The intramolecular exchange interactions within the single-molecule magnet (SMM) “butterfly” molecule [Fe${}_3$Ln(${\mu }_3$-O)${}_2$(CCl${}_3$COO)${}_8$(H${}_2$O)(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 $\mathrm{Ln}=\mathrm{Gd}$ and Dy, which represent extreme cases where the rare earth presents single-ion isotropic and uniaxial anisotropy, on one hand, and with $\mathrm{Ln}=\mathrm{Lu}$ 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 ($T\simeq 2.5$ K) hard x-ray XMCD at the Ln L${}_{2,3}$ 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 $\{{\mathrm{Fe}}_3$LnO${}_2\}$ cluster the Ln-Fe${}_3$ subcluster interaction is determined to be antiferromagnetic in both Dy and Gd compounds, with values $J_{\mathrm{Dy}-{\mathrm{Fe}}_3}=-0.4$ K and $J_{\mathrm{Gd}-{\mathrm{Fe}}_3}=-0.25$ 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 $\{{\mathrm{Fe}}_3$DyO${}_2\}$ 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 (${\mu }_{0}H=4$ T). In contrast, in isotropic $\{{\mathrm{Fe}}_3$GdO${}_2\}$ this reorientation does not occur.

• Received 12 December 2012

DOI:https://doi.org/10.1103/PhysRevB.87.184403