \left(0.2\mathrm{K}\right)H$.

Figure 3

(Color online) Normalized magnetization $(M_{\mathit{sat}}=5583\mathrm{emu})$ measured versus field in a VSM for three characteristic temperatures. The magnetization data at $0.2\mathrm{K}$ are also reported from the inset of Fig. 2. In the inset, for the same temperatures, $M-{\chi }_{i}H$ versus $H∕(T+\theta )$ with ${\chi }_i=1.25\times {10}^{-3}{\mathrm{cm}}^3∕\mathrm{mol}$ Cu and $\theta =1.3\mathrm{K}$.

Figure 4

(Color online) Closed squares: normalized magnetization of Herbertsmithite measured at $1.7\mathrm{K}$ versus field. The solid line is a linear fit of the data for $H>10\mathrm{T}$ which likely reflects the nonsaturated intrinsic susceptibility ${\chi }_i$. Open squares: the defect contribution obtained by subtracting the above linear contribution from the closed square magnetization curve. Dashed line: Brillouin function for 7.7% of free $S=1∕2$ spins. Inset: the shaded area on the SQUID $\chi \left(T\right)$ plot represents the possible values of ${\chi }_i$ from this study.