THE INFLUENCE OF Co SUBSTITUTION ON THE MAGNETIC PROPERTIES AND THE GIANT MAGNETO-IMPEDANCE EFFECT OF (Fe_1-xCo_x)₈₄Zr_3.5Nb_3.5B₈Cu₁

The microstructure, magnetic properties and the giant magneto-impedance (GMI) effect of the nanocrystalline (Fe_1-xCo_x)₈₄Zr_3.5Nb_3.5B₈Cu₁ ribbons have been studied. With increasing Co concentration, the nanocrystallized grain size and volume fraction of the \alpha-Fe(Co) phase decrease under the same annealing condition, while the coercivity H_c increases from 1.92 A/m at x=0.0 to 30.38 A/m at x=0.8. The effective permeability drops sharply with increasing Co concentration so that the GMI effect goes down. A value of more than 380% for the magneto-impedance(MI) ratio was measured at frequency f=2.5 MHz and the sensitivity can reach 0.9% (A/m)^-1 for Fe₈₄Zr_3.5Nb_3.5B₈Cu₁ ribbon. When x is equal to 0.8, the MI ratio drops to about 30% and the sensitivity only reaches a value of lower than 0.025% (A/m)^-1. The concise analysis in theory is also expounded.