SENSITIVE FIELD-DEPENDENT AND FREQUENCY-DEPENDENT MAGNETOIMPEDANCE INNANOCRYSTALLINE FE73.5CUNB3SI13.5B9 RIBBONS

The magnetoimpedance spectra (0 less than or equal to f less than or e qual to 3 MHz) of amorphous and of nanocrystallized (550 degrees C ann ealing for 0.5-18 h) FeCuNbSiB ribbons were measured at room temperatu re using a commercial impedance analyzer as a function of applied magn etic field up to 75 Oe. The magnetoimpedance (MI), Z(f, H), shows stro ng dependence on frequency of the drive, ac current and magnetic field . For all the samples, the MI ratio Delta Z/Z = (Z(H-max - Z(0)\Z(H-ma x) first increases rapidly with frequency (f) and reaches a maximum at about 300-500 kHz: then decreases with f. The maximum value of the MI ratio Delta Z/Z, - 227%, is obtained in the sample annealed at 550 de grees C for 3 h with ac current at 300 kHz. At higher frequency, apeak in field dependence of the MI ratio (Z(H) - Z(0))/Z(H-max) was obtain ed for nanocrystalline samples, but not for as-quenched amorphous coun terparts. The field dependence of effective permeability has been show n to be responsible for their MI effects. (C) 1997 Elsevier Science S. A.