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MolS@MolS Report Summary

Project ID: 321033
Funded under: FP7-IDEAS-ERC
Country: Netherlands

Final Report Summary - MOLS@MOLS (Controlling Molecular Spin at the Molecular Scale)

In this project, the fields of molecular magnetism, molecular electronics and nanotechnology have been merged to establish the fabrication of planar nanodevices containing individual magnetic molecules or nanoparticles. Instead of being introduced by the electrodes as in usual spintronic devices, the magnetic functionality is placed in between the source and drain electrodes. In this way a spin transistor is built in which the electric current through the individual magnetic molecule or nanoparticle is sensitive to its spin properties. Transport through these molecular systems has a quantum mechanical character even at room temperature and the challenge has been to control the electrical properties by external stimuli such as electric fields, mechanical strain and light.
Specifically, a molecular spin transistor consists of a single magnetic molecule or nanoparticle bridging a nanometer sized gap opened in a conductive wire. The gate electrode tunes the electronic levels of the molecule, so that different charge (redox) states become accessible. The three-terminal transistor geometry allows for accurate spectroscopy of the energy levels by measuring current-voltage curves both in the resonant single-electron tunneling regime and the off-resonant tunneling regime by means of inelastic tunneling spectroscopy. Different approaches are followed, each with their own advantages: Transistors are built by passing a high current through a small gold wire (electromigration) for detailed spectroscopy at low-temperatures and for the study of the interplay with superconductivity when the gold wire to be broken is connected to superconductors, with electroburning of single- and multi-layer graphene for room-temperature stable devices and with mechanically controlled break junctions to obtain electrostatic and mechanical control at the same time.
The studied molecular complexes include single-molecule magnets, mixed-valence molecules and clusters, spin-crossover molecules and nanoparticles compounds and a series of mono-, di- and tri-radical complexes. Gate-spectroscopy measurements on single-molecule magnets are used to quantify their anisotropy parameters and first indications for off-diagonal terms (responsible for quantum tunneling of the magnetization) have been found. In two-site molecules, negative differential conductance has been observed and the highest rectification ratio in a single-molecule diode has been demonstrated (following the original idea of Aviram and Ratner that started the field of Molecular Electronics). The spin states of spin cross-over molecules have been manipulated by an electric field and by mechanical strain; in both cases the high spin state is the better conducting one. Finally, the radical compounds turn out to be model systems to study Kondo Physics and engineering the magnetic exchange in detail; regarding the latter aspect, tri-radical molecules exhibit ferromagnetic or anti-ferromagnetic coupling between their three spins depending on the level of distortion of the molecule. In molecular junctions with diradicals, all-organic molecules with two free spins, the redox state of the molecule could be changed in a reproducible way, thereby switching on and off the spin-exchange couplings between the added electron and the two radical spins. This electrically-controlled gating of the intramolecular magnetic interactions constitutes an essential ingredient of a single-molecule SWAP quantum gate. The tools developed for single molecules have been extended to study the properties of molecular magnetic nanoparticles. Assemblies of spin cross-over nanoparticles have been placed in between electrodes and their conductance has been measured as a function of temperature. A clear hysteresis signifies the presence of low- to high-spin transitions and vice versa. Interestingly, in measurements on these assemblies the low-spin state is consistently found to be the better conducting one, the mechanism of which is not understood in detail. Non-contact techniques have been developed to study the spin-dependent hysteresis in more detail; for example electrical transport through graphene has been used as a sensor of the spin-dependent properties.

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