Spin transport in normal and superconducting nanowires
odays conventional electronic devices are based on electron charge transport in semiconductor channels. Spintronics is a rapidly emerging technology, which exploits the spin degree of freedom as well as the charge of the electrons. It is believed that extending conventional electronics to spin-electronics can yield devices with new functionality and result in new large scale applications. Examples of already existing spintronic technology are the magnetic random access memory, magneto-resistive read heads in hard drives and various magnetic field sensors. The fundamental requirement for a working spintronic device is the ability to generate, transport and detect spin currents, which are the subject of this thesis.A current, spin polarized by a ferromagnet and injected into a non-magnetic material remains polarized for the duration of the spin relaxation time. This relaxation time, and consequently the useful distance the injected non-equilibrium spin can be transported in the non-magnetic transport channel, is dependent on the underlying spin relaxation mechanisms in the material. Furthermore, the transport channel can be deviced to exploit the spin-orbit scattering within the channel with the aim to achieve novel spin transport effects, such as the Spin Hall effect. We study such mechanisms and effects in normal and superconducting nanowires. The main results of the work are the following:In thin film devices, the thickness of the electron transport channel can be comparable to the electron's mean free path, which makes the surface scattering the dominant scattering mechanism. To investigate how the additional surface momentum scattering affects spin relaxation, the thickness dependence of the spin relaxation parameters was analyzed. Using spin injection into Al nanowires of various thickness, it was found that the spin flip scattering at the surfaces is substantially weaker compared to that within the bulk of Al.A five terminal device having a pair of spin sensitive detector electrodes placed symmetrically about the injection point was used to directly demonstrate the decoupling of spin and charge currents in a one-dimensional transport channel. The spin accumulation is shown to be strictly symmetric about the injection point and independent of the direction of the charge current. For superconducting nanowires, it is found that the spin accumulation is enhanced by up to 5 orders of magnitude compared to that in the normal state of the wire. In contrast, the spin diffusion length is found to decrease by an order of magnitude on transition in to the superconducting state. This is interpreted as due to magnetic impurity rather than spin-orbit dominated spin-flip scattering in the nanowires studied. We additionally observe a giant spin Hall effect in superconductors, which is more than 5 orders of magnitude stronger than the values reported recently for Al nanowires in the normal state.
School:Kungliga Tekniska högskolan
Source Type:Doctoral Dissertation
Keywords:NATURAL SCIENCES; Physics
Date of Publication:01/01/2007