Dimensional crossover in the XY-compound YBa2Cu3O6+x

The magnetic ordering of the bilayered Heisenberg antiferromagnet YBa2Cu3O6+x with weak XY-anisotropy was investigated for temperatures close to the magnetic transition temperature TN. We find that the magnitude of the sublattice magnetization is well described by a power law ∝εβ[ε=(TN-T)/TN] with critical exponent β2D=0.26(1) in the region 5 x 10-2>ε>5 x 10-3 and with β3D=0.35(5) in the region 5 x 10-3>ε>5 x 10-4, reflecting a dimensional crossover at T∼0.995TN. We present results which suggest the presence of the magnetic reflections (1/2h,1/2h,0), indicative of a small component of the electron moments perpendicular to the ab-plane. (orig.)