DIMENSIONAL CROSSOVER IN THE XY-COMPOUND YBA2CU3O6+X

The magnetic ordering of the bilayered Heisenberg antiferromagnet YBa2 Cu3O6+x with weak XY-anisotropy was investigated for temperatures clos e to the magnetic transition temperature T-N. We find that the magnitu de of the sublattice magnetization is well described by a power law si milar to epsilon(0) [epsilon = (T-N -T)/T-N] with critical exponent be ta(2D) = 0.26(1) in the region 5 x 10(-2) > epsilon > 5 x 10(-3) and w ith beta(3D) = 0.35(5) in the region 5 x 10(-3) > epsilon > 5 x 10(-4) , reflecting a dimensional crossover at T approximate to 0.995T(N). We present results which suggest the presence of the magnetic reflection s (1/2h,1/2h,0), indicative of a small component of the electron momen ts perpendicular to the ab-plane. (C) 1998 Elsevier Science B.V. All r ights reserved.