Single Molecule Magnets (SMMs) have shown, since the beginning of the 90s, fascinating quantum phenomena, such as the tunneling of magnetization through an anisotropy barrier and quantum interference (Berry’s phase). So it is straightforward to wonder whether supramolecular chemistry is able to engineer molecules in order to tailor magnetic states and low-lying energy levels, sufficiently decouple the electron spins from the environment, in particular from the nuclear spins and assemble molecular dimers interacting through suitable linkers. As compared with other candidate systems for quantum computation, two advantages of this molecular approach are evident : (1) chemical synthesis may provide a large number of identical nano-objects in a relatively cheap way with respect to top-down methods and (2) molecules are larger than single ion impurities, thus relaxing the constraints for a local read-out.
Moreover, SMMs come in a variety of shapes and sizes and permit selective substitutions of the ligands in order to alter the coupling to the environment. It is also possible to exchange the magnetic ions, thus changing the magnetic properties without modifying the structure and the coupling to the environment. While grafting SMMs on surfaces has already led to important results, even more spectacular results emerge from the rational design and tuning of single SMM-based junctions. From a physics viewpoint, SMMs combine the classical macroscale properties of a magnet with the quantum properties of a nanoscale entity. They have crucial advantages over magnetic nanoparticles in that they are perfectly monodisperse and can be studied in molecular crystals. They display an impressive array of quantum effects with important consequences on the physics of spintronic devices. Although the magnetic properties of SMMs can be affected when they are deposited on surfaces or between leads, these systems remain a step ahead of non-molecular nanoparticles, which show large size and anisotropy distributions, for a low structure versatility.
In this context, a strong activity of the group is related to the magnetic characterization of molecular nanomagnets, synthesized in about 20 chemist groups, using worldwide unique home-built micro-SQUID and micro-Hall-probe facilities.
Our studies can be divided into several parts :
The field sweep rate and temperature dependence of the magnetization reversal of a single-chain magnet is studied. It is shown at low temperatures that the reversal of the magnetization is induced by a quantum nucleation of a domain wall.
Molecular nanomagnets or single-molecule magnets (SMMs) are mainly organic molecules that have one or several metal centers with unpaired electrons. Their magnetization can relax via thermally activated quantum tunneling and, in some cases, via pure ground state quantum tunneling.