Spin manipulation in semiconductor spin devices
Final Report Abstract
In this project, we explored the topic of spin manipulation in semiconductors by means of an electric field. This is a fundamental topic in the field of spintronics, which aims to increase the functionality of electronic devices by exploiting the spin of an electron in addition to its electric charge. Spin is a quantum mechanical property of elementary particles, such as an electron, which behaves like an angular momentum. The magnetic moment associated with this momentum can have only two orientations in the external magnetic field, parallel or antiparallel to the direction of that field. The aim of the spintronic research is to generate spin-polarized currents in non-magnetic conductors, i.e. currents of electrons with predominantly one spin orientation, and to manipulate the flow of these currents by acting on the spin of the electrons and not on their charge. The paradigm spintronic device has been a spin field-effect transistor (sFET) proposed in 1990. This device has a structure similar to that of a conventional FET. It consists of a twodimensional semiconducting channel between the source and the drain electrodes, formed in a semiconductor heterostructure, and capacitively coupled to the metallic gate, which is separated from the channel by a thin layer of an insulator. In the sFET, however, the source and drain contacts are to be made of a ferromagnetic material (FM), so that the current flowing in the channel is assumed to be spin polarized and the switching between the high and low conductance states is realized by rotating the spins by 180 degrees on their way from the source to the drain. This manipulation takes place via the spin-orbit coupling (SOC), the effect by which the spin magnetic moment is coupled to the magnetic moment due to the orbital motion of an electron. This coupling can be described by an effective magnetic field that interacts with the spin magnetic moment. In the presence of this field, the spins can rotate as they move through the sFET channel. If the strength of the spin-orbit coupling can be controlled by the electric field, the orientation of the spins can be controllably reversed as they travel between the source and drain, and the conduction state of the sFET can be switched. Despite extensive research performed on the topic, the practical realization of the sFET has so far been hampered by various constraints imposed in the original proposal. One of these is the requirement of ballistic transport , which means that the electrons should move between the source and drain without being scattered, i.e., they should travel a distance shorter than the mean free path of the electrons. It was thought that only in this case the spinpolarization of all electrons could be preserved and the orientation of the entire spin ensemble efficiently rotated. In this project, we investigated the possibility of controlled spin rotation in the non-ballistic channels, longer than the mean free path. We have successfully demonstrated that by making such channels very narrow, one can achieve similar control over spin rotation as in the ballistic channels. This shows that functional sFETs are not limited to short ballistic channels with large spin-orbit coupling, as postulated in the original proposal, which significantly increases their applicability. This could be particularly important for the realization of sFETs in a novel van der Waals material platform, where short mean free paths are typically observed. Since the mean free path decreases with temperature, our observation could also open a path to sFET operation at elevated temperatures.
Publications
-
Diffusive Spin Transport in Narrow Two-Dimensional-Electron-gas Channels. Physical Review Applied, 16(1).
Eberle, F.; Schuh, D.; Bougeard, D.; Weiss, D. & Ciorga, M.
-
Control of spin relaxation and spin precession in diffusive 2DEG channels, Ph.D. thesis, University of Regensburg
Franz Eberle
-
Controlled Rotation of Electrically Injected Spins in a Nonballistic Spin-Field-Effect Transistor. Nano Letters, 23(11), 4815-4821.
Eberle, Franz; Schuh, Dieter; Grünewald, Benedikt; Bougeard, Dominique; Weiss, Dieter & Ciorga, Mariusz
-
Linear and non-linear spin-to-charge conversion in a quantum point contact, Ph.D. thesis, University of Regensburg
Benedikt Grünewald
-
Perspective on the spin field-effect transistor. Journal of Physics D: Applied Physics, 58(1), 012001.
Ciorga, Mariusz
