Rotation-field annealing effect on magnetoimpedance of Co-based amorphous ribbons

Abstract

A new annealing method, called the RFA method, is introduced to improve the magnetoimpedance (MI) effect in Co-based amorphous ribbons; e.g. VAC6025. This method includes applying a sufficiently large rotating field hR, with a frequency F from 1 to 100 Hz, when the ribbon sample has been at the annealing temperature TA (from 160 to 280 °C) for 1 h, and then is field-cooled to room temperature. The best achievements are that: [I] after the F = 1 or 50 Hz and TA = 200 °C RFA treatment, the maximum MI ratio (ΔZ/Z)m = 72% and the maximum operating frequency fm = 0.6 MHz; [II] after the F = 50 Hz and TA = 160 °C RFA treatment, the maximum field sensitivity Sm = 4.5%/Oe and the highest figure of merit FOM = 1.8% MHz/Oe; [III] since both of the optimal TAs above are much lower than the crystallization temperature (TX = 500 °C), the samples remain ductile after RFA. In conclusion, we believe that the RFA treatment can greatly improve the MI performance of the VAC6025 ribbons, since hR, operating at TA and/or below, can effectively randomize the magnetic anisotropy, and thereby reduce the effective (or average) anisotropy energy 〈K〉. This is equivalent to stating that the RFA method is effective in enhancing the permeability or the ΔZ/Z ratio of any amorphous ribbon sample, including VAC6025.