Long-range ferromagnetism in hybrid compounds: The role of dipolar interactions

The influence of dipolar interactions, as compared with quantum through-bond exchange coupling, is discussed in hybrid organic-inorganic layered ferromagnets with large interlayer spacing. A model is proposed to explain the occurrence of 3D ferromagnetic order, with a sizeable transition temperature, despite the fact that exchange interactions between layers are negligible. In such low-dimensional systems, it is shown that the exponential divergence of the in-plane correlation length at low temperature promotes superspins within the magnetic layers which couple ferromagnetically through sizeable dipolar interactions. The model evidences the occurrence of a spontaneous magnetisation for a critical temperature that depends weakly on the distance between magnetic layers but mostly on the rate of divergence of the 2D correlation length, i.e. on the strength of the in-plane exchange coupling. The behaviour of copper(II) layered ferromagnets with an interlayer spacing ranging from 29 to 40 Angstrom and Curie temperatures ranging from 21 to 19 K, respectively, is shown to be well described by such a model. (C) 1998 Elsevier Science B.V. All rights reserved.