Spin transport across a customized multi-decker molecule – SPINCOMM

More than two decades ago has emerged a new technology exploiting electron spins to convey information within an electric circuitry. This technology, known as spintronics, translates in advantages such as nonvolatile storage technology, fast-data processing speed and low-power consumption. The working principle of a spintronic device is to generate a non-equilibrium spin population and to detect it. However, creation and detection occurs in different regions of the device. During the transfer process from one region to the other, the spin population tends to relax towards its non spin-polarized equilibrium state weakening then the efficiency of the device. One of the central research areas in spintronics therefore aims at perfecting this transfer process. Present efforts involve improving existing technology or finding novel radical ways of manipulating spin-polarized electrons. The SPINCOMM project is in line with this second approach and falls in the context of molecular spintronics.

The purpose of project SPINCOMM is to carry out the first fundamental investigation of spin transport across a single organometallic wire. To achieve this ambitious goal, a pioneering bottom-up approach will be implemented through four innovating strategies: 1) SIMPLIFICATION: The wires will have a multi-decker architecture where single transition-metal atoms alternate with cyclopentadienyl rings (C5H5). Strikingly, these wires have been predicted to display a 100% spin-filtering efficiency over a wide bias range. 2) CONTROL: Transport measurements will be carried out with a low-temperature scanning tunneling microscope (STM) operated in ultrahigh vacuum. The molecules will be deposited onto a well-calibrated surface and then contacted by the STM tip. Junction formation with a single multi-decker molecule will be greatly facilitated by the upstanding adsorption geometry onto the surface. Precise information about the binding properties of the multi-decker molecule to the electrodes will be available. XMCD measurements will be carried out independently to carefully characterize the magnetic status of the molecules. 3) CUSTOMIZATION: The chemical composition of the molecule and its length will be modified directly in the STM junction to optimize spin transport. Moreover, the material of tip and surface will be changed in order to tackle different aspects of spin transport. These essentially consist in the Kondo effect (non-magnetic tip and surface) and its interplay with spin-polarized electrons (ferromagnetic tip and a non-magnetic surface), as well as a transport across a single-molecule spin-valve (ferromagnetic tip and surface). 4) SIMULATION: Given the unprecedented microscopic control exerted over the junction and the simplified molecular architecture employed, the experimental data will be highly amenable to first-principle calculations. State-of-the-art density functional theory and transport calculations will be used to unravel the key mechanisms governing spin transport, along with non-equilibrium and correlated calculations to treat the Kondo problem.

With the know-how acquired, the mono-decker architecture of the molecule will be exploited for developing a new spin-sensitive microscopy. A molecular tip comprising a mono-decker molecule will be used to record “contact images” of the surface. Surfaces with opposite magnetizations are expected to produce a higher contrast than the one accessible to SP-STM due to the nearly ideal spin-filtering effect of the mono-decker molecule. With spin-polarized contact microscopy it will be possible to map the spin-polarized properties of surfaces and nanostructures with atomic-scale spatial resolution and to assess the impact of defects, surface impurities, and electronic inhomogeneities on spin transport. We expect this technique to develop quickly and to have a success similar to one of SP-STM in these last ten years.