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

TL;DR

This study combines inelastic neutron scattering experiments with theoretical modeling to understand the magnetic properties of the Ni4Mo12 molecule, revealing limitations of both spin Hamiltonian and Hubbard models in describing its magnetism.

Contribution

It provides a comprehensive experimental and theoretical analysis showing the inadequacy of standard models to fully explain Ni4Mo12's magnetic behavior.

Findings

INS spectra partially match theoretical models

Spin Hamiltonian approach fails to explain all features

Hubbard model also cannot fully describe magnetism

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.