Design, fabrication, and characterization of magnetic nanostructures

FACULTY OF ENGINEERING, SCIENCE AND MATHEMATICS SCHOOL OF ELECTRONICS AND COMPUTER SCIENCE Doctor of Philosophy by David Claudio Gonzalez For several years, thin films of ferromagnetic materials with metallic spacer layers showing giant magnetoresistance (GMR) were the technological basis used in the read-heads of hard disk drives. Similarly, tunnelling magnetoresistance (TMR), which is an effect typically larger than giant magnetoresistance, occurs when the metallic spacer layers are substituted by an insulating layer. Read-heads based on tunnelling magnetoresistance have been available to the consumer market for the last couple of years. Furthermore, nonvolatile random access memories, also known as magnetic random access memory (MRAM) have also been possible thanks to the use of the tunnelling magnetoresistance effect and have recently been introduced to the consumer market. These technological advances have been possible thanks to years of extensive study and optimization devoted to such effects. Following this logic, we arrive at the conclusion that an effect that produces higher magnetoresistance ratios but uses lower magnetic fields or even only electric currents is highly desired and could be useful for the design and fabrication of spintronic devices of tomorrow. In this thesis, the use of electron beam lithography (EBL) and a bilayer liftoff process to fabricate magnetic Ni nanostructures with constrictions in the range of 12 to 60 nm is reported. These structures were fabricated based upon the constricted nanowire (CNW) and nanobridge (NB) geometries. High control and reproducibility in the fabrication of such geometries have been achieved with the introduced bilayer liftoff process. This is important because it provides the opportunity to study the statistics of the domain wall magnetoresistance (DWMR) effect and assess its reproducibility. Additionally, micromagnetic simulations of the fabricated structures were carried out and it was found that domain walls (DWs) with reduced widths down to 48 and 42.5 nm, can be achieved using the CNW and NB geometries, respectively. The magnetoresistance effect due to the presence of a DW has been estimated using dimensions achieved experimentally. Furthermore, the anisotropic magnetoresistance (AMR) effect was obtained numerically and it was found to be smaller than DWMR. This opens the possibility of using the fabricated structures for more systematic studies of DWMR. Declaration of Authorship I, David Claudio González, declare that the thesis entitled Design, Fabrication and characterisation of Magnetic Nanostructures and the work presented in it are my own. I confim that: • this work was done wholly or mainly while in candidature for a research degree at this University; • where any part of this thesis has previously been submitted for a degree or any other qualification at this University of any other institution, this has been clearly stated; • where I have consulted the published work of others, this is always clearly attributed • where I have quoted from the work of others, the source is always given. With the exception of such quotations, this thesis is entirely my own work; • I have acknowledged all main sources of help; • where the thesis is based on work done by myself jointly with others, I have made clear exactly what was done by others and what I have contributed myself; • parts of this work have been published in research journals as indicated in the list of publications provided with this manuscript.