${}^{63}\mathrm{Cu}$, ${}^{35}\mathrm{Cl}$, and ${}^1\mathrm{H}$ NMR in the $S=\frac{1}{2}$ Kagome Lattice ${\mathrm{ZnCu}}_3(\mathrm{OH}{)}_6{\mathrm{Cl}}_2$

${\mathrm{ZnCu}}_3(\mathrm{OH}{)}_6{\mathrm{Cl}}_2$ ($S=\frac{1}{2}$) is a promising new candidate for an ideal Kagome Heisenberg antiferromagnet, because there is no magnetic phase transition down to $\sim 50\mathrm{mK}$. We investigated its local magnetic and lattice environments with NMR techniques. We demonstrate that the intrinsic local spin susceptibility decreases toward $T=0$, but that slow freezing of the lattice near $\sim 50\mathrm{K}$, presumably associated with OH bonds, contributes to a large increase of local spin susceptibility and its distribution. Spin dynamics near $T=0$ obey a power-law behavior in high magnetic fields.

Figure 1

Left: A Kagome lattice. Right: ${\mathrm{Cu}}^{2+}$ Kagome layer in ${\mathrm{ZnCu}}_3(\mathrm{OH}{)}_6{\mathrm{Cl}}_2$. ${\mathrm{Cl}}^{-}$ site is above or below the middle of the ${\mathrm{Cu}}^{2+}$ triangles.

Figure 2

${}^{35}\mathrm{Cl}$ NMR line shapes of the $I_z=+\frac{1}{2}$ to $-\frac{1}{2}$ central transition in 8.4 Tesla in a partially ($\sim 20\%$) uniaxially aligned powder sample. The sharp peak near 35.02 MHz marked as “$B//c$” originates from the particles whose $c$ axis is aligned along the external magnetic field. Vertical lines specify the $c$-axis peak and edge corresponding to ${}^{35}K$. Vertical dashed lines specify the frequency corresponding to ${}^{35}K_{1/2}$.

Figure 3

${}^{35}\mathrm{Cl}$ NMR Knight shift (left scale). ${}^{35}K$ (blue circles). ${}^{35}K_{1/2}$ (red triangles). The blue solid line is a fit to Curie-Weiss behavior, ${}^{35}K=(22\pm 7)/(T-{\theta }_{\mathrm{CW}})+{}^{35}K_{\mathrm{chem}}$, where ${\theta }_{\mathrm{CW}}=-240\pm 80\mathrm{K}$. The constant background ${}^{35}K_{\mathrm{chem}}=0.018\%$ is shown by a dashed line. ${\chi }_{\mathrm{bulk}}$ measured by SQUID (dotted line) is also overlaid (right scale).

Figure 4

Temperature dependence of ${}^{35}\mathrm{Cl}$ NMR spin-lattice relaxation rate ${}^{35}(1/T_1)$ at various magnetic fields (filled symbols). Solid line represents a fit to a power law, ${}^{35}(1/T_1)=T^{\eta }$ with $\eta =0.47$ (8.3 T), 0.44 (4.4 T), 0.2 (2.4 T and 1.0 T). ${}^1\mathrm{H}$ relaxation rate in low field (0.9 T), ${}^1(1/T_1)$, and ${}^{63}\mathrm{Cu}$ relaxation rate in high field (8 T), ${}^{63}(1/T_1)$, are also superposed on ${}^{35}(1/T_1)$ measured in comparable magnetic field.