Dedicated to Professor Dante Gatteschi on the occasion of his 60th birthdayThe study of paramagnetic metal-ion aggregates has been of increasing interest since the observation that such molecules can exhibit magnetic memory effects. [1][2][3] Termed singlemolecule magnets or SMMs, the important factors leading to such properties derive from the combination of a large ground-state spin and a large magnetic anisotropy of the Ising (easy-axis) type. Studies have largely been based on transition-metal compounds since they typically exhibit both of the aforementioned features. The incorporation of lanthanides into these complexes has been investigated to take advantage of the potentially large number of unpaired f-electrons available. [4][5][6][7] However, very little work has been done to date on purely lanthanide-based systems. [8,9] The origin of SMM behavior in lanthanide-containing compounds is more complicated than that of d-block transition-metal ions since there is likely to be a significant orbital component. In the lanthanide-containing phthalocyanine complexes reported in the literature the ligand environment induces a large splitting of the ground J manifold, whereas in SMMs large-spin ground states arising from magnetic interactions between the metal centers of the cluster can enhance the weaker single-ion

The high axiality and Ising exchange interaction efficiently suppress quantum tunneling of magnetization of an asymmetric dinuclear Dy(III) complex, as revealed by combined experimental and theoretical investigations. Two distinct regimes of blockage of magnetization, one originating from the blockage at individual Dy sites and the other due to the exchange interaction between the sites, are separated for the first time. The latter contribution is found to be crucial, allowing an increase of the relaxation time by 3 orders of magnitude.

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