N.A. Buznikov

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.

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 and magneto-impedance of cold-drawn permalloy-copper composite wires

The permalloy-copper composite wires consisting of high conductivity Cu inner core and NiFe outer shell were fabricated by a modified cold-drawn technique, The diameter of the Cu inner core was in 20-120 /spl mu/m range, and the NiFe outer shell was 10-40 /spl mu/m thick. The composite wires exhibit soft magnetic behavior and the giant magneto-impedance effect. The amplitude of the giant magneto-impedance at high frequencies is comparable to that of conventional amorphous wires and significantly larger than that of NiFe wires without Cu inner core. Nevertheless, the sensitivity of these wires does not reach the theoretical predictions based on the simple models for composite wires with the axial or circumferential types of magnetic anisotropy.

The effect of stripe domain structure on dynamic permeability of thin ferromagnetic films with out-of-plane uniaxial anisotropy

Abstract The permeability is calculated for a thin ferromagnetic film with the stripe domain structure and out-of-plane uniaxial magnetic anisotropy. Analytical expressions for the frequency dependence of components of permeability tensor are derived with the use of the Smit–Beljers method, with the thickness of domain walls and the domain wall motion being neglected. The effect of the domain width and the angle between the anisotropy axis and the film plane on the frequency dependence of the permeability is analyzed. General equations relating the static permeability components and the ferromagnetic resonance frequencies are found. The results of the approach are applied to the derivation of the constraint for the microwave permeability of thin ferromagnetic films. The analysis of the constraint as a function of the axis deviation angle, the domain aspect ratio and the damping parameter allows the conditions to be found for maximal microwave permeability. The results obtained may be useful in connection with the problem of developing high-permeable microwave magnetic materials.

Domain-walls motion in glass-coated CoFeSiB amorphous microwires

A method for observation of domain-walls motion in amorphous microwires with circular magnetic anisotropy is proposed. Using the method, the magnetization reversal of glass-coated Co-based microwires induced by current pulses of high amplitude is studied. The magnetization reversal is shown to occur due to the nucleation of the domain walls at the sample ends and their subsequent motion along the microwire. The dependencies of the domain-wall velocity on the current pulse amplitude and a longitudinal DC magnetic field are measured. A model describing main features of experimental data is presented.

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

Abstract 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.

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

Abstract 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 linearized 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 GMI in field-annealed amorphous ribbons.

Dynamic permeability of ferromagnetic thin films with stripe domain structure

Summary form only given. A relationship constraining the static permeability and resonance frequency of thin ferromagnetic films has been established recently. This relationship is great importance in view of the problem of developing the materials possessing high permeability at microwaves. But the relationship is of limited applicability, being derived for a single domain film with uniaxial inplane anisotropy only. This paper deals with more practical occasion, where the effect of the stripe domain structure and of out-of-plane magnetic anisotropy on the dynamic permeability of a ferromagnetic thin film.

Nonlinear magnetoimpedance effect in soft magnetic amorphous wires extracted from melt

The nonlinear mode of the magnetoimpedance in Co-based amorphous wires extracted from the melt is studied. The frequency spectrum of a voltage induced in a pick-up coil wound around the wire is analyzed as a function of an ac current amplitude and a longitudinal dc magnetic field. It is observed that at sufficiently high ac current amplitudes even harmonics dominate in the pick-up voltage spectrum. A high sensitivity of even harmonics to the dc magnetic field is demonstrated. The studied nonlinear magnetoimpedance is promising for the design of weak magnetic field sensors.

High-frequency magnetic permeability of nanocomposite film

Abstract The high-frequency magnetic permeability of nanocomposite film consisting of the single-domain spherical ferromagnetic particles in the dielectric matrix is studied. The permeability is assumed to be determined by rotation of the ferromagnetic inclusion magnetic moments around equilibrium direction in AC magnetic field. The composite is modeled by a cubic array of ferromagnetic particles. The magnetic permeability tensor is calculated by solving the Landau–Lifshits–Gilbert equation accounting for the dipole interaction of magnetic particles. The permeability tensor components are found as functions of the frequency, temperature, ferromagnetic inclusions density and magnetic anisotropy. The obtained results show that nanocomposite films could have a rather high value of magnetic permeability in the microwave range.