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 h R, with a frequency F from 1 to 100 Hz, when the ribbon sample has been at the annealing temperature T A (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 T A = 200 °C RFA treatment, the maximum MI ratio (Δ Z/ Z) m = 72% and the maximum operating frequency f m = 0.6 MHz; [II] after the F = 50 Hz and T A = 160 °C RFA treatment, the maximum field sensitivity S m = 4.5%/Oe and the highest figure of merit FOM = 1.8% MHz/Oe; [III] since both of the optimal T As above are much lower than the crystallization temperature ( T X = 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 h R, operating at T A 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 Δ Z/ Z ratio of any amorphous ribbon sample, including VAC6025.