Magnetic Excitations in Polyoxometalate Clusters Observed by Inelastic Neutron Scattering: Evidence for Anisotropic Ferromagnetic Exchange Interactions in the Tetrameric Cobalt(II) Cluster [Co4(H2O)2(PW9O34)2]10-. Comparison with the Magnetic and Specific Heat Properties

Abstract

The ground-state properties of the tetranuclear Co2+ cluster [Co4(H2O)2(PW9O34)2]10- were investigated by combining specific heat, magnetic susceptibility, and magnetization measurements with a detailed inelastic neutron scattering (INS) study on a fully deuterated sample of K10[Co4(H2O)2(PW9O34)2]·22H2O. As a result of the single ion anisotropy of the octahedral Co2+, the appropriate exchange Hamiltonian of the Co4 spin cluster is anisotropic. INS turns out to be essential for the determination of energy splittings in the ground state resulting from the coupling. Besides the energy pattern, INS provides information about the wave functions of the split ground-state components of the spin cluster. Based on the Hamiltonian ĤAN = ∑i=x,y,z −2Ji(S1iS3i + S1iS4i + S2iS3i + S2iS4i) − 2J‘i(S1iS2i) the following set of parameters is obtained: Jz = 1.51 meV, Jx = 0.70 meV, Jz‘ = 0.46 meV, Jx‘ = 0.44 meV, r = 1.6, where r = Jx/Jy = Jx‘/Jy‘. These parameters also reproduce the magnetic specific heat, susceptibility...