Magnetic and neutron spectroscopic properties of the tetrameric nickel compound [Mo12O28(μ2-OH)9(μ3-OH)3{Ni(H2O)3}4]•13H2O

We present results of inelastic neutron scattering experiments performed for the compound [Mo12O28(mu(2)-OH)(9)(mu(3)-OH)(3){Ni(H2O)(3)}(4)]center dot 13H(2)O, which is a molecular magnet with antiferromagnetically coupled Ni2+ ions forming nearly ideal tetrahedra in a diamagnetic molybdate matrix. The neutron spectroscopic data are analyzed together with high-field magnetization data (taken from the literature), which exhibit four steps at non-equidistant field intervals. The experimental data can be excellently described by antiferromagnetic Heisenberg-type exchange interactions as well as an axial single-ion anisotropy within a distorted tetrahedron of Ni2+ ions characterized by x-ray single-crystal diffraction. Our analysis contrasts to recently proposed models, which are based on the existence of extremely large biquadratic (and three-ion) exchange interactions and/or on a strong field dependence of the Heisenberg coupling parameters.