Fig. 1

Spin-1/2 Heisenberg square-lattice model in Sr₂CuTeO₆ and Sr₂CuWO₆. a Phase diagram of the J₁–J₂ square-lattice model. J₁ is the nearest-neighbor interaction and J₂ the next-nearest-neighbor interaction. The classical ground states are ferromagnetic (FM), Néel antiferromagnetic (NAF) and columnar antiferromagnetic (CAF) ordering. The highly frustrated J₂/J₁ ≈ 0.5 and J₂/J₁ ≈ –0.5 regions are located at the NAF–CAF and CAF–FM boundaries, respectively. b The double perovskite structure of Sr₂CuTeO₆ and Sr₂CuWO₆. Sr, Cu, B” (Te/W) and O are represented by green, blue, dark yellow and red spheres, respectively. The blue (dark yellow) octahedra represent CuO₆ (B”O₆). c The Néel antiferromagnetic structure of Sr₂CuTeO₆ and the columnar antiferromagnetic structure of Sr₂CuWO₆ with the view down the c-axis. The dominant antiferromagnetic interactions are shown