Slow relaxation in a one-dimensional rational assembly of antiferromagnetically-coupled [Mn4] single-molecule-magnets

Four discrete MnIII/MnII tetra-nuclear complexes with double-cuboidal core were synthesized. dc magnetic measurements show that both Mn2+ - Mn3+ and Mn3+ - Mn3+ magnetic interactions are ferromagnetic in three samples leading to an S = 9 ground state for the Mn4 unit. Furthermore, these complexes are Single-Molecule Magnets (SMMs) clearly showing both thermally activated and ground state tunneling regimes. Slight changes in the [Mn4] core geometry result in an S = 1 ground state in fourth sample. A one-dimensional assembly of [Mn4] units was obtained in the same synthetic conditions with the subsequent addition of NaN3. Double chair-like N3- bridges connect identical [Mn4] units into a chain arrangement. This material behaves as an Ising assembly of S = 9 tetramers weakly antiferromagnetically coupled. Slow relaxation of the magnetization is observed at low temperature for the first time in an antiferromagnetic chain, following an activated behavior with 47 K and tau_0 = 7x10^-11 s. The observation of this original thermally activated relaxation process is induced by finite-size effects and in particular by the non-compensation of spins in segments of odd-number units. Generalizing the known theories on the dynamic properties of poly-disperse finite segments of antiferromagnetically coupled Ising spins, the theoretical expression of the characteristic energy gaps were estimated and successfully compared to the experimental values.