The magnetic order of the Tb ions in polycrystalline Pb(2)Sr(2)TbCu3O(8) has been studied by neutron-diffraction and ac-susceptibility measurements. Magnetic correlations are observed to develop below T approximate to 9 K, and the correlation length diverges at T-N=5.3 K. At high temperatures the in-phase component of the ac susceptibility chi' exhibits Curie-Weiss behavior, while it departs from this behavior as the magnetic correlations between the Tb ions develop, and exhibits a cusp at lower temperatures characteristic of antiferromagnetic ordering. The T-N determined from the critical behavior of the correlation length measured by neutron diffraction is in good agreement with that determined by the relative maximum in d chi'/dT. Both two-dimensional (2D) and three-dimensional Bragg peaks are observed, but the magnetic correlations are predominantly 2D in nature. The sublattice magnetization drops sharply as the critical point is approached, as is typical for low-dimensional systems, and the value of 1.00(5) obtained for the critical exponent nu agrees very well with nu=1 for the S=1/2 2D Ising model.
