Influence of nanocrystallization on the evolution of domain patterns and the magnetoimpedance effect in amorphous Fe73.5Cu1Nb3Si13.5B9 ribbons

The evolution of the magnetic domain patterns and the giant magnetoimpedanc e (MI) effect in the Fe73.5Cu1Nb3Si13.5B9 amorphous ribbons are studied as a function of the annealing time t(a) at 550 degreesC. It is shown that by annealing at 550 degreesC for t(a) from 1 to 150 h, very fine nanocrystalli ne bcc-FeSi grains are homogeneously formed in the amorphous matrix. Althou gh with increasing t(a) the grain size remains very fine (less than or equa l to 11 nm), the coercivity H-c increases rapidly from 0.0014 Oe for t(a) = 3 h to 1.67 Oe for t(a) = 150 h. For the nanocrystalline ribbons with t(a) = 3 h, the domain structure is characterized by a few broad longitudinal t ogether with some broad transverse domain patterns, connected to the minimu m coercitive field. With increasing annealing time t(a)less than or equal t o 10 h, only transverse domain patterns are observed and the transverse dom ain width gradually becomes narrow. Considering the different magnetic soft ness, domain structure, and magnetization process, various types of the MI effect are observed in these nanocryastalline ribbons. A maximum value for the MI ratio DeltaZ/Z = [Z(H)-Z(H-max)]/Z(H-max), of about 400%, and a maxi mum sensitivity of the MI ratio, of about 170% Oe-1, are obtained in the na nocrystalline samples annealed at 550 degreesC for 3 and 10 h, respectively . These maximum MI effects are related to the optimum combination of soft p roperties with the transverse domain patterns. It is revealed that the magn etic softness together with the domain width of a transverse domain pattern are the key parameters to realize a large MI ratio. (C) 2001 American Inst itute of Physics.