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 Cu${}_4$(OH)${}_6$FBr. Magnetic susceptibility gives ${\theta }_{CW}$ = $-$136 K, while long-range order does not happen until $T_N$ = 15 K with a weak ferromagnetic moment $\mu$ $<$ 0.1${\mu }_B$/Cu. A 60 T magnetic field induces a moment less than 0.5${\mu }_B$/Cu at $T$ = 0.6 K. Specific-heat measurements have observed multiple phase transitions at $T\ll$ $\mid$${\theta }_{CW}$$\mid$. The magnetic entropy of these transitions is merely 18% of $k_B$ln2 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.