Antiferromagnetism in kagome $\alpha$-Cu${}_3$Mg(OH)${}_6$Br${}_2$

The antiferromagnetism in $\alpha$-Cu${}_3$Mg(OH)${}_6$Br${}_2$ was studied by magnetic-susceptibility, specific-heat and neutron-diffraction measurements. The crystal structure consists of Cu${}^{2+}$ kagome layers with Mg${}^{2+}$ ions occupying the centers of the hexagons, separated by Br${}^{1-}$ ions. The magnetic system orders antiferromagnetically at 5.4 K with the magnetic moments aligned ferromagnetically within the kagome planes. The ordered moment is 0.94 ${\mu }_B$, suggesting little quantum and geometrical fluctuations. By comparing the magnetic and specific-heat properties with those of the haydeeite, we suggest that $\alpha$-Cu${}_3$Mg(OH)${}_6$Br${}_2$ may be described by the two-dimensional spin-$1\mathrm{/}2$ Heisenberg kagome model and is in the region of the ferromagnetic-order side of the phase diagram.