Perfect kagome-lattice antiferromagnets with J${}_{eff}$ = 1/2: The Co${}^{2+}$-analogs of copper minerals volborthite and vesignieite

We report the synthesis, crystal structure, and magnetic properties of Co${}^{2+}$ kagome magnets Co${}_3$V${}_2$O${}_7$(OH)${}_2$$\cdot$2H${}_2$O and BaCo${}_3$(VO${}_4$)${}_2$(OH)${}_2$, which can be recognized as Co-analogues of the intensively researched quantum kagome magnet volborthite Cu${}_3$V${}_2$O${}_7$(OH)${}_2$$\cdot$2H${}_2$O and vesignieite BaCu${}_3$(VO${}_4$)${}_2$(OH)${}_2$. For each compound, the ground state is seemingly A-type antiferromagnetic order. At low temperatures, applying a magnetic field causes a metamagnetic-like transition described by the transition in which antiferromagnetically-aligned canted moments change to ferromagnetically-aligned ones. These ground and field-induced states include a canted ferromagnetic component perpendicular to the kagome planes favored by Dzyaloshinskii-Moriya interactions. These magnetic properties are well characterized by the J${}_{eff}$ = 1/2 physics. Our findings will be the first step toward clarifying the J${}_{eff}$ = 1/2 kagome physics, which has been little studied experimentally or theoretically.