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.