Role of dipolar interactions in three-dimensional magnetic ordering of chain compounds with very large interchain spacing - art. no. 134418

A model that takes into account the dipolar interaction between quantum spi n chains is proposed to explain the occurrence of three-dimensional (3D) or dering in magnetic chain compounds, where superexchange interaction can be neglected because of large interchain spacing. The divergence of the in-cha in correlation length at low temperature promotes correlated spin blocks th at can interact from chain to chain through sizeable dipole-dipole coupling . Using a modified mean-field approach we show that the strength of this in teraction is large enough to induce 3D magnetic ordering. The model is appl ied to the Mn-porphyrin-based magnets exhibiting ferrimagnetic chains well separated in space (up to 30 Angstrom apart). Different ground-state spin a lignments are analyzed. In the framework of the proposed approach, the orde ring temperatures that are calculated compare well with the experimental fi ndings. It is shown that the rate of increase of the correlation length at low temperature is the essential parameter in order to reach the observed t ransition temperatures. Indeed an exponential divergence of the 1D correlat ion length is required, which implies the existence of single-ion anisotrop y, whereas the power-law divergence for 1D Heisenberg coupled spins yields transition temperatures one order of magnitude smaller than observed.