Our basic building blocks for molecular quantum spintronics devices are called single-molecule magnets (SMMs). SMMs are constituted by an inner magnetic core with a surrounding shell of organic ligands1 that can be tailored to bind them on surfaces or into junctions.2 In order to strengthen magnetic interactions between the magnetic core ions, SMMs often have delocalized bonds, which can enhance their conducting properties. 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.3 It is also possible to exchange the magnetic ions, thus changing the magnetic properties without modifying the structure and the coupling to the environment.4 While grafting SMMs on surfaces has already led to important results, even more spectacular results will emerge from the rational design and tuning of single SMM-based devices.
Fig.: Few famous single-molecule magnets
SMMs combine the classic 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 (that are observable up to higher and higher temperatures due to progress in molecular designs), ranging from quantum tunnelling of magnetization to Berry phase interference and quantum coherence with important consequences on the physics of spintronic devices.5 Although the magnetic properties of SMMs can be affected when they are deposited on surfaces or between leads3, these systems remain a step ahead of non-molecular nanoobjects, which show large size and anisotropy distributions, for a low structure versatility.
We have a renowned expertise in nanoscale magnetism and have evidenced many of the quantum effects listed above. The specific features of SMMs compared to crystalline nanoparticles have led us to focus on spintronic devices based on these unique molecular systems. Moreover, for quantum applications, it is necessary to reduce the number of spins in the system and SMMs offer the possibility to obtain a few spins object when working at the single molecule level. This possibility has motivated us to turn towards single-molecule devices and to develop the challenging devices required for this implementation.
Fig.: (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.
1 Christou, G.; Gatteschi, D.; Hendrickson, D. N.; Sessoli, R. MRS Bull. 25, 66 (2000); Gatteschi, D., Sessoli, R. & Villain, J.: Molecular Nanomagnets (Oxford University Press, New York, 2007)
2 Cornia, A., et al., Struct. Bond. 122, 133 (2006); Fleury, B., et al. Chem. Comm. 2020 (2005); Naitabdi , A., et al., Adv. Mater. 17, 1612 (2005); Coronado, E., et al. Angew. Chem. Int. Ed. 43, 6152-6156 (2004)
3 Bogani, L., et al. Adv. Mater. 19, 3906-3911 (2007)
4 Ishikawa, N., et al. J. Phys. Chem. B 108, 11265 (2004)
5 Review: L. Bogani, W. Wernsdorfer, Nature Mater. 7, 179 (2008)