Antiferromagnetic ordering in single-crystal Cu₃B₂O₆

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