Magnetic and thermal properties of 4f-3d ladder-type molecular compounds

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We report on the low-temperature magnetic susceptibilities and specific heats of the isostructural spin-ladder molecular complexes L₂[M(opba)]3· xDMSO· yH₂O, hereafter abbreviated with 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 TC 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.