Brands, Mario:

Elektronischer Transport in nanostrukturierten ferromagnetischen Leiterbahnen

Duisburg (2005), II, 135 S. : Ill., graph. Darst.
Dissertation / Fach: Physik
Fakultät für Physik
Duisburg, Essen, Univ., Diss., 2005
Abstract:
Within the present thesis the influence of the magnetization on the resistance behavior of ferromagnetic nanowires is investigated. Special emphasis lies on the quantitative determination of the domain wall resistance (DWR) of single domain walls and the analysis of quantum transport phenomena. For this, two systems of in-plane magnetized Co-nanowires and out-of-plane magnetized (Co/Pt)_n-multilayer-nanowires are investigated. The nanowires are fabricated using a combination of both high-resolution multi-step electron beam lithography with lift-off-technique and electron beam evaporation. Some of the wires are capped in situ with insulating carbon to protect them from oxidation. The magnetoresistance at low temperatures is dominated by the anisotropic magnetoresistance (AMR). Upon variation of the orientation of the magnetic field both the characterization of the magnetic configuration and the relevant magnetization reversal processes as a function of the wire thickness and width are obtained. For in-plane magnetized cobalt nanowires domain wall pinning is accomplished by means of lateral nanostructuring. However, the resistance of a single domain wall can be mainly attributed to the AMR. For (Co/Pt)_n-multilayers with perpendicular magnetic anisotropy contributions from the AMR can be excluded and the resistance of a single domain wall at T = 4,2 K is positive with DWR ~ 2,5%. The positive MR is explained quantitatively within a model incorporating spin-dependent scattering, whereas a negative MR resulting from delocalization of electrons is not observed. The low temperature resistance behavior of the ferromagnetic nanowires shows deviations from the classical resistance behavior of a typical metal such that the resistance exhibits a logarithmic increase with decreasing temperature, which can be attributed to quantum corrections due to enhanced electron-electron interactions (EEI) in two dimensions. Upon reduction of the wire width down to 32 nm a cross-over from two- to one-dimensional behavior with respect to EEI-contributions is observed. The data analysis does not yield corresponding resistance corrections due to weak electron localization (WEL). Even by systematic variations of both the wire thickness and the material as well as the degree of disorder WEL-effects remain undetected. Moreover, quantum corrections for ferromagnetic (Co/Pt)_n-multilayer-wires with perpendicular magnetic anisotropy are analyzed for the first time, for which also only contributions from enhanced electron-electron interactions are found. The absence of weak localization effects in polycrystalline nanowires is explained by local inhomogeneities of the magnetization.