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Jeudi 10 Avril 2014, 14h
Amphi Holweck, Esc C, 1ème etage
Spin-Orbit Effects in Electron Transport
B J Hickey
E C Stoner Lab, Department of Physics and Astronomy
University of Leeds
The spin-orbit interaction seems unavoidable in many aspects of Condensed Matter Physics and in spintronics it is ubiquitous. Many of the effects are understood and have been with us for a long time, the anisotropic magnetoresistance (AMR) of ferromagnets for example, but recently, it was proposed [1] that the AMR can exist in thin layers of Pt where only one atomic layer of has an induced magnetisation by proximity. This is controversial however, as others think that the effect is actually the spin Hall magnetoresistance. The interaction between electron spins and magnons in an insulating ferromagnet can give rise, through the spin orbit interaction, to a magnetoresistance that derives from the spin Hall and inverse spin Hall effects. We have studied this effect in platinum on Yttrium Iron garnet (YIG) thin films. The YIG films have been made by RF magnetron sputtering grown on Gadolinium Gallium garnet substrates (GGG). We have used these samples to measure the temperature dependence of the spin Hall magnetoresistance and fitted the data using a recently published theory[2]. The spin-orbit interaction is evident in the origin of the effect and also in the temperature dependence through the Elliot-Yafet mechanism which we suggest is responsible for the spin relaxation.
In a very different system we have been studying the spin-orbit effect that gives rise to quite unexpected results. Lateral spin valves made from carbon nanotubes are interesting candidates for spin dependent transport because they possess many of the desirable features of graphene and are already in a convenient shape for use as a conductor. Our lateral spin valves use single-wall nanotubes contacted by permalloy electrodes with a tunnel barrier between the metal and the nanotube. Thus the tube is a quantum dot and can be operated within or outwith the Coulomb blockade (CB)regime. Outside the CB regime we measure a TMR of about 10% but more interestingly, within the CB regime where the device functions at the single electron level, we measure magnetoresistance in excess of 300%. Due to the sensitivity of the dot to its environment, the conductance of the dot can be changed electrostatically and the large MR is associated with tuning the dot on and off a CB resonance, but without changing either the gate voltage or the source-drain bias. Using magnetic fields directed along the tube axis, we have also demonstrated the full spin-orbit induced splitting of the dot energy levels and show how these act as a spin filter for single electron transport.
[1] Lu YM et al PRL 110 147207 (2013)
[2] Chen, YT et al Phys Rev B 87, 144411, 2013