Design, fabrication, and characterization of magnetic nanostructures

Abstract

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 magnetoresistanceeffect 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.

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Identifiers

ORCID for C. de Groot: orcid.org/0000-0002-3850-7101

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