Experimental and micromagnetic study of magnetisation behaviour in isolated ferromagnetic nanowires

Abstract

A full understanding of the magnetism of isolated ferromagnetic nanowires is still an unresolved issue and presents a research challenge for researchers. Understanding the intrinsic magnetic properties of such nanowires is significant from a scientific point of view and will be an issue for potential applications. Therefore, the aim of the work presented here is to investigate the magnetic and magnetisation reversal behaviour of electrodeposited template released isolated ferromagnetic nanowires complemented by detailed micromagnetic simulations.
To understand the fabrication techniques, characterisation systems and the materials used to fabricate the nanowires, the elemental composition, surface topography and magnetic properties of a range of electrodeposited thin films were investigated first.
In order to emphasis the reasons behind studying the magnetic behaviour of such isolated nanowires, the statistical distribution of template pore size and spacing, nanowire dimension distributions, and the morphology of such nanowires have been measured using high resolution scanning electron microscope analysis. To determine the crystal structure and the crystallite sizes within these nanowires, which might have a great influence on their magnetic properties, both the template deposited and selected areas of individual nanowires have been studied using a combination of X-ray diffraction and high resolution transmission electron microscopy incorporating with electron diffraction, respectively.
To align the template released nanowires with respect to the prefabricated micromarkers and electrical contact pads, magnetic field assisted alignment was developed. The effect of field strength, nanowire composition and aspect ratio on the percentage of aligned nanowires have been statistically analysed and the results explained.
The surface and bulk magnetisation reversal behaviour of isolated individual, small bundles and clusters of nanowires is technically challenging but have been successfully measured here using magneto-optical Kerr effect magnetometry and magnetoresistance, respectively. The magnetic switching behaviour obtained from these techniques is compared with each other, the literature and analytical models.
In addition, detailed micromagnetic simulations have been utilised to determine the magnetic behaviour of two wire geometries: cylindrical and planar Ni81Fe19 nanowires with a range of diameters and thicknesses, respectively. For both geometries, the effect of nanowire diameter and thickness on the magnetic properties such as switching fields, remanent magnetisation and magnetic state before and during the switching behaviour, have been analysed and compared. The switching field of the simulated nanowires are compared with their counterparts results obtained from both MOKE and MR measurements. The magnetisation reversal behaviour of these wires geometries was also investigated and compared with the classical analytical curling model of magnetisation reversal.