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Combined Ligand Field and Density Functional Theory Analysis of the Magnetic Anisotropy in Oligonuclear Complexes Based on FeIII−CN−MII Exchange-Coupled Pairs

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journal contribution
posted on 07.04.2008, 00:00 by Mihail Atanasov, Peter Comba, Claude A. Daul
Magnetic anisotropy in cyanide-bridged single-molecule magnets (SMMs) with FeIII−CN−MII (M = Cu, Ni) exchange-coupled pairs was analyzed using a density functional theory (DFT)-based ligand field model. A pronounced magnetic anisotropy due to exchange was found for linear FeIII−CN−MII units with fourfold symmetry. This results from spin–orbit coupling of the [FeIII(CN)6]3− unit and was found to be enhanced by a tetragonal field, leading to a 2Eg ground state for FeIII. In contrast, a trigonal field (e.g., due to τ2g Jahn–Teller angular distortions) led to a reduction of the magnetic anisotropy. A large enhancement of the anisotropy was found for the FeIII−CN−NiII exchange pair if anisotropic exchange combined with a negative zero-field splitting energy of the S = 1 ground state of NiII in tetragonally compressed octahedra, while cancellation of the two anisotropic contributions was predicted for tetragonal elongations. A recently developed DFT approach to Jahn–Teller activity in low-spin hexacyanometalates was used to address the influence of dynamic Jahn–Teller coupling on the magnetic anisotropy. Spin Hamiltonian parameters derived for linear Fe−M subunits were combined using a vector-coupling scheme to yield the spin Hamiltonian for the entire spin cluster. The magnetic properties of published oligonuclear transition-metal complexes with ferromagnetic ground states are discussed qualitatively, and predictive concepts for a systematic search of cyanide-based SMM materials are presented.