Lattice- and spin-dimensionality crossovers in a linear-chain-molecule-based ferrimagnet with weak spin anisotropy

[MnOEP][HCBD], a member of the metalloporphyrin family of donor-acceptor molecule-based magnets, consists of isolated ferrimagnetic chains of alternating S=2, [MnOEP], and s=1/2, [HCBD] units (OEP=octaethylporphyrinato and HCBD=hexacyanobutadiene). Analysis of the exchange pathways reveals an exchange along one interchain axis (Jinter) almost 3 orders of magnitude weaker than the intrachain exchange (Jintra), and a negligible exchange along the other interchain axis. From the susceptibility and magnetization data we determine Jintra and Jinter to be antiferromagnetic with values of -172 K and -0.4 K, respectively. At 19.6 K, the system undergoes both lattice and spin dimensionality crossovers in which the ferrimagnetic chains couple antiferromagnetically as two-dimensional Ising sheets. [S0163-1829(97)01625-1]