Two-dimensional magnetic order in Pb₂sr₂tbcu ₃o₈

Neutron diffraction techniques have been used to investigate the magnetic ordering of the Tb ions in polycrystalline Pb₂Sr₂TbCu ₃O₈. Significant magnetic correlations are found to develop below ∼10 K, and these correlations are two dimensional in nature, representative of the strongly anisotropic magnetic interactions in these layered materials. The correlation length increases with decreasing temperature, and long-range order is observed to develop at T_N≊5.5 K. However, the sawtooth Bragg profile for the scattering demonstrates that the long-range order is two dimensional, rather than three dimensional, with a spin configuration where nearest-neighbor spins are antiparallel. The scattering profile can be explained quantitatively by assuming long-range order within the ab plane, with no significant correlations along the c axis to the lowest temperatures measured (1.36 K). Our results suggest that the system is well described by a two-dimensional Ising model. The only other (pure) system where a crossover to three-dimensional behavior is not observed is DyBa ₂Cu₄O₈.