Magnetic metamaterials for innovative solutions to the inductive tagging problem

Abstract

In this thesis, two Electronic Article Surveillance (EAS) systems will be studied, focusing on the security tag element. Using metamaterial concepts, an acousto-magnetic tag that resonates at 58 kHz with a smaller footprint will be presented. This will be achieved by patterning the resonant magnetostrictive ribbons within the tag with air holes, as a finite analogue to a Phononic Crystal (PnC), and measuring the relationship between the air hole geometry and the resonance frequency, amplitude, and the number of resonant modes that are excited. This will be achieved with experimental and computational studies, where Finite Element Method (FEM) models will be used to investigate more geometries than can be experimentally realised. The effect of the air holes on the magnetic properties of the magnetostrictive ribbons will then be understood through imaging the domain structure of the ribbons. It will be shown that patterning the ribbons disrupts the domain structure, and there is a correlation between the disrupted domain structure and the reduced resonance amplitude that is observed with the patterned ribbons. Magnetic field annealing will then be used to attempt to recover the resonance amplitude of the patterned ribbons. The second security tag that will be studied is the 8.2 MHz Radio Frequency (RF) tag, which does not currently operate on conductive materials. The conventional 8.2 MHz security tag comprises of an LC circuit that is excited to resonance by the component of the magnetic field perpendicular to the plane of the tag. By changing the component of the magnetic field that excites resonance in the tag, so that the tag is excited by the component of the magnetic field parallel to the plane of the tag, two new tags are presented that are based off resonant LC circuits with a ferrite at the core. It will be shown that both of the geometries presented in this thesis are comparable to the conventional 8.2 MHz tag in resonance frequency and amplitude.