Surface Magnetization Processes Research Articles

Magnetic Microwires with Unique Combination of Magnetic Properties Suitable for Various Magnetic Sensor Applications.

There is a pressing demand to improve the performance of cost-effective soft magnetic materials for use in high performance sensors and devices. Giant Magneto-impedance effect (GMI), or fast single domain wall (DW) propagation can be observed in properly processed magnetic microwires. In this paper we have identified the routes to obtain microwires with unique combination of magnetic properties allowing observation of fast and single DW propagation and GMI effect in the same microwire. By modifying the annealing conditions, we have found the appropriate regimes allowing achievement of the highest GMI ratio and the fastest DW dynamics. The observed experimental results are discussed considering the radial distribution of magnetic anisotropy and the correlation of GMI effect, and DW dynamics with bulk and surface magnetization processes. Studies of both Fe- and Co-rich microwires, using the magneto-optical Kerr effect, MOKE, provide information on the magnetic structure in the outer shell of microwires. We have demonstrated the existence of the spiral helical structure in both studied microwires. At the same time, torsion mechanical stresses induce helical bistability in the same microwires, which allow us to consider these microwires as materials suitable for sensors based on the large Barkhausen jump.

Surface magnetization processes in soft magnetic nanowires

The surface magnetization processes taking place in simple permalloy (Py) and FeGa nanowires, Py/Cu, CoFeB/Cu, CoNiP/Cu, FeGa/Py, and FeGa/CoFeB multilayered nanowires have been studied by magneto-optical Kerr effect (MOKE) magnetometry. The results indicate a strong correlation between the direction of the anisotropy axis relative to the direction of the applied field and the plane of incidence of the laser spot, as well as the effect of dipolar interactions between the nanowires or between the ferromagnetic layers on the magnetization reversal. The larger laser spots are inducing more noise in the MOKE hysteresis loops because of the dimensional imperfections along the nanowires.

Surface Magnetization Processes in Amorphous Microwires

Surface magnetization processes in ferromagnetic glass-coated amorphous microwires with positive, negative, and nearly zero magnetostriction have been investigated. The results indicate helical magnetization in the surface region of positive magnetostrictive microwires and enhanced soft magnetic properties on the axial direction. The typical bistable behavior of such microwires is maintained within the surface region. Nearly zero magnetostrictive microwires with diameters larger than 20 ¿m also display bistability in the surface region, whilst negative magnetostrictive ones are not bistable. The values of the metallic nucleus diameter and glass coating thickness affect the surface magnetoelastic anisotropy, as emphasized by the changes in the values of the axial and transverse switching fields. The obtained results are important for a better understanding and control over the surface magnetization processes for high frequency applications of amorphous microwires.

Giant magnetoimpedance effect in soft magnetic wire families

New results on the giant magnetoimpedance effect in soft magnetic amorphous and nanocrystalline wires and microwires are presented. The relation between the magnitude of the effect and the dynamic surface magnetization processes is analyzed. The special role of magnetostriction is discussed with a special emphasis on the possibilities it offers for improving the magnetoimpedance response of wire-shaped materials.

Continuous change of surface magnetization direction from perpendicular to planar in soft magnetic CoP multilayers

Amorphous soft magnetic CoP electrolytic multilayers have been obtained with a controlled surface magnetic anisotropy. The surface magnetization easy direction can be continuously altered from an initial perpendicular position to a final arrangement within the longitudinal plane. This continuous change of magnetization direction is controlled during the sample production stage and, therefore, there is no need to subject the films to subsequent treatments such as, for example, thermal, stress, or magnetic field annealing. The magnetic domain configurations, as observed by means of the Bitter technique, have revealed a continuous variation from an initial stripe domain structure to a final situation in which the magnetization lies within the plane of the films and magnetic domain walls are observed. The magneto-optic behavior of these multilayers has also revealed this continuous evolution from an initial perpendicular easy direction for the magnetization to a configuration in which the surface magnetization has a planar longitudinal direction. The surface magnetization processes are hence modified from the initial rotation magnetization to final processes corresponding to magnetic wall displacements. Control of magneto-optic behavior can be of interest for technical applications, given there would be no need for sense windings. A phenomenological discussion about the evolution of the surface magnetization easy direction is also presented.

Surface magnetization processes investigated by the combined surface magneto-optical Kerr effects in Fe/Cu(100) thin films

Surface and ultrathin-film anisotropies in epitaxial fcc Fe thin films grown on room temperature Cu(100) single crystals in an ultrahigh vacuum has been investigated, in situ, by the combined surface magneto-optical Kerr effects (SMOKE). When the applied field is perpendicular to the film surface, rotation processes occurring between out-of-plane and in-plane directions suggest the presence of an induced uniaxial (canted) anisotropy directed out of the plane of the film. This hypothesis is tested with a simple coherent rotation model.

Surface hysteresis curves of Fe(110) and Fe(100) crystals in ultrahigh vacuum — evidence of adsorbate influences

We have investigated hysteresis curves of silicon-stabilized Fe(100) and Fe(110) crystals using Polarized Secondary Electron Emission (PSEE) for surface magnetization processes and Magneto-Optical Kerr Effect (MOKE) for bulk hysteresis curves. In particular, the influence of adsorbates (oxygen and sulphur) and surface roughness on the magnetization reversal has been studied. For surface hysteresis curves we found adsorbate-induced reductions of coercive force of up to 50%; the bulk hysteresis curves showed significantly smaller deviations. Similar changes were caused by surface disorder on an atomic scale induced by ion bombardment. The results agree qualitatively with a previous experiment on Ni(110).

Wetting transitions in the semi-infinite Ising ferromagnet

Surface magnetization processes in a semi-infinite Ising ferromagnet with a surface field applied oppositely to the magnetization direction in the bulk are investigated by the use of the effective-field theory with correlations. The first-order (wetting) transitions are obtained and discussed in the light of current theories.

Mossbauer effect study of surface magnetization processes in iron

Magnetization processes at the surface of iron plates were investigated in a variable external field by means of Mossbauer emission and scattering spectroscopy of57Fe. It is shown that the 90‡ walls of closure domains do not move until an externally applied field reaches a certain value. The “starting” values of the field determined from the Mossbauer measurements agree with those at which a sudden decrease in the growth rate of the bulk magnetization starts. Blocking of the surface domain wall movement occurs in a layer of at least 10 Μm thickness.

On the influence of defects on the magnetization processes taking place on the surface of polycrystalline Fe-Si samples

In work with the magnetooptical method, using Kerr's transverse effect, magnetization processes on the surfaces of polycrystalline Fe-Si sheets near to the (110) planes are studied. The influence of defects and the degree of the defects of the surface on the course of the surface hysteresis loops are studied in particular. It is found that the presence of defects on the surface of a ferromagnet influence the course of surface magnetization processes in a pronounced and varying way.