Electron Transport through Disordered Domain Walls: Coherent and Incoherent Regimes

We study electron transport through a domain wall in a ferromagnetic nanowire subject to spin-dependent scattering. A scattering matrix formalism is developed to address 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 disorder scattering increases the amount of spin-mixing of transmitted electrons, reducing the adiabaticity. This leads, in the incoherent case, to a reduction of conductance through the domain wall as compared to a uniformly magnetized region which is similar to the giant magnetoresistance effect. In the coherent case, a reduction of weak localization, together with a suppression of spin-reversing scattering amplitudes, leads to an enhancement of conductance due to the domain wall in the regime of strong disorder. The total effect of a domain wall on the conductance of a nanowire is studied by incorporating the disordered regions on either side of the wall. It is found that spin-dependent scattering in these regions increases the domain wall magnetoconductance as compared to the effect found by considering only the scattering inside the wall. This increase is most dramatic in the narrow wall limit, but remains significant for wide walls.Comment: 23 pages, 12 figure

Coulomb blockade and quantum conductance in ferromagnetic nanostructures

This thesis offers an investigation into two recently discovered effects within the quantum area of Spin Electronics. The work begins with the introduction outlining the basis of single electronics, followed by the summary of the research carried out in the field of spin polarized Coulomb blockade, which began 6 years ago. During the course of this thesis, four separate novel ferromagnetic single electron transistors (FSETs) were designed and fabricated and their description is given in Chapter 6. Out of the four presented, two have operating temperatures above 4 K, which, further to optimization, offers a doorway for possible room temperature operation of metallic single electron devices. Chapter 4 considers the experimental design involved in measuring single electron and quantum conductance effects. A quantum mechanical treatment for the coupling of noise to tunneling electrons is used to predict an optimum attenuation coefficient for our measurements. A custom made filter was designed and fabricated using single step electron beam lithography and tests have shown its efficiency to be comparable to the previously reported version, in addition to it being simpler to fabricate. Chapter 7 investigates the magnetoresistance of ferromagnetic point contacts in the quantum conductance regime using the recently developed 'break junction' technique. The results obtained are explained within the framework of the spin-orbit coupling induced change of orbital overlap at the junction. The resistive jumps in the magnetoresistance curves are related to a field induced change of spin configuration within the few atoms composing the contact. This compliments the previous findings of quantum conductance experiments. Chapter 8 offers a possible explanation behind several unexplained findings from measurements of lateral cobalt-silicon structures whose initial aim was to assess the spin injection efficiency in semiconductors. The variation of the magnetoresistive response of the system with the applied field is found to be consistent with that of a paramagnetic granular system weakly coupled by a dipolar interaction. Finally, in Chapter 9 an analysis of spin injection efficiency is presented that is of general application and relevant to a wide range of spin electronic devices. By applying simple band structure ideas to a single interface between a metallic ferromagnet and a three dimensional semiconductor, two conflicting figures of merit are identified - spin accumulation and polarisation of injected current - and their validity to the analysis of different device types is discussed. (author)Available from British Library Document Supply Centre- DSC:DN061477 / BLDSC - British Library Document Supply CentreSIGLEGBUnited Kingdo

The defining length scales of mesomagnetism: A review

This review is intended as an introduction to mesomagnetism, with an emphasis on what the defining length scales and their origins are. It includes a brief introduction to the mathematics of domains and domain walls before examining the domain patterns and their stability in 1D and 2D confined magnetic structures. This is followed by an investigation of the effects of size and temperature on confined magnetic structures. Then, the relationship between mesomagnetism and the developing field of spin electronics is discussed. In particular, the various types of magnetoresistance, with an emphasis on the theory and applications of giant magnetoresistance and tunneling magnetoresistance, are studied. Single electronics are briefly examined before concluding with an outlook on future developments in mesomagnetism