Juan Modesto Clemente-Juan, Eugenio Coronado, Alejandro Gaita-Ariño, Carlos Giménez-Saiz, Hans-Ulrich Güdel, Andreas Sieber, Roland Bircher and Hannu Mutka
Magnetic Polyoxometalates: Anisotropic Exchange Interactions in the Co^II₃ Moiety of [(NaOH₂)Co₃(H₂O)(P₂W₁₅O₅₆)₂]^17-
Inorg. Chem. 44, 3389-3395 (2005)                       

Abstract: The magnetic exchange interactions in a Co^II₃ moiety encapsulated in Na₁₇ [(NaOH₂)Co₃(H₂O)(P₂W₁₅O₅₆)₂] (NaCo₃) were studied by a combination of magnetic measurements (magnetic susceptibility and low-temperature magnetization), with a detailed Inelastic Neutron Scattering (INS) investigation. The novel structure of the salt was determined by X-ray crystallography. The ferromagnetic Co₃O₁₄ triangular cluster core consists of three octahedrally oxo-coordinated Co^II ions sharing edges. According to the single-ion anisotropy and spin-orbit coupling usually assumed for octahedral Co^II ions, the appropiate exchange Hamiltonian to describe the ground-state properties of the isosceles triangular Co₃ spin cluster is anisotropic and is expressed as = - 2 _a=x,y,zx,y,z (_1a2a + _2a3a + _1a3a), where J_a are the components of the exchange interactions between the Co^II ions. To reproduce the INS data, nonparallel anisotropic exchange tensors needed to be introduced, which were directly connected to the molecular symmetry of the complex. The following range of parameters (value ± 0.5 cm^–1) was found to reproduce all experimental information while taking magnetostructural relations into account: = = 8.6 cm^–1; = = 1.4 cm^–1; = = 10.0 cm^–1; = = 6.5 cm^–1 and = 3.4 cm^–1.

Cover Picture: The anisotropic exchange interactions in a ferromagnetic Co^II₃ moiety encapsulated in Na₁₇[(NaOH₂)Co₃(H₂O)(P₂W₁₅O₅₆)₂] were studied by a combination of magnetic measurements with a detailed Inelastic Neutron Scattering investigation. The cover shows the whole molecule and how the magnetic fragment interacts with a neutron beam, giving excitations in the low-energy magnetic levels.