Electronic spin manipulation in all solid state devices

Overview & Synopsis

Manipulation of electronic spin is challenging both from a scientific point of view with complex physics ruling transport mechanisms, but also with perspectives regarding applications in the field of spintronics. Full control of spin orientation of electrons in materials could help reaching further a three-dimensional design of spintronic devices desired for energy-efficient next-generation of memory bits or even computing systems. Manipulation of electronic spin is tightly connected with transport mechanisms taking place in the materials crossed by charge carriers. These charge carriers can be affected either by fields or by other particles and see their both their travel path and their spin orientation being modified. Understanding the physics lying between these different interactions can be the key to access and further control electronic spin. In this work, we aimed to study manipulation of electronic spin for hot ballistic electrons. While the term hot refers to out-of-equilibrium charge carriers whose energy is higher than the Fermi level of the metal, ballistic means the particle traveled without suffering any interaction. To do so, a Magnetic Tunnel Transistor (MTT) was designed with specificities required for this study. It consists in a spin-valve sandwiched between a tunnel barrier for spin-polarized hot electrons injection on one side, and a Schottky barrier with a Cu/Si contact to only collect hot ballistic electrons on the other. MTT structure was used for this study in order to investigate hot electrons physics in the low-energy range. Other ways to inject hot electrons into metals are limited by the vacuum level of the materials, thus giving a physical lower boundary for such studies. Our all solid state device also allows a sharp filtering of non-ballistic electrons with the presence of the Schottky barrier to collect hot electrons. The first aspect investigated in this study is the effect of electron-electron interactions on hot electrons transport. While traveling through a metal, electrons can suffer interactions with those from the Fermi sea of the materials. When doing so, they lose a huge amount of their initial energy. Our MTT measurement exhibit strong feature related to electron-electron interactions. Based on experimental results, a model to take these effects into account is developed. We proved that secondary electrons generated during such interactions contribute to the signal for high injection voltage. When hot electrons are crossing ferromagnets, electron-electron interactions and secondary electrons generation become spin-dependent. Based on the density of states calculations of studied materials inserted in this MTT, it is possible to connect these features to scattering mechanisms in our model. This work opens perspectives for deep studies of electron-electron scattering including possible spin-flip processes and enhancement of spin-polarization of hot electrons current. Another strategy to manipulate electronic spin is to induce a precession motion. Under the effect of a field, the spin of hot electrons can start a precessional motional while traveling through a material. This effect was already demonstrated for several kind of materials, including ferromagnets. Yet only one study, conducted in our team, proved this effect to exist for low-energy electrons in the energy range of actual spintronic devices. Following this pioneering work, a new strategy was implemented to control the associated precession angle using an external parameter, the temperature. Variation of temperature in our device allows to tune precession mechanism in a ferromagnetic layer. Using a specific MTT with a three-dimensional magnetizations configuration, precession angles were extracted from several temperature and could be explained in the scope of temperature-dependent precessional motion of electronic spin.