Our research in spintronics is aimed at understanding the basic steps needed for spin-based information storage and processing. In spintronics devices, the spin degree of freedom provides functionality in addition to the charge of the electron. We are constructing devices made from ferromagnetic metals combined with semiconductors and are investigating the injection of spin-polarized charges, as well as the transport, manipulation and detection of spin. In addition, the aspect of spin coherence is investigated for possible quantum information processing devices.
Our devices are made of inorganic semiconductors, semiconductor quantum structures, and organic materials. Planar and layered structures are fabricated and measured using in-plane transport as well as tunneling across heterostructures.
Spin injection from a ferromagnetic metal into a semiconductor can be achieved by electrical transport across a tunneling barrier or a Schottky contact. The role of a tunneling barrier is twofold.
1. It serves as a buffer layer and hence prevents chemical reactions between metal and semiconductor layers.
2. It provides a means to match energy levels for efficient spin-injection efficiency.
Studies of spin-injection from the ferromagnetic tip of a scanning tunneling microscope allows us to spatially and energetically resolve the injection efficiency. An alternative approach to spin injection is to use optical selection rules, which relate the helicity of photons with the spin-orientation of conductance-band electrons.
For device operation, spin has to be transported between two contacts without losing its polarization. With this respect, the coherence time and the related spin coherence length are relevant.
Typically, spin is manipulated by exposing it to a magnetic field. Magnetic fields can be generated and controlled externally or in the device itself in the form of an effective magnetic field originating from hyperfine interaction with polarized nuclei, effective g-factors or spin-orbit interaction (Dresselhaus and Rashba fields).
Various techniques are used to detect spin-polarized charge carriers. Owing to optical selection rules, the electroluminescence is polarized according to the spin-polarization of the recombining charges. Kerr and Faraday rotation measurements on semiconductors allow detecting spin polarization with sub-picosecond time resolution. Ultimately, device operation has to rely on electrical detection of spin polarization using spin-filtering at the interface to a ferromagnetic metal.