Molecular magnetism

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.

(a) Schematic representation of the energy landscape of a SMM with a spin ground state S = 10. The magnetization reversal can occur via quantum tunnelling between energy levels (blue arrow) when the energy levels in the two wells are in resonance. Phonon absorption (green arrows) can also excite the spin up to the top of the potential energy barrier with the quantum number M = 0, and phonon emission descends the spin to the second well. (b) Hysteresis loops of single crystals of [Mn12O12(O2CCH2C(CH3)3)16(CH3OH)4] SMM at different temperatures. The loops exhibit a series of steps, which are due to resonant quantum tunnelling between energy levels.

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 :

  1. Carrying out extensive characterization studies of the hysteresis properties, quantum tunneling of magnetization, and relaxation profiles of the compounds. This allows us to quickly select the best compounds for a deeper study.
  2. Carrying out special methods in order to learn more about environmental decoherence effects. For example, the quantum hole digging method that we developed recently can measure the effect of hyperfine, dipolar or exchange coupling. It can also be used to select a sub-group of molecules in a crystal, allowing us to overcome problems related to distributions of molecule environments. The Landau-Zener method allows us to determine tunnel splitting.
  3. Developing new measurement schemes for possible applications for information storage and quantum computing.
Cohérence quantique - CQ

Cohérence quantique - CQ

Révéler des phénomènes quantiques dans des circuits électroniques nanométriques
Hélium : du fondamental aux applications - HELFA

Hélium : du fondamental aux applications - HELFA

Hélium comme système modèle, hydrodynamique et turbulence, spatial et astrophysique, instrumentation et développement cryogénique.
Magnétisme et Supraconductivité - MagSup

Magnétisme et Supraconductivité - MagSup

Equipe Magnétisme et supraconductivité à l’Institut NEEL - Systèmes impliquant différents degrés de liberté comme la charge, le spin ou le réseau.
Optique et Matériaux - OPTIMA

Optique et Matériaux - OPTIMA

Rassembler une chaine de compétence complète qui va de la synthèse et l’élaboration de matériaux nouveaux à l’étude des propriétés optiques non linéaires et plasmoniques
Matériaux, Rayonnements, Structure - MRS

Matériaux, Rayonnements, Structure - MRS

Compréhension des propriétés physico-chimiques de matériaux complexes sur la base d’une connaissance fine de leur structure
Micro et NanoMagnétisme - MNM

Micro et NanoMagnétisme - MNM

Complementary expertise in fabrication, characterisation, and simulations for studies in nanomagnetism with applications in spin electronics and micro-systems
Nano-Electronique Quantique et Spectroscopie - QNES

Nano-Electronique Quantique et Spectroscopie - QNES

Transport électronique et spectroscopie locale de structures quantiques
Nano-Optique et Forces - NOF

Nano-Optique et Forces - NOF

Nano - optique et forces
Nanophysique et Semiconducteurs - NPSC

Nanophysique et Semiconducteurs - NPSC

Élaboration de nanostructures de semi-conducteurs III-V et II-VI et étude de leurs propriétés physiques en vue de nouvelles fonctionnalités
Nanospintronique et Transport Moléculaire - NanoSpin

Nanospintronique et Transport Moléculaire - NanoSpin

Studying magnetism at the nanoscale, where classical and quantum properties can be combined and used for molecular quantum spintronics
Semi-conducteurs à large bande interdite - SC2G

Semi-conducteurs à large bande interdite - SC2G

De la physique du diamant et autres semi-conducteurs à grand gap vers les applications en électronique et biotechnologies
Surfaces, Interfaces et Nanostructures - SIN

Surfaces, Interfaces et Nanostructures - SIN

Etudes expérimentales et théoriques de systèmes de basse dimensionnalité
Systèmes Hybrides de basse dimensionnalité - HYBRID

Systèmes Hybrides de basse dimensionnalité - HYBRID

Propriétés électroniques, optiques, vibrationnelles, mécaniques, et leur couplage à l’échelle quantique, de nouveaux systèmes hybrides (nanotubes, graphène, matériaux bi-dimensionnels, fonctionnalisés) que l’équipe développe.
Théorie de la Matière Condensée -TMC

Théorie de la Matière Condensée -TMC

Phénomènes physiques nouveaux dans les matériaux et systèmes modèles.
Thermodynamique et biophysique des petits systèmes - TPS

Thermodynamique et biophysique des petits systèmes - TPS

Instrumentation ultrasensible pour sonder les propriétés électronique et thermique : de la matière condensée à basse température aux systèmes biologiques à l’ambiante.
Théorie Quantique des Circuits - ThQC

Théorie Quantique des Circuits - ThQC

Étude théorique du transport électronique dans des dispositifs nanométriques aux propriétés quantiques remarquables.
Ultra-basses températures - UBT

Ultra-basses températures - UBT

La physique quantique à la limite des ultra-basses températures.

Quantum nucleation in a single-chain magnet

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.

Overview of molecular nanomagnets

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.

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