Doping-induced spin reorientation in kagome magnet <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:mrow> <mml:msub> <mml:mi>TmMn</mml:mi> <mml:mn>6</mml:mn> </mml:msub> <mml:msub> <mml:mi>Sn</mml:mi> <mml:mn>6</mml:mn> </mml:msub> </mm

The kagome-lattice compounds $R{\mathrm{Mn}}_{6}{\mathrm{Sn}}_{6}$ ($R$ is a rare-earth element), where the Mn atoms form a kagome net in the basal plane, are currently attracting a great deal of attention as they have been shown to host complex magnetic textures and electronic topological states strongly sensitive to the choice of the $R$ atom. Among the magnetic $R$ atoms, ${\mathrm{TmMn}}_{6}{\mathrm{Sn}}_{6}$ orders with the easy-plane magnetization forming a complex magnetic spiral along the $c$ axis. Previous neutron studies carried out on polycrystalline samples found that Ga doping changes the magnetic anisotropy from easy-plane to easy-axis. Here we present magnetic and magnetotransport measurements on a single crystal and first-principles calculations in the doping series of ${\mathrm{TmMn}}_{6}{\mathrm{Sn}}_{6\ensuremath{-}x}{\mathrm{Ga}}_{x}$. We find that the magnetic properties are highly sensitive even to a small concentration of Ga. With minimal Ga substitution, the easy-plane anisotropy is maintained, which gradually changes to the easy-axis anisotropy with increasing Ga. We discuss these observations with respect to the effect of Ga doping on magnetocrystalline anisotropy and Tm crystal field.