An Investigation of the Structure, Pinning and Magnetoresistance of Domain Walls in Ni81Fe19 Planar Nanowires

Abstract

The research and development of Ni81Fe19 thin films and planar nanowire structures has attracted considerable interest in recent years; in terms of improving the fundamental understanding of the basic physical processes and also for the development of potential applications. Example applications include sensors and the data storage devices. The optimisation of such devices requires detailed knowledge of the thickness dependence and microstructural influences on the magnetic and magnetoresistance properties, along with a thorough understanding of the effect of geometrical confinement on domain wall (DW) structure and pinning behaviour in nanowire structures. The out-of-plane structural properties of thermally evaporated Ni81Fe19 thin films on pre-oxidised silicon substrates have been investigated using x-ray scattering techniques and transmission electron microscopy (TEM). These techniques have been used to provide information on the out-of-plane lattice parameter, the presence and degree of texture and also to quantify the width of the SiO2/Ni81Fe19 interface. Magneto-optical Kerr effect (MOKE) magnetometry, differential phase contrast TEM imaging, micromagnetic simulations and anisotropic magnetoresistance measurements (AMR) have been used to make a detailed study of the thickness dependence of the magnetic behaviour of both thin films and nanowire structures. The resistivity of thin films produced in this study is found to exhibit a higher value and lower mean free path than has previously been reported in the literature, which is attributed to the presence of a microstructure characterised by a small crystallite grain structure. The AMR is strongly thickness dependent for t < 10 nm, and tends toward zero for t < 7 nm. It is suggested that this is due to strain at the SiO2/Ni81Fe19 interface, which changes the magnetostriction and is related to the AMR by spin-orbit effects. The structure and pinning behaviour of DWs has been systematically investigated as a function of nanowire width, thickness and notch geometry. Although the wall structure is sensitive to the nanowire cross-sectional area, the DW depinning behaviour is relatively insensitive to notch geometry and instead is highly sensitive to wall type and chirality. A detailed model has been developed to make predictions for the AMR of individual DWs in nanowires. The model incorporates experimentally derived thickness dependent resistivity parameters and detailed DW spin structures from micromagnetic simulations. The magnitude of DW resistance is sensitive to wire width and the AMR ratio, and is found to be extremely sensitive to the magnitude of the magnetoresistance