Etude théorique et expérimentale de l'anisotropie magnétique : une approche multi-échelle – TEMAMA

This project focuses on the study of magnetic anisotropy in Single Molecule Magnets (SMMs). A crucial stake for the scientific community of this domain consists in controlling and tunning the anisotropy for both technological devices (information storage and quantum information) and fundamental purposes (tunneling, coherence and quantum interferences). The project rests on joined efforts of complementary groups: Experimentalists (Chemists and Physicists) have in charge the synthesis and characterization of a consistent series of complexes (mono-, bi- and poly-metallic) while theoreticians should conceive a strategy to simulate properties of SMMs. The ultimate target of this collaboration is a deep understanding of the structural and electronic factors governing the anisotropy (both its nature and magnitude) and the proposition of new architectures having higher blocking temperatures. The purpose consists in a multi-scale approach of the problem, for both experimentalists and theoreticians. Indeed, SMMs can a priori be divided in fragments consisting of single-ion complexes. The magnetic anisotropy energy (MAE) of a poly-nuclear complex depends on the MAE of each center and of synergistic effects between them. A first task of chemist experimentalists is the synthesis of complexes such that the anisotropy of the metal ion (governed by judiciously chosen ligands) is transferable from the mono-nuclear to the poly-nuclear species, the target being to build SMMs from these elementary building-blocks. Characterizations from EPR spectroscopy will provide both the g and D tensors as well as the proper axes (single-crystal studies). A first objective of theoreticians will be the implementation of a correlated variational ab initio tool involving both spin-orbit and spin-spin couplings. A systematic comparison of the results obtained for mono- and bi-nuclear complexes with experimental data should validate the use of this code. For poly-nuclear complexes a two-step multi-scale approach is compulsory. In a first step, a multi-spin model Hamiltonian involving the relevant electronic interactions (g and D tensors as well as inter-site exchange integrals J) will be extracted from ab initio calculations performed on bi-nuclear fragments using the embedded cluster method and the effective Hamiltonian theory. An automatic procedure of extraction will be implemented in the POLY-ANISO code developed by L. Chibotaru. Thermodynamic properties and EPR spectroscopy simulations will then be determined using the model multi-spin Hamiltonian. The validity of the procedure will be checked by confrontations to experimental data on tetra-nuclear complexes for which the exact resolution of this Hamiltonian is possible. For larger size SMMs the CORE method, related to the real space renormalization group theory, should be implemented in order to make possible the approximate diagonalization of the corresponding multi-spin Hamiltonian matrix. Finally a last part of the project is devoted to the conception, synthesis and characterization of hetero-nuclear SMMs expected to present more performing properties. The rational and gradual handling of the complexity and the step-by-step control and interaction between theory and experiment should make possible the success of this ambitious project.