A. Maričić

The Effect of Temperature on the Magnetic Properties of the Electrochemically Obtained Ni92.8 Mo7.2 Powder Pressed at Different Pressures

The Ni92.8Mo7.2 alloy powder was obtained by electrochemical codeposition from an ammonium solution of NiSO4 and (NH4)6 Mo7O24 at j = 100 mA m-2, on a titanium cathode. DSC measurements and determination of the dependence of electrical resistivity on temperature did not reveal any changes in powder structure in the temperature range from 293 to 460 K. Therefore, in this range, there was no significant change in magnetic susceptibility either. Structural relaxation took place in the temperature interval from 460 to 560 K causing an increase in magnetic susceptibility. At temperatures higher than 570 K, magnetic susceptibility rapidly decreased. The Curie temperature of the powder was 660 K. With the increasing powder pressing pressure the magnetic susceptibility increased while the electric resistivity decreased. With the pressing pressure increase, the pore size was decreased and a better contact between powder particles was established. This caused electrical resistivity decrease and magnetic susceptibility increase.

Magnetoimpedance Effect in Fe89.8Ni1.5Si5.2B3C0.5 Metallic Glass Ribbons

The magnetoimpedance (MI) effect in magnetically soft Fe89.8Ni1.5Si5.2B3C0.5 metallic glass ribbons is studied in this paper. As soft magnetic properties were determined by field induced anisotropy (and therefore by magnetic domain structure), ribbon samples were annealed under tensile stress in order to enhance induction of transverse anisotropy and to improve magnetoimpedance effect. Stress-annealing (SA) technique up to 420 0C/475 MPa/30 min. was used for tailoring electrical resistivity (ρ) and magnetic permeability (μ), i.e. two material properties that in classical skin effect determine the penetration depth δm and therefore have influence on MIeffect. The critical frequency of about 600 kHz was observed as the point with the initial increase of MI. Significant improvement of MI-response reaching the value Z/Z ≈ 25 %, after annealing at 420 0C, was recorded at driving frequency 4 MHz. The highest MI-element sensitivity was found for low magnetic field intensity where values of about 12 % / kA/m were attained.

Correlation between thermal, kinetic, magnetic and electric effects in the crystallization process of amorphous magnetic alloys

Abstract By use of differential scanning calorimetry the mechanism of crystallization (devitrification) was investigated for magnetic amorphous alloys of compositions Fe 82 B 13 Si 5 (A) and Fe 75 B 10 Si 15 (B) in the temperature range between ambient and 600 °C. It was shown that both alloys crystallize in two stages between 480° and 520 °C. The energies of activation of the process, E 1 = 435.2 kJ / mol and E 2 = 909.5 kJ / mol for alloy A were derived, with corresponding frequency factors Z 1 = 1.1×10 30 min −1 and Z 2 = 8.1 × 10 60 min −1 , where subscripts 1 and 2 denote the first and second stage of crystallization respectively. With alloy B the corresponding figures are E 1 = 660.2 kJ / mol , E 2 = 870.3 kJ / mol , Z 1 = 4.9 × 10 45 min −1 and Z 2 = 7.6 × 10 58 min 1 . The temperature dependence of the crystallization rate constants was followed with values at exo maxima of k 1 = 1.73 min −1 and k 2 = 4.17 min −1 for alloy A and k 1 = 3.1 min −1 and k 2 = 6.6 min −1 for alloy B. Parallel to the thermal and kinetic investigations, measurements of changes of the relative electric resistance and the relative magnetic permeability were followed. A clear correlation was observed between the changes in crystal structure (X-ray diffractograms) with the changes in electron and magnetic structures, particularly in the temperature range of the principal processes of alloy devitrification. The enthalpies of crystallization are rather close and are − ΔH =128 J/g for alloy A and 125 J/g for alloy B.