L. Kraus

Theory of giant magneto-impedance in the planar conductor with uniaxial magnetic anisotropy

Abstract An electrodynamic theory of GMI, based on a simultaneous solution of linearized Maxwells equations and Landau–Lifshitz equation of motion is presented. An analytical solution is found for the case of uniformly magnetized planar film with uniaxial in-plane magnetic anisotropy. The exchange-conductivity effect and two types of magnetic relaxation in the equation of motion are considered. The AC current is generally transmitted by four pairs of plane waves of mixed electromagnetic and spin-wave character. An analytical formula for the tensor of complex impedance is derived and is used for numerical calculations of longitudinal GMI effect. The influence of various material parameters on the magnitude and frequency dependence of GMI is discussed.

The theoretical limits of giant magneto-impedance

Abstract An electrodynamic theory of GMI-effect in planar film conductors with uniaxial in-plane anisotropy is reported. Exchange interactions and two types of magnetic relaxation are considered. Domain wall movements are neglected. The theoretical magnitudes of GMI about 104% can be obtained for exactly transversal anisotropy. Fluctuation of easy direction and the exchange-conductivity effect cause substantial reduction of GMI at frequencies below 1 GHz.

Microstructural and magnetic investigation of partially crystallized amorphous ribbons

Abstract We investigated microstructural and magnetic properties of partially crystallized Fe 64 Co 21 B 15 amorphous ribbons, prepared by different heat treatments. We found a deep correlation between microstructure (α-Fe nanograins ranging from 40 to 70 nm) and hysteresis properties. With the help of Preisach modelling we were able to identify, from the presence of two separated local coercive field peaks in the reconstructed Preisach distribution, two separate magnetization mechanisms associated with the amorphous and the crystalline phase. Both local coercive field peaks rescale according to a function of the mean grain size only. This result can be interpreted as a consequence of domain wall pinning by the nanograins.

Coercivity contributions and their temperature dependence in stress-annealed amorphous alloys

Abstract We have examined the uniaxial anisotropy induced by stress-annealing in amorphous or partially crystallized samples of ferromagnetic Fe-rich alloys. Several coercivity characteristics were measured and investigated in the temperature range 10–300 K. The coercivity contribution due to domain wall translations was found to be strongly dependent on the amount of the crystalline phase and on the magnitude of the induced anisotropy. The coercivity contribution connected predominantly with the domain nucleation appears to be independent of these two factors. All of the magnetic properties (magnetization, anisotropy and coercivity parameters) of the studied alloys were very stable within the temperature ranges investigated.

Magnetic properties of the crystalline and amorphous components of a nanocrystalline FeNbB alloy

Abstract Nanocrystallization of a rapidly quenched Fe 80.5 Nb 6.9 B 12.6 alloy was investigated. The crystalline fraction v er of a α - Fe (∼ 10 nm) was controlled by varying the annealing temperature from 450 to 580°C. Curie temperatures of both the amorphous matrix and the crystalline phase increase with increasing v cr , the latter being lower than T c of pure α -FE. The room temperature magnetization of the matrix decreases with v cr . The observed behaviour is explained by the presence of Nb in the crystalline grains.

Magnetoelastic properties of stress/field annealed Fe80Cr2B14Si14 amorphous alloy

Abstract The influence of stress/field annealing on magnetic domain structure and field dependence of Young modulus (Δ E -effect) of amorphous Fe 80 Cr 2 B 14 Si 4 ribbons was studied. The change of induced magnetic anisotropy is well reflected in magnetomechanical behaviour. The steepest reduction of Δ E is observed in the same region of the annealed stress σ a where the character of the domain structure changes. The experimentally observed behaviour is qualitatively well explained by the theoretical model developed under the assumption that the stress/field-induced anisotropy is given by a superposition of the stress-induced and field-induced components.

Influence of Si and Nb content on magnetostriction and creep-induced magnetic anisotropy of nanocrystalline FeNbCuSiB alloys

Abstract The saturation magnetostriction and creep-induced magnetic anisotropy of FeNb x Cu l Si y B alloys ( x = 3, 4.5; 0 ≤ y ≤ 16) nanocrystallized under tensile stress were investigated. The experimental results suggest that both the saturation magnetostriction λ s and the anisotropy constant K u are influenced by the Nb concentration. The observed effect of Nb on K u is discussed in terms of the model published recently by Herzer, according to which K u originates from magnetoelastic coupling of the α -FeSi nanocrystals with internal back stresses.

Creep-induced magnetic anisotropy and magnetostriction in partly nanocrystalline Fe74Nb3Cu1Si13B9 alloy

Abstract The magnetic anisotropy and saturation magnetostriction constant of Fe 74 Nb 3 Cu 1 Si 13 B 9 devitrified under applied stress have been investigated. Various annealing times and temperatures are used to control the amount of nanocrystalline phase. The interpretation of creep-induced anisotropy by magnetoelastic coupling with internal back-stresses in FeSi grains leads to the conclusion that gradual enrichment of FeSi phase in Si takes place with increasing crystalline fraction.

A magnetoelastic study of a transverse creep-induced magnetic anisotropy in an FeNb-based amorphous alloy

Abstract The hard ribbon axis anisotropy induced in the magnetostrictive amorphous alloy Fe 74.7 Nb 3 Si 12.8 B 9.5 by stress annealing at temperatures 633–693 K and under tensile stresses up to 204 MPa has been studied by several experimental methods (measurement of dc hysteresis loops, field dependence of engineering magnetostriction λ e and Youngs modulus E , and domain structure observations). The experimental data are discussed in terms of the phenomenological model developed earlier by Squire. The most significant improvement in the magnetoelastic response was found for the highest annealing temperature T a = 693 K. The maximum ΔE effect (27%) observed is much lower than the maximum estimated theoretical value (87%). The λ e and domain studies suggest that this difference is most probably due to the dispersion of easy axes and/or anisotropy values. Even though the phenomenological model does not take into account local variations in the anisotropy, it gives a reasonable quantitative description of a slightly perturbed hard ribbon axis case.