A. Furrer, K. W. Krämer, Th. Strässle, D. Biner, J. Hauser and H. U. Güdel
Magnetic and neutron spectroscopic properties of the tetrameric nickel compound [Mo₁₂O₂₈(m₂-OH)₉(m₃-OH)₃{Ni(H₂O)₃}₄]·13H₂O
Phys. Rev. B 81, 214437/1-10 (2010)                 

Abstract: We present results of inelastic neutron scattering experiments performed for the compound [Mo₁₂O₂₈(m₂-OH)₉(m₃-OH)₃{Ni(H₂O)₃}₄]·13H₂O, 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.