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Magnetic and neutron spectroscopic properties of the tetrameric nickel compound $[Mo_{12}O_{28}(μ_2-OH)_9(μ_3-OH)_3{Ni(H_2O)_3}_4] $\cdot$ 13H_2O$

We present results of inelastic neutron scattering experiments performed for the compound Magnetic and neutron spectroscopic properties of the tetrameric nickel compound $[Mo_{12}O_{28}(μ_2-OH)_9(μ_3-OH)_3{Ni(H_2O)_3}_4] $\cdot$ 13H_2O$, 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.