Barlowite: A Spin-$1/2$ Antiferromagnet with a Geometrically Perfect Kagome Motif

We present thermodynamic studies of a new spin-$1/2$ antiferromagnet containing undistorted kagome lattices—barlowite ${\mathrm{Cu}}_4(\mathrm{OH}{)}_6\mathrm{FBr}$. Magnetic susceptibility gives ${\theta }_{\mathrm{CW}}=-136\mathrm{K}$, while long-range order does not happen until $T_N=15\mathrm{K}$ with a weak ferromagnetic moment $\mu <0.1{\mu }_B/\mathrm{Cu}$. A 60 T magnetic field induces a moment less than $0.5{\mu }_B/\mathrm{Cu}$ at $T=0.6\mathrm{K}$. Specific-heat measurements have observed multiple phase transitions at $T\ll \mid {\theta }_{\mathrm{CW}}\mid$. The magnetic entropy of these transitions is merely 18% of $k_B\ln 2$ per Cu spin. These observations suggest that nontrivial spin textures are realized in barlowite with magnetic frustration. Comparing with the leading spin-liquid candidate herbertsmithite, the superior interkagome environment of barlowite sheds light on new spin-liquid compounds with minimum disorder. The robust perfect geometry of the kagome lattice makes charge doping promising.

Figure 1

Crystal structure of barlowite when looking perpendicular (a) and almost parallel (b) to the kagome lattice. The symbols are kagome Cu1, blue; interkagome Cu2, purple; O, red; H, salmon; F, green; Br, brown. In (a), the thick black lines denote Cu-Cu grids. (c) Magnetic susceptibility of polycrystalline (explained in the text) barlowite measured at ${\mu }_{0}H=0.1\mathrm{T}$. The data are compared with calculations [17] as described in the text. Inset: Inverse susceptibility and a Curie-Weiss fit. (d) Magnetization versus field measured at liquid-helium temperatures. Inset: Hysteresis loop at ${\mu }_{0}H<0.08\mathrm{T}$.

Figure 2

(a) Magnetization versus temperature measured after cooling from room temperature in zero field (ZFC) and a field (FC). (b) Negative derivative of the temperature dependence of magnetization in the proximity of $T_N$. (c) Power-law fitting to the spontaneous magnetization.

Figure 3

(a) Specific heat measured on a single crystal sample of barlowite at fields parallel to the kagome lattice and compared with zero-field specific heat of herbertsmithite. (b) Magnetic specific heat after subtracting the background. (c) Magnetic entropy integrated from $T=2\mathrm{K}$ and normalized as a fraction of the total value per Cu spin.