Quantum effects in Molecular Nanomagnets: from Theory to Applications

Abstract

Molecular nanomagnets have attracted the attention of the scientific community since they represent model systems to study many quantum phenomena, such as quantum entanglement and decoherence. They are also interesting for their envisaged technological applications, including magnetic refrigeration, high-density information storage and quantum information processing. Driven by these motivations, the present Thesis focuses on the study of magnetic properties and spin dynamics of different classes of molecular nanomagnets, with the purpose to understand their quantum behavior and their potential applications in future technologies. Magnetic frustration is at the origin of many exotic phenomena in matter. Here frustration-induced properties are studied and analyzed for the clusters Ni7, Fe7, Fe6 and Mn6, together with their effects on phonon-induced relaxation dynamics. Indeed, the comprehension of relaxation mechanisms is crucial in order to address the implementation of these systems in the fields of quantum information processing or information storage. Molecular nanomagnets are also promising materials for very-low-temperature magnetic refrigeration due to a potentially enhanced magnetocaloric effect. By explicitly considering Carnot refrigeration cycles, the theoretical recipe to design the best molecules for cryogenic magnetic refrigeration is identified. Another important class of molecular nanomagnets is that of antiferromagnetic rings. Of these, Cr7Ni is of great importance for applications in quantum information processing. The local spin density distribution in Cr7Ni is here obtained from 53Cr-NMR and confirmed by theoretical calculations. The origin of magnetic anisotropy of these rings grafted on surfaces is investigated using XMCD and theoretical calculations. Antiferromagnetic “purple” rings are completely characterized by the comparison of our calculations with inelastic neutron scattering and EPR data. The characterization of purple rings represents a first step in the description of a new family of “purple-green” entangled dimers, which represent model systems to study spin entanglement and its application in the field of quantum computation.