DYNAMIC MAGNETIC EFFECTS IN AMORPHOUS MICROWIRES FOR SENSORS AND CODING APPLICATIONS

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

Off-diagonal impedance in amorphous wires and application to linear magnetic sensors

The magnetic-field behaviour of the off-diagonal impedance in Co-based amorphous wires is investigated under the condition of sinusoidal (50 MHz) and pulsed (5 ns rising time) current excitations. For comparison, the field characteristics of the diagonal impedance are measured as well. In general, when an alternating current is applied to a magnetic wire the voltage signal is generated not only across the wire but also in the coil mounted on it. These voltages are related with the diagonal and off-diagonal impedances, respectively. It is demonstrated that these impedances have a different behaviour as a function of axial magnetic field: the former is symmetrical and the latter is antisymmetrical with a near linear portion within a certain field interval. In the case of the off-diagonal response, the dc bias current eliminating circular domains is necessary. The pulsed excitation that combines both high and low frequency harmonics produces the off-diagonal voltage response without additional bias current or field. This suits ideal for a practical sensor circuit design. The principles of operation of a linear magnetic sensor based on C-MOS transistor circuit are discussed.Comment: Accepted to IEEE Trans. Magn. (2004

Stress effect on magnetoimpedance (MI) in amorphous wires at GHz frequencies and application to stress-tunable microwave composite materials

The effect of tensile stress on magnetoimpedance (MI) in CoMnSiB amorphous wires at microwave frequencies (0.5-3 GHz) is investigated both experimentally and theoretically. In the presence of the dc bias magnetic field of the order of the anisotropy field, the impedance shows very large and sensitive change when the wire is subjected to a tensile stress: 100% and 60% per 180 MPa for frequencies 500 MHz and 2.5 GHz, respectively. It is demonstrated that this behavior owes mainly to the directional change in the equilibrium magnetization caused by the applied stress and field, which agrees well with the theoretical results for the surface impedance. This stress effect on MI is proposed to use for creating microwave stress-tunable composite materials containing short magnetic wires. The analysis of the dielectric response from such materials shows that depending on the stress level in the material, the dispersion of the effective permittivity can be of a resonant or relaxation type with a considerable change in its values (up to 100% at 600 MPa). This media can be used for structural stress monitoring by microwave contrast imaging

Ferromagnetic Wires Composite Media with Tunable Scattering Spectra at Microwaves

We demonstrate composite media with ferromagnetic wires that exhibit a frequency region at the microwave regime with scattering spectra strongly dependent on an external magnetic field or stress. These tunable composite materials have recently been proposed theoretically; however, no direct experimental verification has been reported. We used composite materials with predominantly oriented CoFeCrSiB glass-coated amorphous wires having large magnetoimpedance at GHz frequencies. The free space measurements of reflection and transmission coefficients were conducted in the frequency range 1-8 GHz in the presence of an external static magnetic field or stress applied to the whole sample. In general, the transmission spectra show greater changes in the range of 10dB for a relatively small magnetic field of few Oe or stress of 0.1 MPa. The obtained results are quantitatively consistent with the analytical expressions predicted by the effective medium arguments. The incident electromagnetic wave induces an electrical dipole moment in each wire, the aggregate of which forms the effective dipole response of the whole composite structure in the radiative near or far field region. The field and stress dependences of the effective response arise from a field or tensile stress sensitivity of the ac surface impedance of a ferromagnetic wire. In the vicinity of the antenna resonance the variations in the magneto-impedance of the wire inclusions result in large changes of the total effective response. A number of applications of proposed materials is discussed including the field tunable microwave surfaces and the self-sensing media for the remote non-destructive evaluation of structural materials

Laser processing effect on magnetic properties of amorphous wires

A study was conducted to observe the laser processing effects on the magnetic properties of amorphous wires. Weekly interacting heterogeneous structures with different magnetic properties were formed by the local annealing by argon laser. Favourable changes were observed due to the creation of local stresses and structural interfaces

Current distribution and giant magnetoimpedance in composite wires with helical magnetic anisotropy

The giant magnetoimpedance effect in composite wires consising of a non-magnetic inner core and soft magnetic shell is studied theoretically. It is assumed that the magnetic shell has a helical anisotropy. The current and field distributions in the composite wire are found by means of a simultaneous solution of Maxwell equations and the Landau-Lifshitz equation. The expressions for the diagonal and off-diagonal impedance are obtained for low and high frequencies. The dependences of the impedance on the anisotropy axis angle and the shell thickness are analyzed. Maximum field sensitivity is shown to correspond to the case of the circular anisotropy in the magnetic shell. It is demonstrated that the optimum shell thickness to obtain maximum impedance ratio is equal to the effective skin depth in the mahnetic material.Comment: 23 pages, 7 figure