A TEM investigation of patterned ferromagnetic nanostructures by lithographic techniques

This PhD research project encompasses an investigation into the controlled behaviour of magnetic domain walls in patterned ferromagnetic nanostructures using advanced nanofabrication techniques and characterised by the techniques of transmission electron microscopy. By fabricating Permalloy (Py) nanowires using electron beam lithography (EBL) and focused ion beam (FIB) milling a comparative study has been made in which key differences in the magnetic behaviour have been identified. Nominally identical Py nanowires, with widths down to 150 nm, were fabricated onto a single electron transparent Si3N4 membrane. Transmission electron microscopy (TEM) experiments were performed to compare the nanostructures produced by these two techniques in what we believe is the first direct comparison of fabrication techniques for nominally identical nanowires. Both EBL and FIB methods produced high quality structures with edge roughness being of the order of the mean grain size 5 -10 nm observed in the continuous films. However, significant grain growth was observed along the edges of the FIB patterned nanowires. Lorentz TEM \emph{in situ} imaging was carried out to compare the magnetic behavior of the domain walls (DWs) in the patterned nanowires with anti-notches present to pin DWs. The overall process of DW pinning and depinning at the anti-notches showed consistent behaviour between nanowires fabricated by the two methods with the FIB structures having slightly lower characteristic fields compared to the EBL wires. However, a significant difference was observed in the formation of a vortex structure inside the anti-notches of the EBL nanowires after depinning of the domain walls. No vortex structure was seen inside the anti-notches of the FIB patterned nanowires. Whilst the two fabrication methods show that well defined structures can be produced for the dimensions considered here, the differences in the magnetic behavior for nominally identical structures may be an issue if such structures are to be used as conduits for domain walls in potential memory and logic applications. In this project, investigations were also carried out on ion irradiation of nanowires in which DW pinning sites arise from a controlled local modification of the magnetic properties of the nanowire. The nanowires comprised a multilayer thin film of Cr(3 nm)/Py(10 nm)/Cr(5 nm) in which the local magnetic properties were varied by irradiation with Ga ions in a focused ion beam microscope. Alloying Py with Cr is known to significantly change its magnetic properties. The nanowires were patterned and irradiated in the FIB, a single irradiation line was used to create a pinning site at an angle of 45 degrees to the wire length. Observation of the magnetic state of the nanowires was made using Lorentz microscopy. A transverse DW (TDW) was created at the end of a 500 nm wide nanowire and then a field applied to move it towards the pinning site. This TDW was pinned at the irradiated line for low doses. For lines with increased dose the TDW was often seen to transform into a vortex DW. The change of micromagnetic wall structure and the dependence of the subsequent depinning field on the dose and wall type were investigated together with initial simulation results modeling these features. In addition to the control of the DWs, our findings indicate potential for engineering and filtering DWs of certain types as they pass irradiated features at predefined locations in nanowires, dependent on the dose associated with these features in magnetic nanowires sandwiched between metallic layers. On a continuous 16.9±0.8 nm thick Py film, magnetically softer and harder stripes, which are in direct lateral contact by means of exchange coupling, were fabricated by focused Ga+ irradiation. The irradiation dose was 6.24x1015 ions/cm2 and the width of the alternate exposed and unexposed stripes were varied from 1000 nm to 200 nm. Low angle electron diffraction experiment confirmed that due to this amount of ion dose, saturation magnetic induction of the irradiated stripe is reduced to 72.0±0.7 % of the unirradiated stripe, assuming a thickness reduction of 2.9±0.7 nm determined from TEM cross-sectional image. Magnetisation reversal experiments were carried out using high resolution Lorentz microscopy. Starting from a stripe width of 1000 nm a pronounced two step reversal with nearly anti-parallel orientated magnetisation in neighboring stripes is observed. Differential phase contrast (DPC) images demonstrate a transition from the discrete switching of the wider nanostripes to the tendency of collective switching of the narrower stripes of width 200 nm. This transition is associated with vanishing ability of hosting neighboring high angle domain walls between adjacent stripes.