  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}(\mu_2-OH)_9(\mu_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.
