High-frequency giant magnetoimpedance in joule-heated Co-based amorphous ribbons and wires

The giant magnetoimpedance (GMI) effect in Co68.25Fe4.5Si12.25B15 melt-spun amorphous wires and ribbons was studied in the microwave region (45 MHz≤f≤6 GHz). Selected samples were submitted to dc joule-heating in vacuum, exploiting the heat released to the sample by a dc electrical current flow, to induce circular and transverse magnetic anisotropy. The high-frequency impedance was measured using a technique based on the S-parameter measurements of a specific transmission line by means of an automatic vectorial network analyser. Annealed materials, either in wire or ribbon form, display a linear increase of ΔZ/Z ratio with the applied magnetic field up to reaching a maximum (i.e. ΔZ/Z≈60% at H≈400 A/m) at low field values.