Magnetic and thermal properties of 4f-3d ladder-type molecular compounds -: art. no. 184405

We report on the low-temperature magnetic susceptibilities and specific heats of the isostructural spin-ladder molecular complexes L-2[M(opba)](3).xDMSO.yH(2)O, hereafter abbreviated by L2M3 (where L=La, Gd, Tb, Dy, Ho and M=Cu, Zn). The results show that the Cu containing complexes (with the exception of La2Cu3) undergo long-range magnetic order at temperatures below 2 K, and that for Gd2Cu3 this ordering is ferromagnetic, whereas for Tb2Cu3 and Dy2Cu3 it is probably antiferromagnetic. The susceptibilities and specific heats of Tb2Cu3 and Dy2Cu3 above T-C have been explained by means of a model taking into account nearest-as well as next-nearest-neighbor magnetic interactions. We show that the intraladder L-Cu interaction is the predominant one and that it is ferromagnetic for L=Gd, Tb, and Dy. For the cases of Tb, Dy, and Ho containing complexes, strong crystal field effects on the magnetic and thermal properties have to be taken into account. The magnetic coupling between the (ferromagnetic) ladders is found to be very weak and is probably of dipolar origin.