Antiferromagnetic ordering in single-crystal Cu3B2O6

We have measured the temperature dependences of the magnetic susceptibility in a magnetic field of 1T and the specific heat in magnetic fields of 0-9 T for Cu3B2O6 Single crystals grown by the Floating Zone method. It has bee n found that a three-dimensional antiferromagnetic order with the spin dire ction parallel to the be plane appears at low temperatures below 10 K. The spin entropy is much smaller than the value of the free-spin entropy at tem peratures immediately above 10 K and recovers to the value of the free-spin entropy at approximately 50 K. Accordingly, we conclude that the two-dimen sional magnetic correlation develops with decreasing temperature at low tem peratures below similar to 50K, leading to the three-dimensional antiferrom agnetic phase transition at 10 K.