Finding distinct signatures of a quantum spin liquid (QSL) is an ongoing quest in condensed matter physics, invariably complicated by the presence of disorder in real materials. In this regard the two-dimensional kagome system ${\mathrm{YCu}}_3{\left(\mathrm{OH}\right)}_6\left[{\left({\mathrm{Cl}}_x{\mathrm{Br}}_{(1-x)}\right)}_{3-y}{\left(\mathrm{OH}\right)}_y\right]$ $\left(\mathrm{YCOB}\text{−}\mathrm{Cl}\right)$, where the vast mismatch in size of Y and Cu avoids substitutional disorder, otherwise present in kagome materials, has emerged as a favorable candidate. In crystals of this system, with $x<0.4$ and no long-range order, we report an unusual field-dependent magnetization $M\left(B\right)$, where $M/B$ changes linearly with $\left|B\right|$, the absolute value of the field, in contrast to the expected quadratic behavior. Model calculations with a distribution of ferromagnetic (FM) clusters faithfully capture the observed features. YCOB-Cl exhibits a field-enhanced $T^2$ heat capacity expected in certain spin-liquid models. However, a concomitant linear $T$ behavior in the spin susceptibility has been lacking. Our work suggests that FM clusters, by dominating the contribution to the susceptibility, mask the true underlying behavior.

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• Exotic phases of matter
• Fractionalization
• Magnetic order
• Magnetic susceptibility
• Methods in magnetism
• Quantum phase transitions
• Spin dynamics
• Spinon