This work is devoted to the study of the dynamic properties of magnetic amorphous wires,
in particular, glass-coated microwires, which have small diameters (5-30 microns), outstanding
soft magnetic behaviour with a high permeability and low coercivity, yet, possess a well-defined
magnetic structure.
First part of my PhD research has been devoted to the investigation of a bi-stable
magnetisation reversal in glass-coated amorphous microwires. In contrast to traditional
approaches, where characteristics of the magnetisation reversal are analysed as a consequence of
the eddy current effect, l have applied stochastic methods for modelling the remagnetisation
reversal in the microwires with axial anisotropy. While the eddy current approach, widely
discussed in literature, was based on the single domain model, proposed stochastic approach
takes into account a multi-domain state of studied samples. A modified stochastic Neel-Brown
model of the magnetisation reversal has been proposed enabling the explanation of number of
characteristic parameters of the microwires with axial magnetisation. Such important parameters
of Barkhausen discontinuity as a mean switching field and a standard deviation of the switching
field distribution have been investigated experimentally for understanding the influence of
extrinsic factors such as a slew rate of the alternating magnetic field on applications operation.
A deep understanding of the remagnetisation process in amorphous the microwires with
axial anisotropy was successfully applied in development of a new type of the remote magnetic
interrogation system. My reading system allows the large Barkhausen jump to be detected
without actual contact between the magnetic microwire and the magnetic field detector.
Experiments show that the detection will be possible at a distance of approximately 100-150 mm
from the detecting sensor. A very low cost and easily repetitive amorphous microwires with axial
anisotropy are . incontrovertibly best materials for Electronic Article Surveillance (EAS)
applications.
During the study of the microwires with axial anisotropy and development of the
application based on them, I took part in the investigation of unusual coding methods of the
amorphous microwires using a localised laser annealing treatment. This treatment produces a
multi-pulse code within the wire and therefore adds to the information contained within the wire,
improving reliability and security. I developed and used a magnetic interrogation system
allowing an accurate and reliable test and analysis of the studied samples.
The second part of my PhD research has included investigations of microwires with
circumferential and helical anisotropies. The main interest in these materials is due to their
applications for high-performance magnetic and stress sensors. Within this research project, the
microwires with circumferential/helical anisotropy have been studied in a broad range of
frequencies. A number of dynamic effects have been experimentally obtained and analysed. In
particular, a detailed investigation of dynamic circular hysteresis (10kHz-300kHz) has been
carried out allowing explanation of different behaviour of the materials with
circumferential/helical anisotropy at different frequencies. The experimental curves are proposed
to be analysed in terms of field dependence of characteristic permeabilities: domain wall
displacements (reversible and irreversible) and magnetisation rotation. It was established that
these permeabilities have different field behaviour. That explains different MI patterns at
relatively low frequencies (less than a few MHz) and relatively high frequencies (more than 10
MHz).
Further, some special features of the Magneto-Impedance effect in the microwires with a
circumferential anisotropy such as off-diagonal impedance and microwave impedance have been
considered. In this research, the former presents a considerable interest for development of linear
magnetic sensors and the latter can find application in tuneable microwave materials and
devices. As a result of this study several types of linear, bi-directional MI sensors were
developed. I also developed new MI sensing approaches (such as off-diagonal response) and a
new high performance detection technique allowing us to improve sensitivity, bandwidth, and
linearity at low cost and simple construction ..
The last part of the PhD research has been devoted to an investigation of the stress-impedance
in the ultra high-frequency (UHF) band (300MHz-3 GHz). Based on the experimental
investigation, a new type of a stress-sensitive composite material is proposed. The microwave
effective permittivity of such material depends on mechanical stresses. These composite
materials opens up new possibilities for remote monitoring of stress with the use of microwave
"free-space" techniques. This kind of composite material can be characterised as a "sensing
medium", which images the mechanical stress distribution inside construction or on its surface