Dresden 2009 – scientific programme

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MA: Fachverband Magnetismus

MA 33: Bio- Molecular Magnetism

MA 33.6: Talk

Thursday, March 26, 2009, 11:30–11:45, HSZ 103

Can the Hubbard model explain the steps observed in the magnetization curve of {Ni₄Mo₁₂}? — •Martin Höck and Jürgen Schnack — Universität Bielefeld, Fakultät für Physik, Postfach 100131, D-33501 Bielefeld

The low-temperature magnetization curve of the magnetic molecule {Ni₄Mo₁₂} features four nonequidistant steps which cannot be explained using a Heisenberg model [1]. In his article [2], V. Kostyuchenko presents a spin-1 model with biquadratic and three-spin interactions and claims that it is the strong coupling limit of a certain Hubbard model. This spin-1 model correctly predicts the position of the steps in the magnetization curve.

We investigate whether the Hubbard model proposed in [2] is really capable of describing {Ni₄Mo₁₂}. To this end, we calculate its eigenvalues using numerical exact diagonalization and try to fit its parameters to the experimental magnetization data. We are unable to find suitable fit parameters although the parameter space of the model is only two-dimensional.

Therefore, we analyze the strong coupling limit of the Hubbard model and rederive its effective spin model up to order O(U⁻³). The spin Hamiltonian which we obtain differs from the one presented by Kostyuchenko. We arrive at the final conclusion that the Hubbard model as proposed in [2] is not suited to describe the molecule {Ni₄Mo₁₂}.

[1] J. Schnack et al., Phys. Rev. B 73, 094401 (2006);

[2] V. V. Kostyuchenko, Phys. Rev. B 76, 212404 (2007)