Semiconductor spintronics: switching spins at low voltage

The emerging ability to measure and control electron spins in nano-structured materials down to the level of single spins is at the heart of the research field of spintronics, with potential applications in logic and quantum computation. In current semiconductor-based logic devices, the electron spin is a quantity that is mostly neglected. The switching functionality of a conventional field-effect transistor is based on charging a channel region with electrons. For a given on/off ratio of the source-drain current, the switching requires a minimum voltage swing related to the thermal energy, which sets a lower limit on the active power consumption of the device. Such a principal limitation is not present if the spin direction of electrons is switched. This observation has triggered huge interest in spintronics as a low-power alternative for logic devices. With the example of existing spintronics device concept, the challenges for using spin switches in logic applications will be discussed. Very large spin filtering efficiencies are needed to use a spin switch as a drop-in replacement for FET-based current switches, setting demanding requirements for the processes of spin injection and detection. An alternative approach is to encode the digital information directly into the spin state and omit excess spin-to-charge conversion, which however requires the development of spin amplification to achieve gain in the spin domain. Many spintronics device concepts comprise nonmagnetic regions where non-equilibrium spin polarization is switched by electrical fields. There, the spins have to be processed within the respective spin lifetime. We will discuss how spin-orbit interaction limits the spin lifetime but at the same time is needed for electrical spin switching. Experimental results based on time-resolved magneto-optical Kerr rotation will be shown that demonstrate fast switching of spins in GaAs-based semiconductor quantum structures with specially engineered spin-orbit interaction where the influence on the spin lifetime is lifted.