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Spin currents in magnetic nanostructures

Final Report Summary - MASPIC (Spin currents in magnetic nanostructures)

The study of the interplay between the magnetization of a structure and a spin-polarized charge carrier or pure spin current as well as the dynamics of confined spin structures such as domain walls is a key area of research in the field of spin electronics. From an application standpoint, the ability to manipulate the magnetization state of a magnetic element via the direct interaction with iterant electronic spins is a key prerequisite for a variety of logic, data storage and in also sensing devices for instance based on domain wall motion. This is especially important, since conventional approaches of magnetic state manipulation via externally generated magnetic fields face a variety of drawbacks, such as poor power scaling with the continually required miniaturization of devices. In order to apply these phenomena, however, a detailed fundamental understanding is required to allow the control of the novel physical effects which emerge, such as the new current-related terms which govern the dynamics of such a system for magnetization manipulation and the reciprocal effect that the magnetization configuration has on the transport, e.g. resulting in magnetoresistance effects. It is this which MaSpic has sought to address by a combined development of equipment and experimental techniques, in addition to the development of theory and the adoption of the latest simulation approaches.

Firstly we have looked at the interplay between spin polarized charge currents and magnetization. We have determined key parameters which determine the dynamics, and in turn domain wall velocities, such as the non-adiabaticity of the transport for different systems and materials. Here we have found that the non-adiabaticity has a contribution which is directly related to the spin structure of the magnetic sample. Out-of-plane magnetized multilayers have been found to be particularly promising in terms of high values of non-adiabaticity and corresponding high domain wall velocities for devices. Furthermore, for multilayer stacks with structural inversion asymmetry newly discovered spin-orbit torques offer an alternative avenue for controlling domain wall motion efficiently.

We have also studied the reciprocal influence of the magnetization on the transport and distinguished the intrinsic magnetoresistance effects from other effects such as magnetostriction and contamination. We have been able to determine different regimes of behaviour and for the narrowest domain walls with ballistic transport observe large effects, up to 50%, at zero field in clean and stable contacts, which can be directly related to the presence of the domain wall. These large effects could be taken advantage of in future memory concepts, since as devices continue to shrink in size, such ballistic effects become accessible and start to dominate the transport.

Secondly, the project has investigated the use of pure spin currents in terms of magnetization manipulation, which are a very recently proposed alternative and offer advantages due to the lack of associated Joule heating at the position where the spin currents can act on magnetization. Such spin currents have been successfully applied to de-pin magnetic domain walls in non-local spin valves, with an observed efficiency which is orders of magnitude larger than using spin-polarized charge currents. We have further studied optically generated super-diffusive spin currents which we have seen can lead to ultrafast domain wall profile changes.

Finally we have analyzed domain wall spin structures during wall motion and we have also conceived a new paradigm-shifting approach to magnetization manipulation, employing tailored out-of-plane magnetic field pulses to synchronously move multiple magnetic domains through an understanding of the non-equilibrium dynamics.