Chemically tunable quantum magnetism on the anisotropic triangular lattice in rhenium oxyhalides

The spin-1/2 Heisenberg antiferromagnet on an anisotropic triangular lattice (ATL) is an archetypal spin system hosting exotic quantum magnetism and dimensional crossover. However, the progress in experimental research on this field has been limited due to the scarcity of ideal model materials. Here, we show that rhenium oxyhalides A₃ReO₅X₂, where spin-1/2 Re⁶⁺ ions form a layered structure of ATLs, allow for flexible chemical substitution in both cation A²⁺ (A = Ca, Sr, Ba, Pb) and anion X^- (X = Cl, Br) sites, leading to seven synthesizable compounds. By combining magnetic susceptibility and high-field magnetization measurements with theoretical calculations using the orthogonalized finite-temperature Lanczos method, we find that the anisotropy <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:msup><mml:mrow><mml:mi>J</mml:mi></mml:mrow> <mml:mrow><mml:mo>'</mml:mo></mml:mrow> </mml:msup> <mml:mo>/</mml:mo> <mml:mi>J</mml:mi></mml:math> ranges from 0.25 to 0.45 depending on the chemical composition. Our findings demonstrate that A₃ReO₅X₂ is an excellent platform for realizing diverse effective spin Hamiltonians that differ in the strength of the anisotropy <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:msup><mml:mrow><mml:mi>J</mml:mi></mml:mrow> <mml:mrow><mml:mo>'</mml:mo></mml:mrow> </mml:msup> <mml:mo>/</mml:mo> <mml:mi>J</mml:mi></mml:math> as well as the relevance of perturbation terms such as the Dzyaloshinskii-Moriya interaction and interlayer exchange coupling.