Unique magnetic transition process demonstrating the effectiveness of bond percolation theory in a quantum magnet.

Zheng, Xu-Guang; Yamauchi, Ichihiro; Hagihala, Masato; Nishibori, Eiji; Kawae, Tatsuya; Watanabe, Isao; Uchiyama, Tomoki; Chen, Ying et al. · Nat Commun · 2024

basic_science · Level V

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Abstract

Like the crystallization of water to ice, magnetic transition occurs at a critical temperature after the slowing down of dynamically fluctuating short-range correlated spins. Here, we report a unique type of magnetic transition characterized by a linear increase in the volume fraction of unconventional static short-range-ordered spin clusters, which triggered a transition into a long-range order at a threshold fraction perfectly matching the bond percolation theory in a new quantum antiferromagnet of pseudo-trigonal Cu<sub>4</sub>(OH)<sub>6</sub>Cl<sub>2</sub>. Static short-range order appeared in its Kagome lattice plane below ca. 20 K from a pool of coexisting spin liquid, linearly increasing its fraction to 0.492(8), then all Kagome spins transitioned into a stable two-dimensional spin order at T<sub>N</sub> = 5.5 K. Inspection on the magnetic interactions and quantum magnetism revealed an intrinsic link to the spin liquid material Herbertsmithite, ZnCu<sub>3</sub>(OH)<sub>6</sub>Cl<sub>2</sub>. The unconventional static nature of the short-range order was inferred to be due to a pinning effect by the strongly correlated coexisting spin liquids. This work presents a unique magnetic system to demonstrate a complete bond percolation process toward the critical transition. Meanwhile, the unconventionally developed magnetic order in this chemically clean system should shed new light on spin-liquid physics.