Perfect kagome-lattice antiferromagnets with Jeff=12 : The Co2+ analogs of the copper minerals volborthite and vesignieite

We report the synthesis, crystal structure, and magnetic properties of ${\mathrm{Co}}²⁺$ kagome magnets ${\mathrm{Co}}₃{\mathrm{V}}₂{\mathrm{O}}₇{(\mathrm{OH})}₂\ifmmode\cdot\else\textperiodcentered\fi{}2{\mathrm{H}}₂\mathrm{O}$ and ${\mathrm{BaCo}}₃$(${\mathrm{VO}}₄{)}₂{(\mathrm{OH})}₂$, which can be recognized as Co analogs of the intensively researched quantum kagome magnet volborthite ${\mathrm{Cu}}₃{\mathrm{V}}₂{\mathrm{O}}₇{(\mathrm{OH})}₂\ifmmode\cdot\else\textperiodcentered\fi{}2{\mathrm{H}}₂\mathrm{O}$ and vesignieite ${\mathrm{BaCu}}₃$(${\mathrm{VO}}₄{)}₂{(\mathrm{OH})}₂$. For each compound, the ground state is seemingly $A$-type antiferromagnetic order. At low temperatures, applying a magnetic field causes a metamagneticlike 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 ${J}_{\mathrm{eff}}=\frac{1}{2}$ physics. Our findings will be a step toward clarifying the ${J}_{\mathrm{eff}}=\frac{1}{2}$ kagome physics, which has not been thoroughly studied experimentally or theoretically.