Magnetic Structure of the $S\,{=}\,1/2$ Quasi-Two-Dimensional   Square-Lattice Heisenberg Antiferromagnet Sr$_2$CuTeO$_6$
  The magnetic structure of the double perovskite compound Sr$_2$CuTeO$_6$ was determined from neutron powder diffraction data. This material is magnetically described as an $S\,{=}\,1/2$ quasi-two-dimensional square-lattice Heisenberg model with antiferromagnetic nearest-neighbor and next-nearest-neighbor interactions. Sr$_2$CuTeO$_6$ undergoes a magnetic phase transition at $T_{\rm N}\,{\simeq}\,29\,$ K. The spin structure below $T_{\rm N}$ is N\'{e}el antiferromagnetic on the square lattice, which means that the nearest-neighbor interaction ($J_1$) is stronger than the next-nearest-neighbor interaction ($J_2$), in contrast to other isostructural compounds such as Ba$_2$CuWO$_6$ and Sr$_2$CuWO$_6$, for which $|J_1|\,{<}\,|J_2|$ is realized. 
Tomoyuki Koga, Nobuyuki Kurita, Maxim Avdeev, Sergey Danilkin, Taku J.   Sato, Hidekazu Tanaka
cond-mat.str-el