  We report the magnetic susceptibility and the low-temperature specific heat
of a double perovskite compound, Sr2CuTeO6, which is expected as the spin-1/2
square-lattice Heisenberg antiferromagnet with nearest-neighbor J1 and
next-nearest-neighbor J2 exchange interactions. The specific heat exhibits a
sharp lambda-like anomaly at TN = 4.8 K indicative of magnetic ordering, which
is much lower than those in isostructural A2CuMO6 with A = Ba, Sr and M = W,
Mo. This indicates the strong suppression of magnetic ordering in Sr2CuTeO6. We
estimate the exchange constant J1/kB ~ 80 K and J2/J1 < 0.07 from magnetic
susceptibility data using Pade approximation of high temperature expansion and
quantum Monte Carlo method. The strong suppression of magnetic ordering in
Sr2CuTeO6 should be ascribed to the weekness of the interlayer exchange.
