Field-induced internal Fe and Ln spin reorientation in butterfly {Fe(3)LnO(2)} (Ln = Dy and Gd) single-molecule magnets.

The intramolecular exchange interactions within the single-molecule magnet (SMM) “butterfly” molecule [Fe(3)Ln(mu(3)-O)(2)(CCl3COO)(8)(H2O)(THF)(3)], where Ln(III) represents a lanthanide cation, are determined in a combined experimental [x-ray magnetic circular dichroism (XMCD) and vibrating sample magnetometer (VSM)] and theoretical work. Compounds with Ln = Gd and Dy, which represent extreme cases where the rare earth presents single-ion isotropic and uniaxial anisotropy, on one hand, and with Ln = Lu and Y(III) as pseudolanthanide substitutions that supply a nonmagnetic Ln reference case, on the other hand, are studied. The Dy single-ion uniaxial anisotropy is estimated from ab initio calculations. Low-temperature (T similar or equal to 2.5 K) hard x-ray XMCD at the Ln L-2,L-3 edges and VSM measurements as a function of the field indicate that the Ln moment dominates the polarization of the molecule by the applied field. Within the {Fe(3)LnO(2)} cluster the Ln-Fe-3 subcluster interaction is determined to be antiferromagnetic in both Dy and Gd compounds, with values J(Dy-Fe3) = -0.4 K and J(Gd-Fe3) = -0.25 K, by fitting to spin Hamiltonian simulations that consider the competing effects of intracluster interactions and the external applied magnetic field. In the uniaxial anisotropic {Fe3DyO2} case, a field-induced reorientation of the Fe-3 and Dy spins from an antiparallel to a parallel orientation takes place at a threshold field (mu H-0 = 4 T). In contrast, in isotropic {Fe3GdO2} this reorientation does not occur.