Tuning the S=1/2 square-lattice antiferromagnet Sr2Cu(Te1-xWx)O6 from Neel order to quantum disorder to columnar order

The spin-1/2 square-lattice Heisenberg model is predicted to have a quantum disordered ground state when magnetic frustration is maximized by competing nearest-neighbor J(1) and next-nearest-neighbor J(2) interactions (J(2)/J(1 )approximate to 0.5). The double perovskites Sr2CuTeO6 and Sr2CuWO6 are isostructural spin-1/2 square-lattice antiferromagnets with Neel (J(1) dominates) and columnar (J(2) dominates) magnetic order, respectively. Here we characterize the full isostructural solid solution series Sr2Cu(Te1-xWx)O-6 (0 <= x <= 1) tunable from Neel order to quantum disorder to columnar order. A spin-liquid-like ground state was previously observed for the x = 0.5 phase, but we show that the magnetic order is suppressed below 1.5 K in a much wider region of x approximate to 0.1-0.6. This coincides with significant T-linear terms in the low-temperature specific heat. However, density functional theory calculations predict most of the materials are not in the highly frustrated J(2)/J(1 )approximate to 0.5 region square-lattice Heisenberg model. Thus, a combination of both magnetic frustration and quenched disorder is the likely origin of the spin-liquid-like state in x = 0.5.