Neutron diffraction and AC susceptibility measurements have been performed to study the magnetic ordering of the Pr spins in PrBa2Cu4O8. An anomaly at 15 K, which we believe is associated with the ordering of the Pr spins, is clearly seen in the temperature dependence of the AC susceptibility, chi'(T) The coupling between the Pr spins, as revealed by neutrons, is antiferromagnetic in nature, with a spin structure may be characterized by the {1/2 1/2 1} wave vector. Three-dimensional coupling between the Pr spins is observed, despite the appearance of exact cancellations of the coupling between the adjacent layers along the c-axis due to geometric symmetry. (C) 1998 Elsevier Science B.V. All rights reserved.
