A.S. Antonov

Residual quenching stresses in glass-coated amorphous ferromagnetic microwires

The calculation of the residual stress tensor components in glass-coated amorphous ferromagnetic microwire is carried out on the basis of the theory of viscoelasticity. The approach takes into account the relaxation of the stresses both in a metallic core and a glass shell of the wire within a certain temperature interval near the point of the wires vitrification. The distribution of the residual stresses is investigated as function of mechanical characteristics of metallic core and glass shell at different ratios of the metallic core radius to the total wire radius. The magnetic behaviour of a glass-coated amorphous microwire with small negative magnetostriction is analysed and is shown to be consistent with the experimental data.

Theory of giant magneto-impedance effect in amorphous wires with different types of magnetic anisotropy

Abstract The effect of various types of magnetic anisotropy on the giant magneto-impedance spectra of amorphous wires with low magnetostriction is studied theoretically. For the first of the model considered the easy anisotropy axis is supposed to be parallel to the wire axis, whereas for the second model it has circumferential direction. In case of a wire with axial type of magnetic anisotropy the transverse magnetic permeability is obtained as a function of external magnetic field amplitude H 0 and other magnetic parameters of the wire. A strong dependence of this quantity on H 0 is shown to explain the giant magneto-impedance (GMI) effect in this type of amorphous wire. Besides, the amplitude of the GMI effect depends substantially on the value of a phenomenological damping parameter. In case of a wire with circumferential anisotropy the classical expression [Landau and Lifshitz, Electrodynamics of Continuous Media, 2nd ed, Pergamon, New York, 1984] for the wire impedance is not valid in the range of external magnetic field 0⩽ H 0 ⩽ H a , where H a is the anisotropy field. In this range of external magnetic field the wire impedance becomes a tensor. It has both longitudinal and transverse components that can be measured experimentally [Antonov et al., IEEE Trans. Magn. 33 (1997) 3367]. In case of a wire with circumferential anisotropy the peak of the longitudinal component of wire impedance as the function of H 0 corresponds to the anisotropy field, whereas in case of axial anisotropy it is found at zero magnetic field. The results obtained show that the good magnetic softness of amorphous wire is one of the most important conditions to observe the GMI effect; neither any domain structure nor circumferential anisotropy are strictly necessary.

Theory of giant magneto-impedance effect in composite amorphous wire

Abstract It is shown that for a composite wire consisting of a nonferromagnetic inner core and amorphous ferromagnetic shell layer the amplitude of the GMI effect has raised considerably when the conductivity of the inner core is much larger than that of the shell region.

Nondiagonal impedance of amorphous wires with circular magnetic anisotropy

Abstract The effect of the nondiagonal magnetoimpedance of amorphous wires with low magnetostriction is studied experimentally and theoretically. The nondiagonal effect was observed and investigated. For a wire with circumferential magnetic anisotropy the tensor of magnetic permeability is obtained as a function of external magnetic field amplitude H and other parameters. A strong dependence of this quantity on H is shown in the domain of the external magnetic field, H ⩽ H A where H A is the anisotropy field. The nondiagonal component of the surface impedance is responsible for the measured effect.

Possible origin for the bamboo domain structure in Co-rich amorphous wire

It is shown that the bamboo domain structure in the outer shell of the Co-rich amorphous wire may arise due to small disturbances in the axial symmetry of the wire. As an example, an elliptical deformation of the wire cross-section is considered. If the anisotropy constant has sufficient value near the wire surface, even small elliptical deformation may lead to a finite period of the domain structure of the order of wire radius.

Nonlinear magnetization reversal of Co-based amorphous microwires induced by an ac current

The magnetization reversal of ferromagnetic Co-based amorphous microwires carrying ac current is studied. The frequency spectra of the voltage V induced in the pick-up coil wound around the microwires were analysed and were found to depend on the ac current amplitude I0 and external longitudinal dc magnetic field He. The even harmonics are shown to dominate in the frequency spectra of V if the ac current amplitude exceeds some threshold value, which strongly depends on He. A high sensitivity of the even harmonic amplitudes to the longitudinal dc field is demonstrated. The experimental data are described in terms of the Faraday law and the quasi-static Stoner-Wohlfarth model. The results obtained may be of importance for the design of weak magnetic field sensors.

High-frequency giant magneto-impedance in multilayered magnetic films

We present the results of experimental and theoretical investigations of giant magneto-impedance (GMI) in the thin-film multilayer structure of F/SiO2/Ti/Cu/Ti/SiO2/F, where F stands for the ferromagnetic alloy having a composition of Fe73.5Cu1Nb3Si16.5B6. The layers were deposited under a transversemagnetic field Hp = 200 Oe applied in the plane of the sample perpendicular to its long side and were annealed at 280°C under Hp = 420 Oe for 2h. The GMI effect was measured at frequencies ranging from 0.5 to 1250 MHz using a waveguide methhod. An external magnetic field with a strength of up to 130 Oe was applied along the direction of the alternating current. The GMI is shown to exist in the multilayer structure even if the skin depth in the ferromagnetic layers is of the order of their thickness. Both real and imaginary parts of the impedance vary significantly under external magnetic field. A maximum GMI sensitivity of 10–35%/Oe was achieved at high frequencies.

The features of GMI effect in amorphous wires at microwaves

A new waveguide method is used to investigate high frequency GMI and magnetic permeability in Co-rich amorphous wire. The GMI spectra at microwaves are quite different from those measured at relatively low frequency. The amplitude of magnetic permeability decreases with increasing frequency, however, its relative change under external magnetic field is still large which enables a high sensitivity of GMI sensors of up to 1000 MHz.

Magnetization reversal process and nonlinear magneto-impedance in Cu/NiFe and Nb/NiFe composite wires

The magnetization reversal of Cu/NiFe and Nb/NiFe composite wires carrying AC current is studied. The frequency spectrum of a voltage induced in a pick-up coil wound around the wire is analyzed. The frequency spectrum is shown to consist of even harmonics within a wide range of AC current amplitudes and longitudinal DC magnetic fields. The strong dependencies of the harmonic amplitudes on the DC field are found. The results obtained may be of importance for the design of weak magnetic field sensors.

Magnetic properties of short amorphous microwires

Abstract Magnetic properties of amorphous Fe- and Co-rich wires with different magnetostriction are studied in dependence on their length (8 mm L L * is estimated. It is shown that for L L * the influence of demagnetizing fields on the wire magnetization is substantial and the wire remanent magnetization is proportional to the square of the wire length. The oscillations of the magnetic moment component parallel to the wire axis during the cross remagnetization processes are found.