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

The giant magnetoimpedance effect in composite wires consisting 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 magnetic material.     

The effect of surface crystalline layers on asymmetric off-diagonal magnetoimpedance in field-annealed CoFeSiB amorphous ribbons

The off-diagonal magnetoimpedance in field-annealed CoFeSiB amorphous ribbons was measured using a pick-up coil wound around the sample. One side of a ribbon was etched in hydrofluoric acid solution during various times in order to change the thickness of the surface crystalline layer appearing after annealing. The asymmetric two-peak field dependence of the off-diagonal impedance was observed for all samples. The evolution of the off-diagonal magnetoimpedance with the change in the ribbon thickness is analyzed.     

Effect of torsional stresses on the magnetoimpedance of amorphous wires with negative magnetostriction

A model to describe the effect of torsional stresses on the magnetoimpedance of amorphous wires with negative magnetostriction is suggested. An approximate expression for the impedance with regard to the spatial distribution of magnetoelastic anisotropy induced by torsional stresses is derived. It is shown that the relative variation of the impedance is maximal near a critical stress value at which the surface magnetic structure of the wire changes. The calculated dependences of the impedance on the external magnetic field and torsional stress are in good qualitative agreement with experimental data for amorphous wires with negative magnetostriction.     

Frequency spectrum of the nonlinear magnetoimpedance of multilayer film structures

Magnetization reversal by high-frequency current in FeCuNbSiB/Al/FeCuNbSiB three-layer film structures is studied. The frequency spectrum of the voltage arising in a coil wound on the sample as a function of a permanent magnetic field (nonlinear magnetoimpedance) is taken. It is shown that the frequency spectra of the voltage are qualitatively different for the longitudinal and transverse orientations of the field with respect to the direction of the current. Frequency spectrum harmonics are demonstrated to be highly sensitive to a magnetic field. A simple electrodynamic model to describe experimental data is suggested.     

A model for torsion-stress effect on nonlinear magnetoimpedance in amorphous wires with negative magnetostriction

A model to describe the influence of torsional stress on nonlinear magnetoimpedance in amorphous wires with negative magnetostriction is proposed. The nonlinear voltage response is found in the framework of the low-frequency approximation taking into account the spatial distribution of the circular magnetic field and the magnetoelastic anisotropy induced by the torsional stress. It is demonstrated that the application of torsional stress results in an increase of the second harmonic amplitude in voltage due to a reinforcement of helical anisotropy in the wire. The second harmonic amplitude is analyzed as a function of external field, torsional stress and current amplitude. The ranges of torsional stress and current amplitude to achieve maximal field sensitivity of the second harmonic are found.     

Propagation of a normal-superconductor boundary along a high-T c superconducting film heated by microwave radiation

The dynamics of the superconducting-normal (S-N) transition in a thin high-temperature superconducting film heated by microwave radiation is investigated theoretically. The dependence of the rate of propagation of the normal-superconductor (NS) interphase boundary on the intensity of the radiation is obtained by solving the two-dimensional nonstationary heat conduction equation. It is shown that in calculating this dependence it is important to take into account two-dimensional effects connected with nonlinearity of heating over the substrate thickness, the reverse side of which is stabilized with respect to temperature. The results obtained may be important in investigating S-N transitions in superconducting devices used in the microwave range. Zh. Tekh. Fiz. 68, 111–116 (January 1998)     

The orientation-effect of exchange bias on giant magnetoimpedance in surface crystallized Co66Fe4B15Si15 amorphous ribbons

We report the results of measurements of the low-temperature specific heat C_p(T) and the ac susceptibility χ_ac(T) in low applied magnetic fields for a series of samples of Eu_1-xCa_xB₆. The anomalies in C_p(T), together with the results for χ_ac(T) and M(H), confirm the onset of phase transitions to long range magnetic order for x < 0.7 and provide evidence that for x ≥ 0.7, the Eu moments, which are captured in large magnetic clusters with magnetic moments of the order of 260 μ_B, adopt a spin-glass type ground state. The data set allows to establish the low-temperature [ T,x] phase diagram of this alloy series.     

Effects of helical anisotropy on nonlinear voltage response in amorphous wires

The nonlinear voltage response in soft magnetic amorphous wires exciting by an alternating current is studied. The frequency spectrum of the voltage in the pick-up coil wound around the wire with a helical anisotropy is found in the framework of a model based on quasi-static Stoner−Wohlfarth magnetization reversal. The effect of a deviation of the anisotropy axis from the azimuthal direction on the field dependences of amplitudes of voltage harmonics is analyzed. It is shown that the field sensitivity of even harmonics increases with the anisotropy axis deviation angle. The current amplitude range to obtain a maximal field sensitivity of the second harmonic is found. The influence of the skin effect on the frequency spectrum of the pick-up coil voltage is discussed. The results obtained may be of importance for the development of sensors of a weak magnetic field.