Magnetowiderstand und Ummagnetisierungsprozesse in einzelnen nanostrukturierten magnetischen Leiterbahnen
The main subject of this thesis was the investigation whether or not the magnetoresistance behavior of single nanostructured magnetic nanowires senses directly the magnetization reversal processes. The nanowires are prepared by means of a combination of electron beam lithography (EBL) and electron beam evaporation. Using a special two step EBL process nonmagnetic gold contacts can be attached precisely to the nanowires. Thus a magnetic influence of the reversal processes of the nanowires by the contact structures can be excluded. The single nanowires are in general 40 µm long and the Co film thickness is 30 nm whereas the width of the wires is varied between 100 nm and 10 µm. To avoid surface oxidation the Co wires are covered in situ with a 2 nm thick Pt layer. The resistance is measured using a AC 4 point probe resistance measurement technique with a precision of about 5*10^(-6). As the resistance behavior of the nanowires depends on the magnetic as well as on the structural properties numerous investigations are carried out. The structure and morphology was investigated by means of different scanning probe techniques as e.g. SEM, AFM, STM and using TEM as well as electron diffraction. We reveal a polycrystalline structure of the Co nanowires with a mean grain size of about 7 nm. The crystal structure of the Co is mainly hcp. SQUID investigations of Co nanowire gratings reveal approximately rectangular shaped hysteresis loops. From this a saturation magnetization of 1385 kA/m results which fits very well to the literature value. The imaging of the domain structure using Kerr microscopy and MFM reveals that the magnetization is oriented in plane during the whole magnetization reversal process and that domain walls of the Neel type occur. Dependent on the wire width a transition from a multidomain remanent domain configuration (w>=2µm) to a monodomain remanent domain configuration (w<=1µm) has been observed. The domain structures confirm the magnetization processes deduced from the magnetoresistance measurements taking the anisotropic magnetoresistance into account. The transversal magnetoresistance reflects a coherent rotation of the magnetic moments independent on the wire width. This reversal mechanism can be excluded in the case of the longitudinal magnetoresistance. In this measurement geometry we observe different reversal mechanisms depending on the wire width. Small wires reverse magnetically by end nucleation of a vortex like domain wall and moving of this structure through the wire. For wide wires the reversal process is due to the formation of complex domain structures. These different magnetization reversal mechanisms are confirmed by means of Kerr-microscopy and Monte Carlo simulations. The wire widths dependence of the coercive field shows a 1/w-behavior independent on the various methods that have been used for the investigations, e.g. magnetoresistance, SQUID, stray field measurements as well as Monte Carlo simulations, and can be explained by demagnetising effects and shape anisotropy. From angular dependent investigation of the magnetoresistance a combination of both reversal mechanisms reflected by the transversal and the longitudinal resistance can be deduced. Quantitativ analysis of the magnetoresistance behavior shows the high sensitivity of the resistance measurements to sense the magnetization reversal process in comparison to the imaging methods used. Furthermore these quantitative analyses shows that an additional resistance contribution besides the anisotropic magnetoresistance has to be taken into account. However, this does not influence the interpretation of the magnetization reversal processes.
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