Evolution of the 1/9 magnetization plateau in the kagome system YCu3⁢(OH)6⁢[(Cl𝑥⁢Br1−𝑥)3−𝑦⁢(OH)𝑦]

Abstract- The 1/9 magnetization plateau has recently been observed in the spin-1/2 kagome Heisenberg antiferromagnet (KHA) YCu_3⁢(OH)₆⁢Br₂⁢[Br_1−𝑦⁢(OH)_𝑦] (YCOB), a promising quantum spin liquid (QSL) candidate. However, the 1/9 plateau has so far been reported only in YCOB, and its microscopic origin and low-energy nature remain under debate. In this study, we investigate the magnetic properties of the chlorine-substituted series YCu_3⁢(OH)₆⁢[(Cl_𝑥⁢Br_1−𝑥)_3−𝑦⁢(OH)_𝑦] (YCOB-Cl) and track the evolution of the 1/9 magnetization plateau across the doping series. We observe clear signatures of the 1/9 plateau at multiple nominal Cl concentrations. The temperature dependence of the plateau response evolves systematically with increasing 𝑥, consistent with a progressive suppression of low-energy spectral weight and the emergence of a possible effective gap scale. Complementary magnetic torque measurements further show that the magnetic oscillations accompanying the plateau are present only in the 𝑥=0 sample, indicating that chemical substitution strongly affects the low-energy states associated with the plateau. Taken together, our results establish YCOB-Cl as a tunable platform for exploring how chemical disorder and modified exchange networks reshape the 1/9 plateau and its associated anomalous magnetic responses.