D. P. Makhnovskiy

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

We investigated the magnetic-field behavior of the off-diagonal impedance in Co-based amorphous wires under sinusoidal (50 MHz) and pulsed (5 ns rise time) current excitations. For comparison, we measured the field characteristics of the diagonal impedance 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 a pickup coil wound on it. These voltages are related to the diagonal and off-diagonal impedances, respectively. We demonstrate that these impedances have a different behavior as functions of axial magnetic field: the diagonal impedance is symmetrical, whereas the off-diagonal one is antisymmetrical with a near-linear portion within a certain field interval. For the off-diagonal response, the dc bias current is necessary to eliminate circular domains. In the case of the sinusoidal excitation without a dc bias current, the off-diagonal response is very small and irregular. In contrast, the pulsed excitation, combining both high- and low-frequency harmonics, produces the off-diagonal voltage response without additional biasing. This behavior is ideal for a practical sensor circuit design. We discuss the principles of operation of a linear magnetic sensor based on a complementary metal-oxide-semiconductor transistor circuit.

Field dependent permittivity of composite materials containing ferromagnetic wires

A type of a composite material is proposed, the microwave permittivity of which changes under the effect of a dc magnetic field applied to the whole composite sample. The composite consists of short ferromagnetic wires embedded into a dielectric matrix. A strong field dependence of the permittivity is seen in the vicinity of the antenna resonance, where the dispersion behavior can experience a transformation from a resonant spectrum to a relaxation one under the effect of the field. This permittivity behavior is due to a high sensitivity of the ac surface impedance of a ferromagnetic wire to a magnetic field, known as the magnetoimpedance (MI) effect. If the resonance-like dispersion behavior is realized, the real part of the effective permittivity can be made negative past the resonance for wire inclusion concentrations well below the percolation threshold. Applying a magnetic field, the negative peak continuously decreases as the dispersion tends to become of a relaxation type. The effective permittivit...

Experimental demonstration of tunable scattering spectra at microwave frequencies in composite media containing CoFeCrSiB glass-coated amorphous ferromagnetic wires and comparison with theory

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.

Optomagnetic composite medium with conducting nanoelements

A type of metal-dielectric composites has been proposed that is characterized by a resonancelike behavior of the effective permeability μ e f f in the infrared and visible spectral ranges. This material can be referred toas an optomagnetic medium. It consists of conducting inclusions in the shape of nonclosed contours or pairs of parallel sticks with length of 50-100 nm embedded in a dielectric matrix. The analytical formalism developed is based on solving the scattering problem for considered inclusions with impedance boundary condition, which yields the current and charge distributions within the inclusions. The magnetic properties originated by induced currents are enhanced by localized plasmon modes, which make an inclusion resonate at a much lower frequency than that of the half-wavelength requirement at microwaves. It implies that microstructure can be made on a scale much less than the wavelength and the effective permeability is a valid concept. The presence of the effective magnetic permeability and its resonant properties lead to unusual optical effects and open interesting applications. In particular, the condition for Brewsters angle becomes different resulting in reflectionless normal incidence from air (vacuum) if the effective permeability and permittivity are the same. The resonant behavior of the effective permeability of the proposed optomagnetic medium could be used for creation of optical polarizes, filters, phase shifters, and selective lenses.

Stress effect on magnetoimpedance in amorphous wires at gigahertz frequencies and application to stress-tunable microwave composite materials

The effect of tensile stress on magnetoimpedance (MI) in CoMnSiB amorphous wires at microwave frequencies (0.5–3GHz) 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 180MPa for frequencies 500MHz and 2.5GHz, 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 be used 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...

Two-dimensional analysis of magnetoimpedance in magnetic/metallic multilayers

This article concerns the effect of the in-plane film size on the magnetoimpedance (MI) characteristics in magnetic/metallic multilayers. The problem is approached by a two-dimensional solution of the Maxwell equations in a symmetrical three-layer film. If the edge effect is neglected, the magnetic flux generated by the current flowing through the film is confined within the outer magnetic layers. In a finite width sandwich, the flux leaks through the inner conductor. This process eventually results in a considerable drop in MI ratio if this width is smaller than the critical flux decay length depending on the effective transverse permeability, layer thickness, and frequency.

Effect of induced anisotropy on magnetoimpedance characteristics in NiFe∕Au∕NiFe sandwich thin films

The high frequency magnetoimpedance (MI) has been measured in NiFe∕Au∕NiFe narrow sandwich thin films with transverse, longitudinal, and crossed magnetic anisotropies over a frequency range of 50–500MHz. The MI elements have an opened structure with lengths of 2 or 5 mm, widths of 50–200μm and a total film thickness of 1.5μm. The magnetoimpedance characteristics with different field curve shapes, including asymmetrical, have been realized for certain magnetic anisotropy using a dc bias current. Initially the different MI field characteristics have been demonstrated in ferromagnetic microwires with circumferential, helical, and longitudinal anisotropies. Here we have reintroduced these field characteristics for the MI sandwich thin film elements. In an attempt to induce a particular anisotropy in the films the magnetic layers were rf sputtered in the presence of a strong magnetic field, with a further thermal restress treatment of the final MI elements. The use of thin film technology for MI is preferable ...

Off-diagonal magnetoimpedance in NiFe-Au-NiFe layered film and its application to linear magnetic sensors

We have measured the field dependence of the off-diagonal impedance in the megahertz frequency range for a NiFe-Au-NiFe layered film using a helical microcoil. The film and the coil were deposited by means of radio-frequency sputtering, and a transverse anisotropy in magnetic layers was established by applying a dc magnetic field during the deposition and by postproduction annealing. The film had 5 mm length, 50 /spl mu/m width, and 1.5 /spl mu/m total thickness. The helical microcoil had 23 turns with a 50 /spl mu/m turn width. We applied high-frequency excitation by means of the coil current and measured the induced voltage across the film stripe. This voltage response is directly proportional to the off-diagonal component of the total impedance tensor. We found that the plots of the real and imaginary parts of the off-diagonal impedance, as functions of the applied dc magnetic field, are antisymmetrical with respect to the field direction. The dc bias current through the film plays an important role: without the bias current, the measured signal is very small and irregular. The field antisymmetry demonstrated by the off-diagonal impedance can be utilized in highly sensitive and linear magnetic sensors, and we discuss the principles of operation of such sensors here.

Tunable and Self-Sensing Microwave Composite Materials Incorporating Ferromagnetic Microwires

New types of stress sensitive and magnetic field tunable microwave composite materials are discussed where embedded short ferromagnetic microwire inclusions are used as controllable radiative elements. The dc external magnetic field is applied to the whole composite structure. And, the local stress is transferred to the individual microwires through the accommodating composite matrix. The spatial and angular distributions of microwires can be random, partly ordered, or completely ordered. For a wide frequency range, the free-space microwave response of a wire-filled composite can be characterized by a complex effective permittivity with resonance frequency dispersion. The latter depends on the conductive and magnetic properties of the microwire inclusions that contribute to the ac microwire magnetoimpedance (MI). In the vicinity of the so-called antenna resonance frequency, which is defined by the length of microwires and matrix dielectric constant, any variations in the MI of the microwires will result in large changes of the effective permittivity, and hence the reflection and transmission coefficients for an incident microwave. The field or stress dependence of the effective permittivity arises from the corresponding field or stress sensitivity of the MI in the ferromagnetic microwires with induced circumferential or helical magnetic anisotropy, respectively. The strong field tunable effect in the proposed composite materials can be utilized to introduce reconfigurable microwave properties in coatings, absorbers, and randomizers, and also in new media such as microwave metamaterials and bandgap wire structures. A maximum field tunability of 30 dB was achieved for free-space transmission measurements when the external magnetic field changed from zero to ~40 Oe. The stress sensitivity of reflection and transmission coefficients opens up new possibilities for the distant non-destructive testing and evaluation of composite materials both in the laboratory environment and large scale applications. The stress tunability of transmission coefficient may reach up to 5-8 dB within the elastic limit. The reflection coefficient usually demonstrates less tunability in both cases (field and stress dependent) and may require a multilayer structure to achieve better results, but it is always strong enough for the stress sensing applications.

Stress-dependent magnetoimpedance in Co-based amorphous wires with induced axial anisotropy for tunable microwave composites

A remarkably strong dependence of magnetoimpedance (MI) on tensile stress has been observed in the microwave frequency range for thin CoFeCrSiB glass-coated microwires with an axial anisotropy induced by tension annealing. The relative change in impedance runs into more than 100% at 0.5-1.5 GHz for a characteristic stress of 600 MPa. It was demonstrated that this stress MI is due to the transformation of the anisotropy from the axial toward the circumferential under the effect of applied tension and does not require the assistance of the dc bias magnetic field. Such microwires incorporated into a dielectric matrix may constitute a new sensing medium that is characterized by the stress-dependent effective permittivity. The proposed medium can be used for the microwave visualization of the stress distribution inside of a composite structure or on its surface.