Electron Transport through Domain Walls in Ferromagnetic Nanowires
FALLOON, Peter Edward (2006) Electron Transport through Domain Walls in Ferromagnetic Nanowires. Thèses de doctorat, Université Louis Pasteur.
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In this dissertation we present a theoretical study of electron transport through domain walls, with a particular focus on conductance properties, in order to understand various transport measurements that have been carried out recently on ferromagnetic nanowires. The starting point for our work is a ballistic treatment of transport through the domain wall. In this case conduction electrons are generally only weakly reflected by the domain wall, and the principal effect is a mixing of transmitted electron spins between up and down states. For small spinsplitting of conductance electrons the latter can be characterized by an appropriate adiabaticity parameter. We then incorporate the effect of spindependent scattering in the regions adjacent to the domain wall through a circuit model based on a generalization of the two-resistor theory of Valet and Fert. Within this model we find that the domain wall gives rise to an enhancement of resistance similar to the giant magnetoresistance effect found in ferromagnetic multilayer systems. The effect is largest in the limit of an abrupt wall, for which there is complete mistracking of spin, and decreases with increasing wall width due to the reduction of spin mistracking. For reasonable physical parameter values we find order-of-magnitude agreement with recent experiments. Going beyond the assumption of ballistic transport, we then consider the more realistic case of a domain wall subject to impurity scattering. A scattering matrix formalism is used to calculate conductance through a disordered region with either uniform magnetization or a domain wall. By combining either amplitudes or probabilities we are able to study both coherent and incoherent transport properties. The coherent case corresponds to elastic scattering by static defects, which is dominant at low temperatures, while the incoherent case provides a phenomenological description of the inelastic scattering present in real physical systems at room temperature. It is found that scattering from impurities increases the amount of spin mistracking of electrons travelling through a domain wall. This leads, in the incoherent case, to a reduction of conductance through the domain wall as compared to a uniformly magnetized region. In the coherent case, on the other hand, a reduction of weak localization and spin-reversing reflection amplitudes combine to give a positive contribution to domain wall conductance, which can lead to an overall enhancement of conductance due to the domain wall in the diffusive regime. A reduction of universal conductance fluctuations is found in a coherent disordered domain wall, which can be attributed to a decorrelation between spin-mixing and spin-conserving scattering amplitudes. To treat the total effect of a disordered domain wall on the conductance of a nanowire, we extend the scattering matrix approach to incorporate the regions adjacent to the domain wall, thus providing a microscopic equivalent of the circuit model studied for a ballistic wall. It is found that scattering in the adjacent regions leads to an increase in domain wall magnetoconductance as compared to the effect found by including only the scattering inside the wall. This increase is most dramatic in the limit of narrow walls, but is also significant in the limit of wide walls. Finally, we apply our model to calculate the spin-transfer torque exerted by a spin-polarized current on a domain wall. Within the circuit model we find expressions for the spin-transfer torque and corresponding domain wall velocity, ignoring pinning effects.
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