Two- and three-dimensional magnetic correlations of Tb

The magnetic order of the Tb ions in polycrystalline (Formula presented)(Formula presented)(Formula presented)(Formula presented) has been studied by neutron-diffraction and ac-susceptibility measurements. Magnetic correlations are observed to develop below T≊9 K, and the correlation length diverges at (Formula presented)=5.3 K. At high temperatures the in-phase component of the ac susceptibility χ′ 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 (Formula presented) 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 dχ′/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 ν agrees very well with ν=1 for the S=1/2 2D Ising model.