Spin injection for magnetic spectroscopy and spin transport in single objects – SPIN MASTER

If the remarkable downscaling of data-storage bits were to continue, dimensions will soon reach the size of a molecule. At this level, both the writing and reading processes become extremely challenging and the combination between nanomagnetism, and the emerging field of spintronics will play a central role. The physical involvements range from quantum wires and quantum dots, down to individual nanoclusters, molecules, or even single atoms. Future progress strongly relies on our fundamental understanding of electron/spin transport and magnetic phenomena in reduced dimensions where sizeable quantum effects are expected. Exploring the electron and spin transport properties on the scale of a nanometer has proven to be a hard task. The ballistic magnetoresistance (BMR), in magnetic constrictions offers a promising new avenue for nanometer scale reading. However, the wide range of reported values for the BMR (up to 3000%) for constrictions reduced to a finite number of atoms, has led to a controversy on the validity of these results. An increased control over the geometry of the constriction is required, also for eventually evidencing the quantum origin of the phenomena. Recently the spin has also entered the realm of molecular electronics. Advances in the manipulation of single objects, now permit to contact individual molecules between two electrodes in devices such as break junction. Only in some rare cases could the transport be measured through one single molecule. Similar to BMR, molecular transport is sensitive to contact geometry, leading to great variability in the conductance. On the other hand studies in nanomagnetism have strongly benefited from spin-polarized scanning tunneling microscope (SP-STM) which allows a local and accurate mapping of magnetic properties by means of a magnetic tip. Promising results have been obtained in the imaging of magnetic domains/walls of metal films and nanostructures on surfaces. SP-STM has proven to be sensitive even to the magnetization of single atoms. The scope of the present project is to employ for the first time the SP-STM to accurately study spin-transport through small objects. To do so, a bulk magnetic tip will be progressively moved into contact with a single atom or molecule adsorbed on a magnetic nanostructure, while concomitantly monitoring changes in the transports properties up to currents of the order of 10 μA. Compared to other techniques, STM provides a better knowledge of identity, location, and number of atoms/molecules in between the leads, molecular orientation or binding site, which are expected to significantly affect conductance properties. A low-temperature STM operating in a clean, ultra-high vacuum environment with an in situ vectorial magnetic field will be specifically developed for this purpose in collaboration with Specs Nanotechnology, a leading company in scanning probe microscopes. The impact of the barrier thickness and of the exchange coupling will be closely monitored by varying tip-atom/molecule distance. The research will focus on the BMR of magnetic junctions comprising a single-magnetic, or non-magnetic, atom. We will consider C60 between magnetic leads as the paradigm for molecular conformation dependent spin transport. The rich variety of conformations adopted by C60 upon adsorption on metallic surfaces, will offer the opportunity of performing a comparative spin-transport study over different, but well known, molecule-lead interfaces. In a final step, the C60 will be replaced by a paramagnetic single-metal ion complex whose spin state can be varied through the magnetic field, in collaboration with the Institute of Material Science of Barcelona (CSIC). The project will benefit of the complementary experience of our group in contact STM and in near-field magnetic microscopy and spectroscopy. We expect to improve our understanding of a whole range of spin-related phenomena, which will be increasingly relevant in view of the ongoing downscaling of spintronic devices. In particular the study of spin-transport by STM allows tackling topics such as the spin injection with a high current density. This puts us in a position to study with unprecedented control the phenomenon known as spin-transfer torque, i.e. the current-driven magnetization reversal of a magnetic domain. Current-driven magnetization reversal of the nanoclusters provides a mechanism for a current-controlled magnetic memory element. Ultimately, the information collected through the combination of contact-STM and spin-polarized STM, the cleanliness of the environment, the unprecedented control over the geometry of the junction, and the choice of these model systems will minimize artefacts and produce data amenable to theoretical modelling.