Eur. Phys. J. B 73, 515-526 (2010)
https://doi.org/10.1140/epjb/e2010-00028-3

Inelastic neutron scattering study and Hubbard model description of the antiferromagnetic tetrahedral molecule Ni₄Mo₁₂

¹ Physikalisches Institut, Universität Freiburg, Hermann-Herder-Str. 3, 79104 Freiburg, Germany
² Universität Bielefeld, Fakultät für Physik, Postfach 100131, 33501 Bielefeld, Germany
³ Universität Osnabrück, Fachbereich Physik, 49069 Osnabrück, Germany
⁴ Institut Laue-Langevin, BP 156, 6 Rue Jules Horowitz, 38042 Grenoble Cedex 9, France

Corresponding author: joscha.nehrkorn@physik.uni-freiburg.de

Received: 16 June 2009
Revised: 12 November 2009
Published online: 27 January 2010

Abstract

The tetrameric Ni(II) spin cluster Ni₄Mo₁₂ has been studied by INS. The data were analyzed extensively in terms of a very general spin Hamiltonian, which includes antiferromagnetic Heisenberg interactions, biquadratic 2-spin and 3-spin interactions, a single-ion magnetic anisotropy, and Dzyaloshinsky-Moriya interactions. Some of the experimentally observed features in the INS spectra could be reproduced, however, one feature at 1.65 meV resisted all efforts. This supports the conclusion that the spin Hamiltonian approach is not adequate to describe the magnetism in Ni₄Mo₁₂. The isotropic terms in the spin Hamiltonian can be obtained in a strong-coupling expansion of the Hubbard model at half-filling. Therefore detailed theoretical studies of the Hubbard model were undertaken, using analytical as well as numerical techniques. We carefully analyzed its abilities and restrictions in applications to molecular spin clusters. As a main result it was found that the Hubbard model is also unable to appropriately explain the magnetism in Ni₄Mo₁₂. Extensions of the model are also discussed.