Dipartimento di Fisica e Scienze della Terra  
Many-body models for molecular nanomagnets
A new approach for the ab-initio determination of microscopic interactions in magnetic molecules has been recentely published in Physical Review Letters. The study was performed by scientists of the Department of Physics and Earth Sciences in collaboration with Eva Pavarini at the Juelich Forschungszentrum, whose supercomputers were used to perform numerical calculations.

Magnetic nanostructures (with dimensions of the order of one billionth of meter) are fundamental building blocks in emerging technologies such as spintronics and quantum information processing. With respect to a typical magnetic material, the tiny size of these systems leads to phenomena characteristic of quantum mechanics, which can be exploited in the design of powerful computation algorithms and devices. Molecular nanomagnets are usually described by theoretical models based on the so-called spin Hamiltonian, which describes interactions involving the atomic magnetic moments in terms of "effective spins". These models contain parameters (e.g., exchange couplings or crystal-field coefficients) which are usually determined by comparison with experimental data. However, in many cases this procedure is not possible or not univocal, notably in presence of a large number of relevant parameters. Hence, a method to calculate interactions "ab initio" (i.e., in a fully theoretical approach starting from the atomic and structural characteristics of the molecule) is highly desirable, but first-principles schemes used so far have proven to be rather inaccurate in predicting the correct parameter values. In the work “Many-body models for molecular nanomagnets” a flexible and effective ab-initio scheme to calculate exchange and crystal-field interactions is introduced. The article illustrates a new approach in which the spin Hamiltonian is derived starting from density-functional theory calculations and explicitly including strong electron-electron correlations in the framework of a generalized Hubbard model. The scheme has been applied to three paradigmatic systems, the antiferromagnetic rings Cr8 and Cr7Ni and the single molecule magnet Fe4. In all cases the relevant magnetic interactions are identified and excellent agreement with experiments is obtained. These results represent a remarkable improvement with respect to other first-principle schemes, usually adopted in the literature.

The proposed approach allows one to determine the spin Hamiltonian without any a priori assumptions on its form. It could thus become essential for modeling molecular nanomagnets characterized by complex and anisotropic interactions, particularly in the case of Co or f-electron systems.

Bibliographical reference: A. Chiesa, S. Carretta, P. Santini, G. Amoretti and E. Pavarini, Phys. Rev. Lett. 110, 157204 (2013).
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