A challenge in molecular electronics is to control the strength of the molecule-electrode coupling to optimize device performance. Here we show that non-covalent contacts between the active molecular component (in this case, ferrocenyl of a ferrocenyl-alkanethiol self-assembled monolayer (SAM)) and the electrodes allow for robust coupling with minimal energy broadening of the molecular level, precisely what is required to maximize the rectification ratio of a molecular diode. In contrast, strong chemisorbed contacts through the ferrocenyl result in large energy broadening, leakage currents and poor device performance. By gradually shifting the ferrocenyl from the top to the bottom of the SAM, we map the shape of the electrostatic potential profile across the molecules and we are able to control the direction of rectification by tuning the ferrocenyl-electrode coupling parameters. Our demonstrated control of the molecule-electrode coupling is important for rational design of materials that rely on charge transport across organic-inorganic interfaces.

Molecular diodes operating in the tunnelling regime are intrinsically limited to a maximum rectification ratio R of ∼10. To enhance this rectification ratio to values comparable to those of conventional diodes (R ≥ 10) an alternative mechanism of rectification is therefore required. Here, we report a molecular diode with R = 6.3 × 10 based on self-assembled monolayers with Fc-C≡C-Fc (Fc, ferrocenyl) termini. The number of molecules (n(V)) involved in the charge transport changes with the polarity of the applied bias. More specifically, n(V) increases at forward bias because of an attractive electrostatic force between the positively charged Fc units and the negatively charged top electrode, but remains constant at reverse bias when the Fc units are neutral and interact weakly with the positively charged electrode. We successfully model this mechanism using molecular dynamics calculations.

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A magnetoelectronic device is proposed in which a spin-current pulse produces a rapid reversal of the magnetization of a thin film nanomagnet. A spin-transfer torque induces the reversal and the switching speed is determined by the precession frequency of the magnetization in a thin film element’s demagnetization field. Micromagnetic simulations show that this switching occurs above a threshold pulse current and can be faster than 50 ps. In contrast to present spin-transfer devices, the switching does not require an initial fluctuation or deviation of magnetic layers from collinear alignment and is far more energy efficient. This device operates at room temperature and can be realized with present-day magnetic nanostructure technology.

The symmetry of magnetic quantum tunneling (MQT) in the single molecule magnet Mn12-acetate has been determined by sensitive low-temperature magnetic measurements in the pure quantum tunneling regime and high frequency EPR spectroscopy in the presence of large transverse magnetic fields. The combined data set definitely establishes the transverse anisotropy terms responsible for the low temperature quantum dynamics. MQT is due to a disorder induced locally varying quadratic transverse anisotropy associated with rhombic distortions in the molecular environment (2 nd order in the spin-operators). This is superimposed on a 4 th order transverse magnetic anisotropy consistent with the global (average) S4 molecule site symmetry. These forms of the transverse anisotropy are incommensurate, leading to a complex interplay between local and global symmetries, the consequences of which are analyzed in detail. The resulting model explains: (1) the observation of a twofold symmetry of MQT as a function of the angle of the transverse magnetic field when a subset of molecules in a single crystal are studied; (2) the non-monotonic dependence of the tunneling probability on the magnitude of the transverse magnetic field, which is ascribed to an interference (Berry phase) effect; and (3) the angular dependence of EPR absorption peaks, including the fine structure in the peaks, among many other phenomena. This work also establishes the magnitude of the 2 nd and 4 th order transverse anisotropy terms for Mn12-acetate single crystals and the angle between the hard magnetic anisotropy axes of these terms. EPR as a function of the angle of the field with respect to the easy axis (close to the hard-medium plane) confirms that there are discrete tilts of the molecular magnetic easy axis from the global (average) easy axis of a crystal, also associated with solvent disorder. The latter observation provides a very plausible explanation for the lack of MQT selection rules, which has been a puzzle for many years.

Spin-transfer torque and spin Hall effects combined with their reciprocal phenomena, spin pumping and inverse spin Hall effects (ISHEs), enable the reading and control of magnetic moments in spintronics. The direct observation of these effects remains elusive in antiferromagnetic-based devices. We report subterahertz spin pumping at the interface of the uniaxial insulating antiferromagnet manganese difluoride and platinum. The measured ISHE voltage arising from spin-charge conversion in the platinum layer depends on the chirality of the dynamical modes of the antiferromagnet, which is selectively excited and modulated by the handedness of the circularly polarized subterahertz irradiation. Our results open the door to the controlled generation of coherent, pure spin currents at terahertz frequencies.

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Solid-state molecular tunnel junctions are often assumed to operate in the Landauer regime, which describes essentially activationless coherent tunnelling processes. In solution, on the other hand, charge transfer is described by Marcus theory, which accounts for thermally activated processes. In practice, however, thermally activated transport phenomena are frequently observed also in solid-state molecular junctions but remain poorly understood. Here, we show experimentally the transition from the Marcus to the inverted Marcus region in a solid-state molecular tunnel junction by means of intra-molecular orbital gating that can be tuned via the chemical structure of the molecule and applied bias. In the inverted Marcus region, charge transport is incoherent, yet virtually independent of temperature. Our experimental results fit well to a theoretical model that combines Landauer and Marcus theories and may have implications for the interpretation of temperature-dependent charge transport measurements in molecular junctions.