DIAGONAL AND OFF-DIAGONAL MAGNETO-IMPEDANCE IN FERROMAGNETIC MICROWIRES AND THIN FILMS

The discovery of the giant magneto-impedance (GMI) effect in 1994 had a strong impact on the development of micro magnetic sensors. In certain soft magnetic materials, such as composites of amorphous thin wires, the magneto-impedance change (MI ratio) is in the range of 50-100% in the MHz frequency band for external magnetic fields of few Oe. Special thin-film structures have been proposed to provide the MI effect in miniature elements. In the present work, the concept of the magneto-impedance matrix has been elaborated, which enables the explanation of variety of MI field characteristics in wires and films from the common point of view. The fabrication technologies of the narrow thin film MI samples with different structures also were developed, including layered films and films integrated with a helical planar microcoil. The experimental technique employed in the work allowed us to measure all components of the total magneto-impedance matrix that came as the first verification of the matrix concept of the magneto-impedance. Different methods of getting the asymmetrical and antisymmetrical magneto-impedance behaviours were proposed demonstrating a great success of the impedance matrix concept. In the case of a simple transverse magnetic anisotropy, the diagonal components of the magneto-impedance matrix are symmetric and the off-diagonal components are antisymmetrical with respect to the de longitudinal magnetic field. The asymmetry in MI behaviour can be related to either a certain asymmetric arrangement of the dc magnetic configuration or a contribution to the measured voltage due to the ac cross-magnetisation process, represented by the off-diagonal component. The first case is realised in the wire and film having the helical or crossed anisotropies respectively, which are subjected to an ac current superposed with a de bias current. In the other approach, the asymmetric voltage response can be obtained by applying the ac current in series through the MI element (wire or film) and the small coil surrounded it. No helical anisotropy is required in this case. These kinds of asymmetrical MI are especially important for developing auto-biased linear MI sensors. The thin film with the integrated planar microcoil allowed us to measure the off-diagonal impedance in the sandwiched film. Results obtained for MI in thin films open up the perspective directions for the integrated MI sensors. The applications of the MI effect are not limited only by magnetic sensor technology. In this work, a new type of tuneable composite materials was proposed, the effective microwave permittivity (dielectric constant) of which depends on the de external magnetic field applied to the composite as a whole. The composite consists of the short pieces of ferromagnetic wires embedded into a dielectric matrix. The composite sample can be fabricated in the form of thin slab with thickness less than 1 millimetre. The short wire inclusions play the role of "the elementary scatterers", when the electromagnetic wave irradiates the composite and induces an electrical dipole moment in each inclusion. These induced dipole moments form the dipole response of the composite, which can be characterised by some effective permittivity. The field dependence of the effective permittivity arises from a high field sensitivity of the ac surface impedance of a ferromagnetic wire. In the vicinity of the antenna resonance (related with the short wire inclusions) any variations in the magneto-impedance of wires result in large changes of the effective permittivity. Therefore, this composite demonstrates both the tuneable and resonance properties (selective absorption). Thus, we have demonstrated a possibility of using the MI effect to design field-controlled composites and band-gap structures. A number of applications can be proposed, including selective microwave coatings with the field-dependent reflection/transmission coefficients and selective tuneable waveguides where the composite material may be used as an additional field-dependent coating. In addition, in the final chapter of future work we will take a quick look at tuneable composites with other microstructures and methods of the excitation

Very large Magneto-impedance and its scaling behavior in amorphous Fe73.5Nb3Cu1Si13.5B9 ribbon

Magneto-impedance (MI) effects have been observed for amorphous Fe73.5Nb3Cu1Si13.5B9 ribbon which has been excited by an a.c. magnetic field parallel to the length of the ribbon. Maximum relative change in MI as large as -99% was observed which has never been reported before. The relative change in MI, when plotted against scaled field was found to be nearly frequency independent. A phenomenological formula for magneto-impedance, Z(H), in a ferromagnetic material, is proposed based on Pade approximant to describe the scaled behavior of MI.Comment: 20 pages, 7 figures, article in press, Physica B (2007

Anisotropic Dependence of Giant Magneto-Impedance of Amorphous Ferromagnetic Ribbon on Biasing Field

The magneto-impedance (MI) in amorphous ribbon of nominal composition Fe73.5Nb3Cu1Si13.5B9 has been measured at 1MHz and at room temperature for different configurations of exciting a.c and biasing d.c. fields. A large drop in both resistance and reactance is observed as a function of d.c magnetic field. When the d.c and a.c fields are parallel but normal to the axis of ribbon, smaller magnetic field is needed to reduce the impedance to its small saturated value compared to the situation when fields are along the axis of ribbon. Larger d.c. field is required to lower the impedance when the d.c field acts perpendicular to the plane of the ribbon. Such anisotropy in magneto-impedance is related to the anisotropic response of the magnetization of ribbon. The large change of impedance is attributed to large variation of a.c permeability on the direction and magnitude of the dc biasing field.Comment: 12 pages, 7 figures, to be published in "International Journal of Modern Physics B

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

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

Modeling of asymmetric giant magnetoimpedance in amorphous ribbons with a surface crystalline layer

A model describing the asymmetric giant magnetoimpedance (GMI) in field-annealed amorphous ribbons is proposed. It is assumed that the ribbon consists of an inner amorphous core and surface hard magnetic crystalline layers. The model is based on a simultaneous solution of linearizied Maxwell equations and Landau-Lifshitz equation. The coupling between the surface layers and the amorphous core is described in terms of an effective bias field. Analytical expressions for the frequency and field dependences of the ribbon impedance are found. The calculated dependences are in a qualitative agreement with results of experimental studies of the high-frequency asymmetric giant GMI in field-annealed amorphous ribbons.Comment: 13 pages, 3 figure

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