Spin Waves bridging Spintronics and Photonics
Final Report Abstract
One of the grand challenges in cutting edge quantum and condensed matter physics is to harness the spin degree of electrons for information technologies. While spintronics, based on charge transport by spin polarized electrons, made its leap in data storage by providing extremely sensitive detectors in magnetic hard-drives, it turned out to be challenging to transport spin information without great losses. With magnonics a visionary concept inspired researchers worldwide: Utilize magnons - the collective excitation quanta of the spin system in magnetically ordered materials - as carriers for information. Magnons are waves of the electrons’ spin precessional motion. They propagate without charge transport and its associated Ohmic losses, paving the way for a substantial reduction of energy consumption in computers. While macroscopic prototypes of magnonic logic gates have been demonstrated, magnonics has the potential to go beyond wave-based information processing: The combination of magnons with nano-sized spin textures allows to create networks of non-volatile objects with magnons as non-linear interconnects. Both magnons and spin textures share a common ground set by the interplay of dipolar, spin-orbit and exchange energies rendering them perfect interaction partners. Magnons are fast, sensitive to the spins’ directions and easily driven far from equilibrium. Spin textures are robust, non-volatile and still reprogrammable on ultrashort timescales. The marriage of both, topological spin textures and magnonics could link both, high frequency information carriers and long time information storage in one unified, nanoscale framework. In the first objective of this project, we addressed methods for steering magnons laterally in two-dimensional structures. We focused on two concepts – the active rotation of the magnetization by local, current induced magnetic fields and the channeling of magnon flow inside magnetic domain walls – both approaches based on the anisotropy of the magnon dispersion relation and the magnons’ sensitivity to effective magnetic fields. Our entire group received for this work the HZDR Research Award in 2016 and I was awarded the Walther-Schottky-Award 2017 of the German Physical Society. Further highlights are the Excitation of whispering gallery magnons in vortices and the nonlocal stimulation of these modes. Using the pure spin currents generated by the spin Hall effect to drive magnetic oscillations in nano-structures is one of the milestones in our second objective. We succeeded to fabricate spin Hall nano-oscillators and to phase-lock them to external microwave signals. Furthermore, the idea to operate such oscillators even without the need of externally applied magnetic fields by using magnetic domain walls as active nano-oscillators could be realized experimentally. The breakthrough of our experiment is that it allows to control the oscillators position by magnetic fields or electric currents similar to the magnetic racetrack memory and the possibility to dynamically create or annihilate nano-sized magnon oscillators. The third objective of this project is the interaction between magnons and surface plasmon polaritons. We constructed a microscope for surface plasmon polariton observation, which is integrated into a Brillouin light scattering microscope for magnon detection. Using samples for surface plasmon interferometry from our collaborator we could verify the functionality of this novel type of microscope. However, the preparation of magnon-plasmon hybrid samples faced technical problems due to equipment downtime causing delays with this objective. We used this time to start a Technology Transfer Project for commercializing our approach of a platform independent synchronization and automation software for laboratory equipment and data acquisition. After one year of funding within the Helmholtz-Enterprise initiative we founded the company thaTEC innovations GmbH (www.thatec-innovation.com) in August 2016 with a former postdoc of the group as CEO. While the access to photonics via surface plasmon polaritons seemed to be promising during the planning phase of the project and the first 3 years, unfortunately, after 5 years there was still now breakthrough to be seen. Hence, we followed an exciting opportunity, which finally allowed to establish a link between photonics, quantum spins and spin waves: The method Optically Detected Magnetic Resonance (ODMR) seemed to be straightforward to integrate in our laboratories dedicated for magnon studies, because the frequency and magnetic field range of defect based quantum spins in SiC crystals very well matches with the resonance properties of spin waves. We started an initiative to implement nano magnets and magnonic circuitry on SiC wafers and to use the optical pumping and photoluminescence for reading the state of the quantum spins.
