Non-analytic magnetic response and intrinsic ferromagnetic clusters in a Dirac spin liquid candidate on the kagome lattice

The definitive identification of a quantum spin liquid (QSL) state is complicated due to many reasons. Here kagome materials are a promising playground. In the last few years the 2D kagome system YCu₃(OH)₆[(Cl_xBr(1−x))_3−y(OH)_y] (YCOB-Cl), has emerged as one of the leading candidates hosting a Dirac spin liquid state. In this system the vast mismatch in size of Y and Cu prevents subsitutional disorder, which has plagued other kagome systems such as Herbertsmithite. In YCOB-Cl crystals with x < 0.4 there is no long range order. In such crystals we report an unusual field dependent magnetization M(B), where M/B changes linearly with |B|, the absolute value of the field, in contrast to the expected quadratic behavior. We also present model calculations with a distribution of ferromagnetic (FM) clusters which faithfully capture the observed features. Through heat capacity measurements in a magnetic field it has been shown that YCOB-Cl exhibits a T² heat capacity expected in certain QSL models. However, a concomitant linear T behavior in the spin susceptibility has been lacking. Our work in addition to revealing a novel non-analytic magnetic response suggests that FM clusters by dominate the contribution to the susceptibility and mask the underlying behavior.