Isotropic and anisotropic magnetic interactions and their manipulation by the electric field : from the mononuclear complex to the Herbertsmithite material. – (HEULLY-ALARY Flaurent / LCPQ / Thèse). – 12/12/2025, 9H30

PhD Defence

HEULLY-ALARY Flaurent, LCPQ, Seminar room, Bat. 3R4

Abstract :
This thesis is part of the ongoing effort to understand and control the magnetic properties of molecular materials, whose growing interest arises from their potential applications in information storage, spintronics, and quantum computing. Using ab initio approaches based on wavefunction theory and density functional theory, it explores isotropic and anisotropic magnetic interactions in systems ranging from mononuclear complexes to strongly correlated materials such as Herbertsmithite. The general objective is to determine how the nature of the orbitals, the spin–orbit coupling, and external perturbations, particularly electric fields, affect the effective parameters describing magnetic behavior. The study relies on the analysis of the electronic wavefunction, obtained through advanced numerical methods, in order to establish a direct connection between the electronic structure and measurable spin quantities. Mononuclear complexes provide a privileged framework for the analysis of spin–orbit coupling and magnetic anisotropy. Their finite size allows a complete description of the system and direct comparison with experimental data. This work investigates how the ligand field symmetry, electronic configuration, and spin-orbit coupling determine the magnitude and nature of anisotropy. Two main aspects are developed: the influence of first-order spin–orbit coupling on the parameters of the Zero-Field Splitting model, and the possibility of modulating these parameters through an external electric field, opening the way toward magnetoelectric control of spin states. The study is then extended to polynuclear systems and model materials, where exchange interactions become dominant. A detailed analysis is proposed to identify the mechanisms underlying these isotropic and anisotropic exchange interactions and to evaluate their evolution under the influence of first-order spin–orbit coupling and applied electric field. Finally, the application of this methodology to a real system, Herbertsmithite, ZnCu₃(OH)₆Cl₂, makes it possible to address the physics of quantum spin liquids. In this kagome material, geometric frustration prevents the establishment of magnetic order, leading to a quantum disordered state. Density functional theory calculations combined with multireference embedded fragment approaches were used to determine the isotropic and anisotropic interactions identified as responsible for this behavior, illustrating the ability of ab initio methods to describe correlated quantum systems from their local electronic structure. This work contributes to the development of a rigorous methodology for understanding magnetic anisotropies through a detailed electronic description. By bridging quantum chemistry and the physics of correlated magnetism, it provides a unified framework for modeling spin interactions and open the way for the rational design of molecular systems and quantum materials with magnetic properties controlled by external perturbations.

Supervisors :

• Nathalie Guihéry
• Nicolas Suaud