Disorder-induced spin-cluster magnetism in a doped kagome spin liquid candidate

Abstract: The search for new quantum spin liquid materials relies on systems with strong frustration such as spins on an ideal kagome lattice. However, lattice imperfections can have substantial effects which are as yet not well understood. In recent work the 2D kagome system YCu$_3$(OH)$_6$(Cl$x$Br${(1-x)}$)$_{3-y}$(OH)$_y$ has emerged as a leading candidate hosting a Dirac spin liquid which appears to survive at least for x<0.4, associated with alternating-bond hexagon (ABH) disorder. Here in samples with x=0.58, y=0.1 we report unusual in-plane ferromagnetic canting (FM) of the in-plane antiferromagnet with moreover an expanded regime of short-ranged order, and propose a theoretical model to explain this behavior. We show that Kitaev type exchanges naturally arise on the kagome lattice to second order in the known Dzyaloshinskii-Moriya exchanges, and that these interactions can produce the unusual in-plane FM moments. We propose a spin cluster phenomenological model to describe the short-ranged regime and analyze quantum fluctuations in a toy model to show how ABH disorder helps stabilize this regime. The combination of experimental observation and theory suggests that kagome-Kitaev interactions and ABH disorder are necessary for describing the magnetic fluctuations in this family of materials, with potential implications for the proposed proximate spin liquid phase.