Itinerant Spin Dynamics in Structures of ... - Jacobs University

Chapter 5
Spin Hall Effect
5.1 Introduction
In order to realize spintronic devices like the spin field effect transistor, one needs
to induce spin polarized electrons in low dimensional electron systems (LDES)[icvacFDS04,
DD90]. Spin polarized electrons can be generated by injecting a current with ferromagnetic
metallic leads into the LDES. However, it has been found that in practice the efficiency
of such spin injection is poor because of the conductivity mismatch. Therefore, recently
there has been a strong effort to find new ways for generating polarized spin currents. One
possibility is to use a T-shaped conductor with SOC as proposed by Yamamoto[YDKO06],
whose efficiency is restricted however strongly by impurity scattering and is therefore only
applicable to narrow wires with few channels. Another approach is to dope the semiconductor
with magnetic impurities: When doped with several percent one speaks of dilute
magnetic semiconductors. With transition metal atoms, like Mn, these materials become
ferromagnetic, such as In 1−x Mn x As which has been discovered by Ohno to have a ferromagnetic
phase with a relatively high critical temperature[OMP + 92]. Due to the still relatively
small concentration of impurities this system can still be manipulated in a wide range of
carrier density, impurity concentration and acceptor level energy: As these impurities not
only provide spin, but also dope the system with holes, the density of charge carriers, and
the Fermi energy can be changed, by changing their concentration. By choosing different
elements, also the acceptor level energy can be changed. This together with the disorder
provided by these magnetic impurities does change the magnetic properties substantially,
since the effective magnetic coupling between the magnetic ions does depend itself on the
83