Observation of second-harmonic generation induced by pure spin currents

Extensive efforts are currently being devoted to developing a new electronic technology, called spintronics, where the spin of electrons is explored to carry information. [1,2] Several techniques have been developed to generate pure spin currents in many materials and structures. [3-10] However, there is still no method available that can be used to directly detect pure spin currents, which carry no net charge current and no net magnetization. Currently, studies of pure spin currents rely on measuring the induced spin accumulation with optical techniques [5, 11-13] or spin-valve configurations. [14-17] However, the spin accumulation does not directly reflect the spatial distribution or temporal dynamics of the pure spin current, and therefore cannot monitor the pure spin current in a real-time and real-space fashion. This imposes severe constraints on research in this field. Here we demonstrate a second-order nonlinear optical effect of the pure spin current. We show that such a nonlinear optical effect, which has never been explored before, can be used for the non-invasive, non-destructive, and real-time imaging of pure spin currents. Since this detection scheme does not rely on optical resonances, it can be generally applied in a wide range of materials with different electronic bandstructures. Furthermore, the control of nonlinear optical properties of materials with pure spin currents may have potential applications in photonics integrated with spintronics.Comment: 19 pages, 3 figures, supplementary discussion adde

Spin-Dependent Transport in Fe/GaAs(100)/Fe Vertical Spin-Valves

The integration of magnetic materials with semiconductors will lead to the development of the next spintronics devices such as spin field effect transistor (SFET), which is capable of both data storage and processing. While the fabrication and transport studies of lateral SFET have attracted greatly attentions, there are only few studies of vertical devices, which may offer the opportunity for the future three-dimensional integration. Here, we provide evidence of two-terminal electrical spin injection and detection in Fe/GaAs/Fe vertical spin-valves (SVs) with the GaAs layer of 50 nanometers thick and top and bottom Fe electrodes deposited by molecular beam epitaxy. The spin-valve effect, which corresponds to the individual switching of the top and bottom Fe layers, is bias dependent and observed up to 20 K. We propose that the strongly bias-and temperature-dependent MR is associated with spin transport at the interfacial Fe/GaAs Schottky contacts and in the GaAs membranes, where balance between the barrier profiles as well as the dwell time to spin lifetime ratio are crucial factors for determining the device operations. The demonstration of the fabrication and spin injection in the vertical SV with a semiconductor interlayer is expected to open a new avenue in exploring the SFET

Magnetic two-dimensional systems

Two-dimensional (2D) systems have considerably strengthened their position as one of the premier candidates to become the key material for the proposed spintronics technology, in which computational logic, communications and information storage are all processed by the electron spin. In this article, some of the most representative 2D materials including ferromagnetic metals (FMs) and diluted magnetic semiconductor (DMSs) in their thin film form, magnetic topological insulators (TIs), magnetic graphene and magnetic transition metal dichalcogenides (TMDs) are reviewed for their recent research progresses. FM thin films have spontaneous magnetization and usually high Curie temperature (Tc), though this can be strongly altered when bonded with semiconductors (SCs). DMS and magnetic TIs have the advantage of easy integration with the existing SC-based technologies, but less robust magnetism. Magnetic ordering in graphene and TMDs are even more fragile and limited to cryogenic temperatures so far, but they are particularly interesting topics due to the desired long spin lifetime as well as the outstanding mechanical and optical properties of these materials

Graphene As a Tunnel Barrier: Graphene-Based Magnetic Tunnel Junctions

Graphene has been widely studied for its high in-plane charge carrier mobility and long spin diffusion lengths. In contrast, the out-of-plane charge and spin transport behavior of this atomically thin material have not been well addressed. We show here that while graphene exhibits metallic conductivity in-plane, it serves effectively as an insulator for transport perpendicular to the plane. We report fabrication of tunnel junctions using single-layer graphene between two ferromagnetic metal layers in a fully scalable photolithographic process. The transport occurs by quantum tunneling perpendicular to the graphene plane and preserves a net spin polarization of the current from the contact so that the structures exhibit tunneling magnetoresistance to 425 K. These results demonstrate that graphene can function as an effective tunnel barrier for both charge and spin-based devices and enable realization of more complex graphene-based devices for highly functional nanoscale circuits, such as tunnel transistors, nonvolatile magnetic memory, and reprogrammable spin logic